Introduction to Collections

Overview of Collections

Introduction

A database is collection of values. To create these values, you can use a collection class. Fortunately, instead of creating a class from scratch, the .NET Framework provides a very impressive library of interfaces and collection classes. The built-in interfaces of the .NET Framework lay a foundation that other classes use to implement and customize the desired functionality. The various built-in collection classes are meant to satisfy various purposes. Some classes are available to any application and can be used by any Windows control. Some other collection classes are nested in classes that particularly need them.

The .NET Framework supports collections in various namespaces. While the System.Collections namespace provides regular collection classes, the System.Collections.Generic namespace contains the equivalent generic classes. 

One the most routines operations performed on a database consists of reviewing its values. To assist you with this, the .NET Framework provides the IEnumerator and the IEnumerable interfaces that are defined in the System.Collections namespace. Their generic equivalences can be found in the System.Collections.Generic namespace. After implementing these interfaces, you can use the For Each operator to visit each value of the database.

To implement the System.Collections.IEnumerator interface, you must derive a class from it. Then, you must define the Reset(), the MoveNext() methods, and the Current property. Here is an example:

Imports System.ComponentModel

Public Class Enumerator
    Implements IEnumerator

    Private Names() As String
    Private Cur As Integer

    Public Sub New(ByVal lst() As String)
        Names = lst
        Cur = -1
    End Sub

    Public ReadOnly Property Current() Implements IEnumerator.Current
        Get
            Return Names(Cur)
        End Get
    End Property

    Public Sub Reset() Implements IEnumerator.Reset
        Cur = -1
    End Sub

    Public Function Movenext() As Boolean Implements IEnumerator.MoveNext
        Cur = Cur + 1

        If Cur < Names.Length Then
            Return True
        Else
            Return False
        End If
    End Function
End Class

To implement the System.Collections.IEnumerable interface, you must derive a class from it. When implementing the class, you must define an accessory method and the GetEnumerator() method that returns an IEnumerator object. Here is an example: 

Imports System.Collections

Public Class Enumerable
    Implements IEnumerable

    Private Names() As String

    Public Sub Identify(ByVal Values() As String)
        Names = Values
        Dim i As Integer

        For i = 0 To Values.Length - 1
            Names(i) = Values(i)
        Next
    End Sub

    Public Function GetEnumerator() As IEnumerator _
                    Implements IEnumerable.GetEnumerator
        Return New Enumerator(Names)
    End Function
End Class

Once you have implemented the interfaces, you can use For Each. Here is an example:

Public Class Exercise

    Private Sub Exercise_Load(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles MyBase.Load
        Dim strName As String
        Dim FullNames(7) As String

        FullNames(0) = "Gertrude Monay"
        FullNames(1) = "Paul Bertrand Yamaguchi"
        FullNames(2) = "Hermine Ngaleu"
        FullNames(3) = "Francine Mukoko"
        FullNames(4) = "Joseph Walters"
        FullNames(5) = "Patricia Katts"
        FullNames(6) = "Helen Cranston"
        FullNames(7) = "Paul Motto"

        Dim coll As Enumerable = New Enumerable

        coll.Identify(FullNames)

        For Each strName In coll
            lbxNames.Items.Add(strName)
        Next
    End Sub
End Class

While the IEnumerator and the IEnumerable interfaces serve as valuable accessories that allow a collection class to support enumeration, to actually create a collection class, there are other interfaces you can use to implement the functionality you want for your collection.

When you want to use a collection in your application, you may first check what classes are available in the .NET Framework. If you do not find a suitable class, you can create your own that implements one or more interfaces. As it happens, the .NET Framework ships with many of them and your next step is to choose which one you prefer. Some of the most commonly used interfaces are

• System.Collections.IComparer and System.Collections.Generic.IComparer:If you derive a class from this interface, you can define how two objects would be compared for similarity or difference
• System.Collections.IDictionary and System.Collections.Generic.IDictionary: This interface is used to create a collection class where each item is made of a  key=value combination

One of the primary pieces of information you should provide about a the values in a database is the number of values that a list is (currently) holding. When creating a collection class, to prepare it to provide this valuable information, you can (should) implement an interface named ICollection. The ICollection interface is defined in the System.Collections namespace while its equivalent of the same name is defined in the System.Collections.Generic namespace. This means that, if you are creating a class that implements it, you should include this namespace in the file. Here is an example for the System.Collections.ICollection interface:

Public Class BookCollection
    Implements ICollection

End Class

To assist you with keeping track of the number of items in a collection, the ICollection interface is equipped with a property named Count, which you must implement. To do this, you can create a private member variable that will actually keep a count of the number of items. The Count property can then be used to communicate this information to the clients of the class. Here is an example:

Public Class BookCollection
    Implements ICollection

    Private NumberOfBooks As Integer

    Public Sub New()
        NumberOfBooks = 0
    End Sub

    Public ReadOnly Property Count() As Integer_
                    Implements ICollection.Count
        Get
            Return NumberOfBooks
        End Get
    End Property

End Class

The ICollection interface also allows its implementer to copy some of its items to an array. To provide this functionality, the interface is equipped with a method named CopyTo, which you must implement. The syntax of this method is:

Sub CopyTo(array As Array, index As Integer)

This method takes two arguments. The first argument is the array that will receive the items. The second argument is the index of the item from where the copying operation will begin. Here is an example:

Public Class BookCollection
    Implements ICollection

    Private Books() As String
    Private NumberOfBooks As Integer

    Public Sub New()
        NumberOfBooks = 0
    End Sub

    Public ReadOnly Property Count() As Integer _
                    Implements ICollection.Count
        Get
            Return NumberOfBooks
        End Get
    End Property

    Public Sub CopyTo(ByVal ary As Array, ByVal index As Integer) _
                    Implements ICollection.CopyTo
        Dim i As Integer
        Dim bks(Count) As String

        For i = 0 To Count - 1
            bks(i) = Books(i)
            ary = bks
            i = i + 1
        Next
    End Sub
End Class

If you create a collection class, you can provide the ability to enumerate its items. When this is done, some time to time, you will want to identify or to know what item is currently being accessed. In case other collection classes are using the same function at the time you are accessing this information, you should have an object that is responsible for synchronizing the collection. To do this in your ICollection-based class, you must implement a property named SyncRoot. This property must return an Object object. Here is an example:

Public Class BookCollection
    Implements ICollection

    . . . No Change

    Public ReadOnly Property SyncRoot() As Object _
                Implements ICollection.SyncRoot
        Get
            Return Me
        End Get
    End Property
End Class

Besides the ability to specify the number of items in a collection, a class that implements the ICollection interface must retrieve a value that indicates whether its item is synchronized. To give this information, you must implement a Boolean property named IsSynchronized. Here is an example:

Public Class BookCollection
    Implements ICollection
	. . . No Change

    Public ReadOnly Property IsSynchronized() As Boolean _
                    Implements ICollection.IsSynchronized
        Get
            Return False
        End Get
    End Property
End Class

System.Collections.ICollection (and System.Collections.Generic.ICollection) exdepends the IEnumerable interface. This means that you should be able to use For Each in your ICollection-based class but you must create the functionality yourself, which is done by implementing the GetEnumerator() method. Even if you do not want to support this feature, you still must provide at least a skeleton for this method. Here is an example:

Public Class BookCollection
    Implements ICollection

    Private Books() As String
    Private NumberOfBooks As Integer

    Public Sub New()
        NumberOfBooks = 0
    End Sub

    Public ReadOnly Property Count() As Integer _
                    Implements ICollection.Count
        Get
            Return NumberOfBooks
        End Get
    End Property

    Public Sub CopyTo(ByVal ary As Array, ByVal index As Integer) _
                    Implements ICollection.CopyTo
        Dim i As Integer
        Dim bks(Count) As String

        For i = 0 To Count - 1
            bks(i) = Books(i)
            ary = bks
            i = i + 1
        Next
    End Sub

    Public ReadOnly Property SyncRoot() As Object _
                Implements ICollection.SyncRoot
        Get
            Return Me
        End Get
    End Property

    Public ReadOnly Property IsSynchronized() As Boolean _
                    Implements ICollection.IsSynchronized
        Get
            Return False
        End Get
    End Property

    Public Function GetEnumerator() As IEnumerator _
        Implements IEnumerable.GetEnumerator
        Return Nothing
    End Function
End Class

While it provides the minimum functionality of a collection, the System.Collections.ICollection (and the System.Collections.Generic.ICollection) interface is not equipped to perform the regular operations of a collection class, such as adding, retrieving, or deleting items from a set.

To assist you with creating a collection class as complete as possible, the .NET Framework provides an interface named IList. The IList interface is defined in the System.Collections namespace and its equivalent of the same name is defined in the System.Collections.Generic namespace. The interface is equipped with the methods necessary to add, insert, delete, or retrieve items from a collection. Because the functionalities of these methods may not suit you, to use these features, you must create a class that implements them.

1. Start Microsoft Visual Basic and create a new Windows Forms Application named MusicalInstrumentStore1
2. To create a dialog box, on the main menu, click Project -> Add Windows Form...
3. Set the name to CategoryEditor and click Add
4. Design the form as follows:
    

   Control Text Name Other Properties Label &Category:     TextBox   txtCategory Modifiers: Public Button OK btnOK DialogResult: OK Button Cancel btnCancel DialogResult: Cancel
   Form Property	Value
   FormBorderStyle	FixedDialog
   Text	Category Editor
   StartPosition	CenterScreen
   AcceptButton	btnOK
   CancelButton	btnCancel
   MaximizeBox	False
   MinimizeBox	False
   ShowInTaskbar	False

5. To create a dialog box, on the main menu, click Project -> Add Windows Form...
6. Set the name to TypeEditor and click Add
7. Design the form as follows:
    

   Control Text Name Other Properties Label &Item Type:     TextBox   txtItemType Modifiers: Public Button OK btnOK DialogResult: OK Button Cancel btnCancel DialogResult: Cancel
   Form Property	Value
   FormBorderStyle	FixedDialog
   Text	Type Editor
   StartPosition	CenterScreen
   AcceptButton	btnOK
   CancelButton	btnCancel
   MaximizeBox	False
   MinimizeBox	False
   ShowInTaskbar	False

8. To create a dialog box, in the Solution Explorer, right-click MusicalInstrumentStore2 -> Add -> Windows Form...
9. Set the name to ItemEditor and click Add
10. Design the form as follows:
     

    Control Text Name Other Properties Label &Item #:     TextBox   txtItemNumber   Label &Category:     ComboBox   cbxCategories   Button New C&ategory... btnNewCategory   Label &Item Type:     ComboBox   cbxItemTypes   Button New &Item Type... btnNewItemType   Label Item &Name:     TextBox   txtItemName   Label &Unit Price:     TextBox 0.00 txtUnitPrice TextAlign: Right Button Picture... btnPicture   TextBox   txtPicturePath   PictureBox   pbxPicturePath SizeMode: Zoom Button Create btnCreate   Button Close btnClose DialogResult: Cancel OpenFileDialog (Name): dlgOpen Title: Select Item Picture DefaultExt: jpg Filter: JPEG Files (*.jpg,*.jpeg)|*.jpg|GIF Files (*.gif)|*.gif|Bitmap Files (*.bmp)|*.bmp|PNG Files (*.png)|*.png
    Form Property	Value
    FormBorderStyle	FixedDialog
    Text	Musical Instrument Store - Item Editor
    StartPosition	CenterScreen
    MaximizeBox	False
    MinimizeBox	False
    ShowInTaskbar	False

11. Double-click the New Category button and implement it as follows:
      

    Private Sub btnNewCategory_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles btnNewCategory.Click Dim Editor As CategoryEditor = New CategoryEditor If Editor.ShowDialog() = DialogResult.OK Then If Editor.txtCategory.Text.Length > 0 Then Dim NewCategory As String = Editor.txtCategory.Text ' Make sure the category is not yet in the list If cbxCategories.Items.Contains(NewCategory) Then MsgBox(NewCategory& " is already in the list") Else ' Since this is a new category, add it to the combo box cbxCategories.Items.Add(NewCategory) ' Just in case the user wanted to use this new category ' select it cbxCategories.Text = NewCategory End If End If End If End Sub

12. In the Class name combo box, select btnNewItemType
13. In the Method Name combo box, select Click and implement the event as follows:
      

    Private Sub btnNewItemType_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnNewItemType.Click Dim Editor As TypeEditor = New TypeEditor If Editor.ShowDialog() = DialogResult.OK Then If Editor.txtItemType.Text.Length > 0 Then Dim NewType As String = Editor.txtItemType.Text ' Make sure the type is not yet in the list If cbxTypes.Items.Contains(NewType) Then MsgBox("The list already contains " & NewType) Else cbxTypes.Items.Add(NewType) cbxTypes.Text = NewType End If End If End If End Sub

14. In the Class name combo box, select btnPicture
15. In the Method Name combo box, select Click and implement the event as follows:
      

    Private Sub btnPicture_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnPicture.Click If dlgPicture.ShowDialog() = DialogResult.OK Then txtPicturePath.Text = dlgPicture.FileName pbxStoreItem.Image = Image.FromFile(txtPicturePath.Text) End If End Sub

16. In the Solution Explorer, right-click Form1.vb and click Rename
17. Type MusicStore.vb and press Enter twice (to display the form)
18. Design the form as follows:
     

    Control Text Name Other Properties Label Item Category:     ComboBox   cbxCategories   Label Items Type:     ComboBox   cbxTypes   Label Available Items     Button New Store Item... btnNewStoreItem   ListView   lvwStoreItems View: Details FullRowSelect: True GridLines: True Columns (Name) Text TextAlign Width colItemNumber Item #     colItemName Item Name/Description   320 colUnitPrice Unit Price Right   PictureBox   pbxStoreItem SizeMode: Zoom GroupBox Selected Items     Label Item #     Label Description     Label Unit Price     Label Qty     Label Sub Total     TextBox   txtItemNumber1   TextBox   txtDescription1   TextBox 0.00 txtUnitPrice1 TextAlign: Right TextBox 0 txtQuantity1 TextAlign: Right TextBox 0.00 txtSubTotal1 TextAlign: Right Button Remove btnRemove1   TextBox   txtItemNumber2   TextBox   txtDescription2   TextBox 0.00 txtUnitPrice2 TextAlign: Right TextBox 0 txtQuantity2 TextAlign: Right TextBox 0.00 txtSubTotal2 TextAlign: Right Button Remove btnRemove2   TextBox   txtItemNumber3   TextBox   txtDescription3   TextBox 0.00 txtUnitPrice3 TextAlign: Right TextBox 0 txtQuantity3 TextAlign: Right TextBox 0.00 txtSubTotal3 TextAlign: Right Button Remove btnRemove3   TextBox   txtItemNumber4   TextBox   txtDescription4   TextBox 0.00 txtUnitPrice4 TextAlign: Right TextBox 0 txtQuantity4 TextAlign: Right TextBox 0.00 txtSubTotal4 TextAlign: Right Button Remove btnRemove4   TextBox   txtItemNumber5   TextBox   txtDescription5   TextBox 0.00 txtUnitPrice5 TextAlign: Right TextBox 0 txtQuantity5 TextAlign: Right TextBox 0.00 txtSubTotal5 TextAlign: Right Button Remove btnRemove5   TextBox   txtItemNumber6   TextBox   txtDescription6   TextBox 0.00 txtUnitPrice6 TextAlign: Right TextBox 0 txtQuantity6 TextAlign: Right TextBox 0.00 txtSubTotal6 TextAlign: Right Button Remove btnRemove6   GroupBox Order Summary     Label Items Total:     TextBox 0.00 txtItemsTotal TextAlign: Right Label Tax Rate:     TextBox 7.75   TextAlign: Right Label %     Label Tax Amount:     TextBox 0.00 txtTaxAmount TextAlign: Right Label Order Total:     TextBox 0.00 txtOrderTotal TextAlign: Right Button Save btnSave   Label Receipt #:     TextBox 0.00 txtReceiptNumber   Button &Open btnOpen   Button New Order btnNewOrder   Button Close btnClose

19. To create a new class, in the Solution Explorer, right-click MusicalInstrumentStore2 -> Add -> Class...
20. Set the Name to StoreItem and press Enter
21. Change the file as follows:
      

    <Serializable()> Public Class StoreItem Private nbr As String Private cat As String Private tp As String Private nm As String Private prc As Double Public Property ItemNumber() As String Get Return nbr End Get Set(ByVal value As String) nbr = value End Set End Property Public Property Category() As String Get Return cat End Get Set(ByVal value As String) cat = value End Set End Property Public Property Type() As String Get Return tp End Get Set(ByVal value As String) tp = value End Set End Property Public Property ItemName() As String Get Return nm End Get Set(ByVal value As String) nm = value End Set End Property Public Property UnitPrice() As Double Get Return prc End Get Set(ByVal value As Double) prc = value End Set End Property Public Overrides Function Equals(ByVal Same As Object) As Boolean If (nbr = Same.nbr) And _ (cat = Same.cat) And _ (tp = Same.tp) And _ (nm = Same.nm) And _ (prc = Same.prc) Then Return True Else Return False End If End Function Public Sub New() nbr = "000000" cat = "Accessories" tp = "Accessories" nm = "Unknown" prc = 0.0 End Sub Public Sub StoreItem(ByVal itmNumber As String) nbr = itmNumber cat = "Accessories" tp = "Accessories" nm = "Unknown" prc = 0.0 End Sub Public Sub New(ByVal itmNumber As String, ByVal strCategory As String, _ ByVal strType As String, ByVal strName As String, _ ByVal dPrice As Double) nbr = itmNumber cat = strCategory tp = strType nm = strName prc = dPrice End Sub End Class

22. Save the file

As mentioned above, to create a collection, you can derive it from the IList interface. Here is an example:

Imports System.Collections

Public Class BookCollection
    Implements IList

End Class

This System.Collections.IList interface is declared as follows:

Public Interface IList _
	Implements ICollection, IEnumerable

This System.Collections.Generic.IList interface is declared as follows:

Public Interface IList(Of T) _
	Implements ICollection(Of T), _
	IEnumerable(Of T),  _
	IEnumerable

This means that the IList interface exdends both the ICollection and the IEnumerable interfaces. This also implies that you must implement the members of these parent interfaces. In other words, you must implement the Count property, the SyncRoot property, the IsSynchronized property, and the CopyTo() method of the ICollection interface.

If you use Microsoft Visual Basic 2008 to create the project, as soon as you type the Implements IList line and press Enter, the studio would generate the complete skeleton code for you:

Public Class BookCollection
    Implements IList

    Public Sub CopyTo(ByVal array As System.Array, ByVal index As Integer) _
				Implements System.Collections.ICollection.CopyTo

    End Sub

    Public ReadOnly Property Count() As Integer _
		 Implements System.Collections.ICollection.Count
        Get

        End Get
    End Property

    Public ReadOnly Property IsSynchronized() As Boolean _
		Implements System.Collections.ICollection.IsSynchronized
        Get

        End Get
    End Property

    Public ReadOnly Property SyncRoot() As Object  _
		Implements System.Collections.ICollection.SyncRoot
        Get

        End Get
    End Property

    Public Function GetEnumerator() As System.Collections.IEnumerator _
		Implements System.Collections.IEnumerable.GetEnumerator

    End Function

    Public Function Add(ByVal value As Object) As Integer _
		 Implements System.Collections.IList.Add

    End Function

    Public Sub Clear() Implements System.Collections.IList.Clear

    End Sub

    Public Function Contains(ByVal value As Object) As Boolean _
		 Implements System.Collections.IList.Contains

    End Function

    Public Function IndexOf(ByVal value As Object) As Integer _
		 Implements System.Collections.IList.IndexOf

    End Function

    Public Sub Insert(ByVal index As Integer, ByVal value As Object) _
		 Implements System.Collections.IList.Insert

    End Sub

    Public ReadOnly Property IsFixedSize() As Boolean _
		 Implements System.Collections.IList.IsFixedSize
        Get

        End Get
    End Property

    Public ReadOnly Property IsReadOnly() As Boolean _
		 Implements System.Collections.IList.IsReadOnly
        Get

        End Get
    End Property

    Default Public Property Item(ByVal index As Integer) As Object _
		 Implements System.Collections.IList.Item
        Get

        End Get
        Set(ByVal value As Object)

        End Set
    End Property

    Public Sub Remove(ByVal value As Object) _
		Implements System.Collections.IList.Remove

    End Sub

    Public Sub RemoveAt(ByVal index As Integer) _
			Implements System.Collections.IList.RemoveAt

    End Sub
End Class

You can then customize it. From what we learned with ICollection, here are examples of implementing these members for the System.Collections.IList interface:

Public Class BookCollection
    Implements IList

    Private Counter As Integer
    Private Objects(10) As Object

    Public Sub New()
        Counter = 0
    End Sub
      
    Public Sub CopyTo(ByVal array As System.Array, ByVal index As Integer) _
                      Implements System.Collections.ICollection.CopyTo

    End Sub

    Public ReadOnly Property Count() As Integer _
            Implements System.Collections.ICollection.Count
        Get
            Return Counter
        End Get
    End Property

    Public ReadOnly Property IsSynchronized() As Boolean _
            Implements System.Collections.ICollection.IsSynchronized
        Get
            Return False
        End Get
    End Property

    Public ReadOnly Property SyncRoot() As Object _
            Implements System.Collections.ICollection.SyncRoot
        Get
            Return Me
        End Get
    End Property
End Class

You must also implement the System.Collections.GetEnumerator() (or the System.Collections.Generic.GetEnumerator()) method of the System.Collections.IEnumerable  (or of the System.Collections.Generic.IEnumerable) interface. If you do not have time to completely implement it, you can simply return Nothing. Here is an example for the System.Collections.IList interface:

Public Function GetEnumerator() As System.Collections.IEnumerator _
                Implements System.Collections.IEnumerable.GetEnumerator
        Return Nothing
End Function

1. To create a new class, in the Class View, right-click MusicalInstrumentStore2 -> Add -> Class...
2. Set the Name to StoreItems and press Enter
3. Click the right side of the Public Class StoreItems line and press Enter
4. Type Implements IList and press Enter.
   Notice that a complete skeleton code has been generated
5. Change the file as follows:
     

   <Serializable()> Public Class StoreItems Implements IList Private Counter As Integer Private Items() As Object = New Object Public Sub New() Counter = 0 End Sub Public Sub CopyTo(ByVal array As System.Array, _ ByVal index As Integer) _ Implements System.Collections.ICollection.CopyTo End Sub Public ReadOnly Property Count() As Integer _ Implements System.Collections.ICollection.Count Get Return Counter End Get End Property Public ReadOnly Property IsSynchronized() As Boolean _ Implements System.Collections.ICollection.IsSynchronized Get Return False End Get End Property Public ReadOnly Property SyncRoot() As Object _ Implements System.Collections.ICollection.SyncRoot Get Return Me End Get End Property Public Function GetEnumerator() As System.Collections.IEnumerator _ Implements System.Collections.IEnumerable.GetEnumerator Return Nothing End Function ' This method is used to add a new item to the collection Public Function Add(ByVal value As Object) As Integer _ Implements System.Collections.IList.Add End Function ' This methods deletes all items from the collection Public Sub Clear() Implements System.Collections.IList.Clear End Sub ' This method is used to find out whether the item ' passed as argument exists in the collection Public Function Contains(ByVal value As Object) As Boolean _ Implements System.Collections.IList.Contains End Function ' This method is used to check whether the item passed as ' argument exists in the collection. If so, it returns its index Public Function IndexOf(ByVal value As Object) As Integer _ Implements System.Collections.IList.IndexOf End Function ' This method can be used to insert an item at ' a certain position inside the collection Public Sub Insert(ByVal index As Integer, ByVal value As Object) _ Implements System.Collections.IList.Insert End Sub Public ReadOnly Property IsFixedSize() As Boolean Implements _ System.Collections.IList.IsFixedSize Get End Get End Property Public ReadOnly Property IsReadOnly() As Boolean _ Implements System.Collections.IList.IsReadOnly Get End Get End Property Default Public Property Item(ByVal index As Integer) As Object _ Implements System.Collections.IList.Item Get Return Nothing End Get Set(ByVal value As Object) End Set End Property ' This method first checks the existence of the item passed ' as argument. If the item exists, the method deletes it Public Sub Remove(ByVal value As Object) _ Implements System.Collections.IList.Remove End Sub ' This method is used to delete the item positioned ' at the index passed as argument Public Sub RemoveAt(ByVal index As Integer) _ Implements System.Collections.IList.RemoveAt End Sub End Class

6. Save the file

In the next sections, we will see how to add values to a list of a database. As you add or insert values in a list, the Count property grows. If your collection is array-based, when you start it, you specify the number of values that the list will contain. In theory, you cannot add new values beyond that number. In reality, you can increase the size of an array and then add a new item. If your collection is a linked list, you are also not confined to the laws of space (unless your computer runs out of memory).

If you create a list whose number of values must be constant, the user cannot add values beyond the maximum allowed number. Therefore, before adding a value, you can first check whether the collection has a fixed size or not. To give you this information, the IList interface is equipped with a Boolean read-only property named IsFxedSize. This property simply lets the user know whether the collection has a fixed number of items.

Here is an example of implementing this property for the System.Collections.IList interface:

Public ReadOnly Property IsFixedSize() As Boolean _
            Implements System.Collections.IList.IsFixedSize
        Get
            Return False
        End Get
End Property

1. Change the IsFixedSize() method as follows:
    

   Public ReadOnly Property IsFixedSize() As Boolean Implements _ System.Collections.IList.IsFixedSize Get Return False End Get End Property

2. Save the file

Most databases are meant to receive new values. If you want, you can create a list that cannot receive new values. To support this, the IList interface is equipped with the Boolean IsReadOnly property. If a list is read-only, it would prevent the clients from adding items to it.

Here is an example of implementing the IsReadOnly property for the System.Collections.IList interface:

Public ReadOnly Property IsReadOnly() As Boolean _
            Implements System.Collections.IList.IsReadOnly
        Get
            Return False
        End Get
End Property

1. Change the IsReadOnly method as follows:
    

   Public ReadOnly Property IsReadOnly() As Boolean _ Implements System.Collections.IList.IsReadOnly Get Return False End Get End Property

2. Save the file

As it should be obvious, the primary operation to perform on a list is to populate it with at least one value. To support this, the System.Collections.IList interface is equipped with a method named Add. Its syntax is:

Function Add(value As Object) As Integer

This method takes one argument as the value to add to the list. If your collection is an array, you can first check that there is still enough room in the list to add a new item. In reality, this is never an issue with the System.Collections.IList interface:

• If there is still room in the collection, the value would be added to the list
• If there is not enough room, the value would simply not be added. There would not be a problem and the program would not crash. In fact, no exception would be thrown if the value was not added because of lack of space. On the other hand, since the compiler would not let you know that there was a problem with "logistic", you may not know whether the value was added or not. Therefore, if you are concerned with knowing whether the value was added, you must provide this functionality yourself 

If the method succeeds with the addition, it returns the position where the value was added in the list. This is usually the last position in the list.

Here is an example:

Public Function Add(ByVal value As Object) As Integer _
                    Implements System.Collections.IList.Add
        ' Check whether there is still room in
        ' the array to add a new item
        If Counter < Objects.Length Then
            ' Since there is room, put the new item to the end
            Objects(Counter) = value
            ' increase the number of items
            Counter = Counter + 1
            ' Return the index of the item that was added
            Return Counter - 1
            ' Since the item could not be added, return a negative index
        Else
            Return -1
        End If
End Function

1. Change the code of the StoreItems.Add() method as follows:
    

   ' This method is used to add a new item to the collection Public Function Add(ByVal value As Object) As Integer _ Implements System.Collections.IList.Add ' Find out if the array is getting too small for the next item(s) ' If it is, increase its size by 5 If Count = Items.Length Then Array.Resize(Items, Items.Length + 5) End If If Counter < Items.Length Then Items(Counter) = value Counter = Counter + 1 Return Counter - 1 Else Return -1 End If End Function

2. In the Class View, double-click ItemEditor
3. In the top section of the file, add the following:
    

   Imports System.IO Imports System.Runtime.Serialization.Formatters.Binary Public Class ItemEditor

4. In the Solution Explorer, right-click ItemEditor.vb and click View Code
5. In the Class name combo box, select btnCreate
6. In the Method Name combo box, select Click and implement the event as follows:
    

   Private Sub btnCreate_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnCreate.Click Dim StreamStoreItem As FileStream Dim Item As StoreItem = New StoreItem Dim items As StoreItems = New StoreItems Dim FormatterStoreItem As BinaryFormatter = New BinaryFormatter ' If this directory doesn't exist, create it Directory.CreateDirectory("C:\Musical Instrument Store") ' This is the file that holds the list of items Dim Filename As String = "C:\Musical Instrument Store\StoreItems.mis" ' Create a random number that will be used to identify the item Dim RndNumber As Random = New Random txtItemNumber.Text = RndNumber.Next(100000, 999999) ' Make sure the user had selected a category If cbxCategories.Text.Length = 0 Then MsgBox("You must specify the item's category") cbxCategories.Focus() Exit Sub End If ' Make sure the user had selected a type If cbxTypes.Text.Length = 0 Then MsgBox("You must specify the item's type") cbxTypes.Focus() Exit Sub End If ' Make sure the user had entered a name/description If txtItemName.Text.Length = 0 Then MsgBox("You must enter the name (or a " & _ "short description) for the item") txtItemName.Focus() Exit Sub End If ' Make sure the user had typed a price for the item If txtUnitPrice.Text.Length = 0 Then MsgBox("You must enter the price of the item") txtUnitPrice.Focus() Exit Sub End If ' Before saving the new item, find out if there was ' already a file that holds the list of items ' If that file exists, open it and store its items ' in our StoreItems list If File.Exists(Filename) Then StreamStoreItem = New FileStream(Filename, _ FileMode.Open, _ FileAccess.Read, _ FileShare.Read) Try ' Retrieve the list of items from file items = CType(FormatterStoreItem.Deserialize(StreamStoreItem), _ StoreItems) Finally StreamStoreItem.Close() End Try End If ' Create the music item item.ItemNumber = txtItemNumber.Text item.Category = cbxCategories.Text item.Type = cbxTypes.Text item.ItemName = txtItemName.Text item.UnitPrice = cdbl(txtUnitPrice.Text) ' Call the Add method of our collection class to add the item items.Add(item) ' Save the list StreamStoreItem = New FileStream(Filename, _ FileMode.Create, _ FileAccess.Write, _ FileShare.Write) Try FormatterStoreItem.Serialize(StreamStoreItem, items) If txtPicturePath.Text.Length <> 0 Then Dim flePicture As FileInfo = New FileInfo(txtPicturePath.Text) flePicture.CopyTo("C:\Musical Instrument Store\" & _ txtItemNumber.Text & flePicture.Extension) End If ' After saving the item, reset the form txtItemNumber.Text = RndNumber.Next(100000, 999999).ToString() cbxCategories.Text = "" cbxTypes.Text = "" txtItemName.Text = "" txtUnitPrice.Text = "0.00" txtPicturePath.Text = "" pbxStoreItem.Image = Nothing Finally StreamStoreItem.Close() End Try End Sub

7. Save the file

When you call the System.Collections.IList.Add() method, it adds the new value to the end of the list. Sometimes, you will want the new value to be insert somewhere inside the list. To support this operation, both the System.Collections.IList and the System.Collections.Generic.IList interfaces provide a method named Insert. The syntax of the System.Collections.IList.Insert() method is:

Sub Insert(index As Integer, value As Object)

The syntax of the System.Collections.Generic.IList.Insert() method is:

Sub Insert(index As Integer, item As T)

This method takes two arguments. The second argument is the value that will be inserted into the list. The argument must hold a valid value. Because this method takes an Object object, if your collection is using a different type of value, you may have to cast it to Object. The first argument is the index of the item that will precede the new one.

As mentioned for the System.Collections.IList. Add() method, there are a few things you should know about this operation's success or lack of it:

• If the index argument holds a negative value or a value higher than the allowed number (for example if the list is an array) of the items (depending on how you implement the method), the new value would not be added, the compiler would not throw an exception, and therefore nothing would let you know that the value was not added. If you want to find out whether the value was formally or actually inserted or not, you must create the functionality yourself
• If the value argument is not valid, again depending on how you structure your class, either the value would not be inserted or something else would go wrong. Fortunately, if the value argument is of the type of a class you created yourself, the compiler would produce an error such as letting you know that the argument is holding a value that is not conform to its property or member variable

While using a list, various operations require that you know the object you are currently accessing. To provide this operation, you must create an indexed property. This property should take an index and return the type of object that makes up the list. Here is an example:

Default Public Property Item(ByVal index As Integer) As Object _
                            Implements System.Collections.IList.Item
        Get
            Return Objects(index)
        End Get
        Set(ByVal value As Object)
            Objects(index) = value
        End Set
End Property

After creating this property, you can then access an item using its index and applying the () operator on its instance. Remember that if you want to use For Each, you must appropriately implement the IEnumerable.GetEnumerator() method.

1. In the Class View, click StoreItems
2. In the lower part of the Class View, double-click Item(Integer) As Object
3. Change the code of the property as follows:
    

   Default Public Property Item(ByVal index As Integer) As Object _ Implements System.Collections.IList.Item Get Return Items(index) End Get Set(ByVal value As Object) Items(index) = value End Set End Property

4. In the Solution Explorer, right-click ItemEditor.vb and click View Code
5. In the Class name combo box, select (ItemEditor Events)
6. In the Method Name combo box, select Load and implement the event as follows:
    

   Private Sub ItemEditor_Load(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.Load Dim i As Integer Dim RndNumber As Random = New Random ' Since all values seem ready, prepare to process the item Dim Item As StoreItem = New StoreItem Dim Items As StoreItems = New StoreItems Dim FormatterStoreItem As BinaryFormatter = New BinaryFormatter ' This is the file that holds the list of items Dim Filename As String = "C:\Musical Instrument Store\StoreItems.mis" If File.Exists(Filename) Then Dim StreamStoreItem As FileStream = New FileStream(Filename, _ FileMode.Open, _ FileAccess.Read, _ FileShare.Read) Try ' Retrieve the list of items from file Items = CType(FormatterStoreItem.Deserialize(StreamStoreItem), StoreItems) ' Display the categories in the combo box For i = 0 To Items.Count - 1 Item = CType(Items(i), StoreItem) If Not cbxCategories.Items.Contains(Item.Category) Then cbxCategories.Items.Add(Item.Category) End If Next ' Display the items types in the combo box For i = 0 To Items.Count - 1 Item = CType(Items(i), StoreItem) If Not cbxTypes.Items.Contains(Item.Type) Then cbxTypes.Items.Add(Item.Type) End If Next Finally StreamStoreItem.Close() End Try Else ' Create a random number that will be used ' to identify the item txtItemNumber.Text = RndNumber.Next(100000, 999999).ToString() ' Make sure the user had selected a category cbxCategories.Text = "" cbxTypes.Text = "" End If End Sub

7. In the Solution Explorer, right-click MusicStore.vb and click View Code
8. Make the following changes:
    

   Imports System.IO Imports System.Runtime.Serialization.Formatters.Binary Public Class MusicStore Private Items As StoreItems Private iFilename As Integer Private IsNewCustomerOrder As Boolean Private Sub LoadMusicStore() ' Since all values seem ready, prepare to process the item Dim i As Integer items = New StoreItems Dim FormatterStoreItem As BinaryFormatter = New BinaryFormatter ' This is the file that holds the list of items Dim Filename As String = "C:\Musical Instrument Store\StoreItems.mis" If File.Exists(Filename) Then Dim StreamStoreItem As FileStream = New FileStream(Filename, _ FileMode.Open, _ FileAccess.Read, _ FileShare.Read) Try ' Retrieve the list of items from file Items = CType(FormatterStoreItem.Deserialize(StreamStoreItem), _ StoreItems) ' Display the categories in the combo box For i = 0 To items.Count - 1 Dim item As StoreItem = CType(items(i), StoreItem) If Not cbxCategories.Items.Contains(item.Category) Then cbxCategories.Items.Add(item.Category) End If Next Finally StreamStoreItem.Close() End Try End If End Sub End Class

9. In the Class Name combo box, select btnNewStoreItem
10. In the Method Name combo box, select Click and implement the event as follows:
    

    Private Sub btnNewStoreItem_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnNewStoreItem.Click Dim Editor As ItemEditor = New ItemEditor ' Create a random number to get it ready for ' the user creating a new store item Dim RndNumber As Random = New Random Editor.txtItemNumber.Text = RndNumber.Next(100000, 999999) If Editor.ShowDialog() = DialogResult.Cancel Then LoadMusicStore() End If End Sub

11. In the Class Name combo box, select cbxCategories
12. In the Method Name combo box, select SelectedIndexChanged and implement the event as follows:
     

    Private Sub cbxCategories_SelectedIndexChanged(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles cbxCategories.SelectedIndexChanged Dim i As Integer cbxTypes.Items.Clear() cbxTypes.Text = "" lvwStoreItems.Items.Clear() pbxStoreItem.Image = Nothing ' If the current store inventory is empty, don't do anything If Items.Count = 0 Then Exit Sub ' Get the item selected in the combo box Dim strCategory = CStr(cbxCategories.SelectedItem) ' Before doing anything, remove everything from the Types combo box ' This eliminates the possibility of adding the same item(s) that ' would exist already in the combo box cbxTypes.Items.Clear() ' Check each item from the store inventory For i = 0 To Items.Count - 1 ' Get the current item from the store inventory Dim Item = CType(Items(i), StoreItem) ' If that item is the same as the one selected in the combo box... If Item.Category = strCategory Then ' ... get ready to add its corresponding type ' But check first that the type is not ' already in the Types combo box ' If it's not yet there, then add it If Not cbxTypes.Items.Contains(Item.Type) Then cbxTypes.Items.Add(Item.Type) End If End If Next End Sub

13. In the Class Name combo box, select cbxTypes
14. In the Method Name combo box, select SelectedIndexChanged and implement the event as follows:
     

    Private Sub cbxTypes_SelectedIndexChanged(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles cbxTypes.SelectedIndexChanged Dim i As Integer cbxTypes.Text = "" pbxStoreItem.Image = Nothing Dim lviStoreItem As ListViewItem ' If the current store inventory is empty, don't do anything If Items.Count = 0 Then Exit Sub ' Get the item selected in the Categories combo box Dim strCategory As String = CStr(cbxCategories.SelectedItem) ' Get the item selected in theTypes combo box Dim strType As String = CStr(cbxTypes.SelectedItem) ' Empty the Available Items list view because ' we are about to (re)populate it lvwStoreItems.Items.Clear() ' Check each item from the store inventory For i = 0 To Items.Count - 1 ' Get the current item from the inventory Dim Item As StoreItem = CType(Items(i), StoreItem) ' If the category of that item is the same as the one ' selected in the combo box and the type of that item is ' the same as the one selected in the Types combo box... If (Item.Category = strCategory) And _ (Item.Type = strType) Then ' ... then display it in the Available Items list view lviStoreItem = lvwStoreItems.Items.Add(Item.ItemNumber) lviStoreItem.SubItems.Add(Item.ItemName) lviStoreItem.SubItems.Add(FormatNumber(Item.UnitPrice)) End If Next End Sub

15. In the Class Name combo box, select lvwStoreItems
16. In the Method Name combo box, select SelectedIndexChanged and implement the event as follows:
     

    Private Sub lvwStoreItems_SelectedIndexChanged(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles lvwStoreItems.SelectedIndexChanged Dim i As Integer Dim strItemNumber As String Dim Item As StoreItem = New StoreItem If (lvwStoreItems.SelectedItems.Count = 0) Or _ (lvwStoreItems.SelectedItems.Count > 1) Then Exit Sub End If strItemNumber = lvwStoreItems.SelectedItems(0).SubItems(0).Text For i = 0 To Items.Count - 1 Dim itm As StoreItem = CType(Items(i), StoreItem) If itm.ItemNumber = strItemNumber Then Item = itm End If Next ' Make a list of the picture files Dim FolderName As String = "C:\Musical Instrument Store" Dim dirStoreItems As DirectoryInfo = New DirectoryInfo(FolderName) Dim PictureFiles() As FileInfo = dirStoreItems.GetFiles("*.jpg") ' Look for a file that holds the same name as the item number For Each fle As FileInfo In PictureFiles ' Get the name of the file without its extension Dim fwe As String = Path.GetFileNameWithoutExtension(fle.FullName) If fwe = strItemNumber Then pbxStoreItem.Image = Image.FromFile(FolderName & _ "\" & Item.ItemNumber& ".jpg") End If Next End Sub

17. Under the above End Sub line, create a new procedure as follows;
     

    ' This function calculates the current total order ' and updates the order Private Sub CalculateOrder() Dim SubTotal1 As Double Dim SubTotal2 As Double Dim SubTotal3 As Double Dim SubTotal4 As Double Dim SubTotal5 As Double Dim SubTotal6 As Double Dim ItemsTotal As Double Dim TaxRate As Double Dim TaxAmount As Double Dim OrderTotal As Double ' Retrieve the value of each sub total Try SubTotal1 = CDbl(txtSubTotal1.Text) Catch MsgBox("Invalid Value") End Try Try SubTotal2 = CDbl(txtSubTotal2.Text) Catch MsgBox("Invalid Value") End Try Try SubTotal3 = CDbl(txtSubTotal3.Text) Catch MsgBox("Invalid Value") End Try Try SubTotal4 = CDbl(txtSubTotal4.Text) Catch MsgBox("Invalid Value") End Try Try SubTotal5 = CDbl(txtSubTotal5.Text) Catch MsgBox("Invalid Value") End Try Try SubTotal6 = CDbl(txtSubTotal6.Text) Catch MsgBox("Invalid Value") End Try ' Calculate the total value of the sub totals ItemsTotal = SubTotal1 + SubTotal2 + SubTotal3 + _ SubTotal4 + SubTotal5 + SubTotal6 ' Display the total order in the appropriate text box txtItemsTotal.Text = FormatNumber(ItemsTotal) Try TaxRate = CDbl(txtTaxRate.Text) Catch MsgBox("Invalid Tax Rate") txtTaxRate.Text = "7.75" txtTaxRate.Focus() End Try TaxAmount = ItemsTotal * TaxRate / 100 OrderTotal = ItemsTotal + TaxAmount txtTaxAmount.Text = FormatNumber(TaxAmount) txtOrderTotal.Text = FormatNumber(OrderTotal) End Sub

18. In the Class Name combo box, select txtItemNumber1
19. In the Method Name combo box, select Leave and implement the event as follows:
     

    Private Sub txtItemNumber1_Leave(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles txtItemNumber1.Leave Dim ItemFound As Boolean = False Dim Item As StoreItem = New StoreItem Dim strItemNumber As String = txtItemNumber1.Text For Each Itm As StoreItem In Items If Itm.ItemNumber = strItemNumber Then ItemFound = True txtDescription1.Text = Itm.ItemName txtUnitPrice1.Text = Itm.UnitPrice.ToString("F") txtQuantity1.Text = "1" txtSubTotal1.Text = Itm.UnitPrice.ToString("F") CalculateOrder() End If Next If ItemFound = False Then MsgBox("There is no store item with that number") txtDescription1.Text = "" txtUnitPrice1.Text = "0.00" txtQuantity1.Text = "0" txtSubTotal1.Text = "0.00" End If End Sub

20. Under the above End Sub line, define the following event
     

    Public Sub Item1Leave(ByVal sender As Object, _ ByVal e As EventArgs) _ Handles txtUnitPrice1.Leave, txtQuantity1.Leave Dim Quantity As Integer Dim UnitPrice As Double Dim SubTotal As Double ' Get the quantity of the current item Try Quantity = CInt(txtQuantity1.Text) Catch MsgBox("Invalid quantity value for item 1") End Try ' Get the unit price of the current item Try UnitPrice = CDbl(txtUnitPrice1.Text) Catch MsgBox("Invalid unit price for item 1") End Try ' Calculate the current sub total SubTotal = Quantity * UnitPrice ' Display the new sub total in the corresponding text box txtSubTotal1.Text = FormatNumber(SubTotal) btnRemove1.Enabled = True ' Update the order CalculateOrder() End Sub

21. In the Class Name combo box, select btnRemove1
22. On the form, double-click the top Remove button and implement the event as follows:
     

    Private Sub btnRemove1_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnRemove1.Click txtItemNumber1.Text = "" txtDescription1.Text = "" txtUnitPrice1.Text = "0.00" txtQuantity1.Text = "0" txtSubTotal1.Text = "0.00" btnRemove1.Enabled = False CalculateOrder() End Sub

23. Complete the file as follows:
     

    Public Sub Item2Leave(ByVal sender As Object, _ ByVal e As EventArgs) _ Handles txtUnitPrice2.Leave, txtQuantity2.Leave Dim Quantity As Integer Dim UnitPrice As Double Dim SubTotal As Double ' Get the quantity of the current item Try Quantity = CInt(txtQuantity2.Text) Catch MsgBox("Invalid quantity value for item 2") End Try ' Get the unit price of the current item Try UnitPrice = CDbl(txtUnitPrice2.Text) Catch MsgBox("Invalid unit price for item 2") End Try ' Calculate the current sub total SubTotal = Quantity * UnitPrice ' Display the new sub total in the corresponding text box txtSubTotal2.Text = SubTotal.ToString() btnRemove2.Enabled = True ' Update the order CalculateOrder() End Sub Private Sub btnRemove2_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnRemove2.Click txtItemNumber2.Text = "" txtDescription2.Text = "" txtUnitPrice2.Text = "0.00" txtQuantity2.Text = "0" txtSubTotal2.Text = "0.00" btnRemove2.Enabled = False CalculateOrder() End Sub Public Sub Item3Leave(ByVal sender As Object, _ ByVal e As EventArgs) _ Handles txtUnitPrice3.Leave, txtQuantity3.Leave Dim Quantity As Integer Dim UnitPrice As Double Dim SubTotal As Double ' Get the quantity of the current item Try Quantity = CInt(txtQuantity3.Text) Catch MsgBox("Invalid quantity value for item 3") End Try ' Get the unit price of the current item Try UnitPrice = CDbl(txtUnitPrice3.Text) Catch MsgBox("Invalid unit price for item 3") End Try ' Calculate the current sub total SubTotal = Quantity * UnitPrice ' Display the new sub total in the corresponding text box txtSubTotal3.Text = SubTotal.ToString() btnRemove3.Enabled = True ' Update the order CalculateOrder() End Sub Private Sub btnRemove3_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnRemove3.Click txtItemNumber3.Text = "" txtDescription3.Text = "" txtUnitPrice3.Text = "0.00" txtQuantity3.Text = "0" txtSubTotal3.Text = "0.00" btnRemove3.Enabled = False CalculateOrder() End Sub Public Sub Item4Leave(ByVal sender As Object, _ ByVal e As EventArgs) _ Handles txtUnitPrice4.Leave, txtQuantity4.Leave Dim Quantity As Integer Dim UnitPrice As Double Dim SubTotal As Double ' Get the quantity of the current item Try Quantity = CInt(txtQuantity4.Text) Catch MsgBox("Invalid quantity value for item 4") End Try ' Get the unit price of the current item Try UnitPrice = CDbl(txtUnitPrice4.Text) Catch MsgBox("Invalid unit price for item 4") End Try ' Calculate the current sub total SubTotal = Quantity * UnitPrice ' Display the new sub total in the corresponding text box txtSubTotal4.Text = SubTotal.ToString() btnRemove4.Enabled = True ' Update the order CalculateOrder() End Sub Private Sub btnRemove4_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnRemove4.Click txtItemNumber4.Text = "" txtDescription4.Text = "" txtUnitPrice4.Text = "0.00" txtQuantity4.Text = "0" txtSubTotal4.Text = "0.00" btnRemove4.Enabled = False CalculateOrder() End Sub Public Sub Item5Leave(ByVal sender As Object, _ ByVal e As EventArgs) _ Handles txtUnitPrice5.Leave, txtQuantity5.Leave Dim Quantity As Integer Dim UnitPrice As Double Dim SubTotal As Double ' Get the quantity of the current item Try Quantity = CInt(txtQuantity5.Text) Catch MsgBox("Invalid quantity value for item 5") End Try ' Get the unit price of the current item Try UnitPrice = CDbl(txtUnitPrice5.Text) Catch MsgBox("Invalid unit price for item 5") End Try ' Calculate the current sub total SubTotal = Quantity * UnitPrice ' Display the new sub total in the corresponding text box txtSubTotal5.Text = SubTotal.ToString() btnRemove5.Enabled = True ' Update the order CalculateOrder() End Sub Private Sub btnRemove5_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnRemove5.Click txtItemNumber5.Text = "" txtDescription5.Text = "" txtUnitPrice5.Text = "0.00" txtQuantity5.Text = "0" txtSubTotal5.Text = "0.00" btnRemove5.Enabled = False CalculateOrder() End Sub Public Sub Item6Leave(ByVal sender As Object, _ ByVal e As EventArgs) _ Handles txtUnitPrice6.Leave, txtQuantity6.Leave Dim Quantity As Integer Dim UnitPrice As Double Dim SubTotal As Double ' Get the quantity of the current item Try Quantity = CInt(txtQuantity6.Text) Catch MsgBox("Invalid quantity value for item 6") End Try ' Get the unit price of the current item Try UnitPrice = CDbl(txtUnitPrice6.Text) Catch MsgBox("Invalid unit price for item 6") End Try ' Calculate the current sub total SubTotal = Quantity * UnitPrice ' Display the new sub total in the corresponding text box txtSubTotal6.Text = SubTotal.ToString() btnRemove6.Enabled = True ' Update the order CalculateOrder() End Sub Private Sub btnRemove6_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnRemove6.Click txtItemNumber6.Text = "" txtDescription6.Text = "" txtUnitPrice6.Text = "0.00" txtQuantity6.Text = "0" txtSubTotal6.Text = "0.00" btnRemove6.Enabled = False CalculateOrder() End Sub

24. In the Class Name combo box, select txtTaxRate
25. In the Method Name combo box, select Leave and implement the event as follows:
     

    Private Sub txtTaxRate_Leave(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles txtTaxRate.Leave CalculateOrder() End Sub

26. In the Class Name combo box, select btnNewCustomerOrder
27. In the Method Name combo box, select Click and implement the event as follows:
     

    Private Sub btnNewCustomerOrder_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnNewCustomerOrder.Click ' We will store our files in the following folder Dim FolderName As String = "C:\Musical Instrument Store\Receipts" Dim Folder = Directory.CreateDirectory(FolderName) ' Get the list of files, if any, from our directory Dim ListOfFiles = Folder.GetFiles() Dim Filename As String = "" ' If there is no file in the directory, ' then we will use 1000 as the first file name If ListOfFiles.Length = 0 Then iFilename = 1000 Else ' If there was at least one file in the directory ' Get a reference to the last file Dim LastFile = ListOfFiles(ListOfFiles.Length - 1) ' Get the name of the last file without its extension Dim fwe As String = Path.GetFileNameWithoutExtension(LastFile.FullName) ' Increment the name of the file by 1 iFilename = CInt(fwe) + 1 End If ' Update our global name of the file Filename = FolderName & "\" & CStr(iFilename)& ".cos" txtReceiptNumber.Text = CStr(iFilename) cbxCategories.Text = "" cbxTypes.Text = "" lvwStoreItems.Items.Clear() txtItemNumber1.Text = "" txtDescription1.Text = "" txtUnitPrice1.Text = "0.00" txtQuantity1.Text = "0" txtSubTotal1.Text = "0.00" txtItemNumber2.Text = "" txtDescription2.Text = "" txtUnitPrice2.Text = "0.00" txtQuantity2.Text = "0" txtSubTotal2.Text = "0.00" txtItemNumber3.Text = "" txtDescription3.Text = "" txtUnitPrice3.Text = "0.00" txtQuantity3.Text = "0" txtSubTotal3.Text = "0.00" txtItemNumber4.Text = "" txtDescription4.Text = "" txtUnitPrice4.Text = "0.00" txtQuantity4.Text = "0" txtSubTotal4.Text = "0.00" txtItemNumber5.Text = "" txtDescription5.Text = "" txtUnitPrice5.Text = "0.00" txtQuantity5.Text = "0" txtSubTotal5.Text = "0.00" txtItemNumber6.Text = "" txtDescription6.Text = "" txtUnitPrice6.Text = "0.00" txtQuantity6.Text = "0" txtSubTotal6.Text = "0.00" txtItemsTotal.Text = "0.00" txtTaxRate.Text = "7.75" txtTaxAmount.Text = "0.00" txtOrderTotal.Text = "0.00" End Sub

28. In the Class Name combo box, select btnSave
29. In the Method Name combo box, select Click and implement the event as follows:
     

    Private Sub btnSave_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnSave.Click Dim fwe As String Dim Filename As String Dim LastFile As FileInfo ' We will store our files in the following folder Dim FolderName As String = "C:\Musical Instrument Store\Receipts" Dim Folder = Directory.CreateDirectory(FolderName) ' Get the list of files, if any, from our directory Dim ListOfFiles() As FileInfo = Folder.GetFiles() ' If this is a new customer order, ' get ready to create a name for the file If IsNewCustomerOrder = True Then ' If there is no file in the directory, ' then we will use 1000 as the first file name If ListOfFiles.Length = 0 Then iFilename = 1000 Else ' If there was at least one file in the directory ' Get a reference to the last file LastFile = ListOfFiles(ListOfFiles.Length - 1) ' Get the name of the last file without its extension fwe = Path.GetFileNameWithoutExtension(LastFile.FullName) ' Increment the name of the file by 1 iFilename = CInt(fwe) + 1 End If ' Update our global name of the file Filename = FolderName & "\" & iFilename.ToString()& ".cos" txtReceiptNumber.Text = iFilename.ToString() IsNewCustomerOrder = False ' If a cleaning order was already opened, we will simply update it Else Filename = "C:\Musical Instrument Store\Receipts\" & _ txtReceiptNumber.Text & ".cos" End If Dim wrtCustomerOrder As StreamWriter = New StreamWriter(Filename) Try wrtCustomerOrder.WriteLine(txtItemNumber1.Text) wrtCustomerOrder.WriteLine(txtDescription1.Text) wrtCustomerOrder.WriteLine(txtUnitPrice1.Text) wrtCustomerOrder.WriteLine(txtQuantity1.Text) wrtCustomerOrder.WriteLine(txtItemNumber2.Text) wrtCustomerOrder.WriteLine(txtDescription2.Text) wrtCustomerOrder.WriteLine(txtUnitPrice2.Text) wrtCustomerOrder.WriteLine(txtQuantity2.Text) wrtCustomerOrder.WriteLine(txtItemNumber3.Text) wrtCustomerOrder.WriteLine(txtDescription3.Text) wrtCustomerOrder.WriteLine(txtUnitPrice3.Text) wrtCustomerOrder.WriteLine(txtQuantity3.Text) wrtCustomerOrder.WriteLine(txtItemNumber4.Text) wrtCustomerOrder.WriteLine(txtDescription4.Text) wrtCustomerOrder.WriteLine(txtUnitPrice4.Text) wrtCustomerOrder.WriteLine(txtQuantity4.Text) wrtCustomerOrder.WriteLine(txtItemNumber5.Text) wrtCustomerOrder.WriteLine(txtDescription5.Text) wrtCustomerOrder.WriteLine(txtUnitPrice5.Text) wrtCustomerOrder.WriteLine(txtQuantity5.Text) wrtCustomerOrder.WriteLine(txtItemNumber6.Text) wrtCustomerOrder.WriteLine(txtDescription6.Text) wrtCustomerOrder.WriteLine(txtUnitPrice6.Text) wrtCustomerOrder.WriteLine(txtQuantity6.Text) wrtCustomerOrder.WriteLine(txtItemsTotal.Text) wrtCustomerOrder.WriteLine(txtTaxRate.Text) Finally wrtCustomerOrder.Close() End Try End Sub

30. In the Class Name combo box, select
31. Double-click the Open button and implement its event as follows:
     

    Private Sub btnOpen_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnOpen.Click Dim FolderName = "C:\Musical Instrument Store\Receipts" Dim strFilename As String = "C:\Musical Instrument Store\Receipts\1000.cos" If txtReceiptNumber.Text = "" Then Exit Sub Else Try strFilename = FolderName & "\" & _ txtReceiptNumber.Text & ".cos" Dim rdrCustomerOrder As StreamReader = New StreamReader(strFilename) Try txtItemNumber1.Text = rdrCustomerOrder.ReadLine() txtDescription1.Text = rdrCustomerOrder.ReadLine() txtUnitPrice1.Text = rdrCustomerOrder.ReadLine() txtQuantity1.Text = rdrCustomerOrder.ReadLine() Item1Leave(sender, e) txtItemNumber2.Text = rdrCustomerOrder.ReadLine() txtDescription2.Text = rdrCustomerOrder.ReadLine() txtUnitPrice2.Text = rdrCustomerOrder.ReadLine() txtQuantity2.Text = rdrCustomerOrder.ReadLine() Item2Leave(sender, e) txtItemNumber3.Text = rdrCustomerOrder.ReadLine() txtDescription3.Text = rdrCustomerOrder.ReadLine() txtUnitPrice3.Text = rdrCustomerOrder.ReadLine() txtQuantity3.Text = rdrCustomerOrder.ReadLine() Item3Leave(sender, e) txtItemNumber4.Text = rdrCustomerOrder.ReadLine() txtDescription4.Text = rdrCustomerOrder.ReadLine() txtUnitPrice4.Text = rdrCustomerOrder.ReadLine() txtQuantity4.Text = rdrCustomerOrder.ReadLine() Item4Leave(sender, e) txtItemNumber5.Text = rdrCustomerOrder.ReadLine() txtDescription5.Text = rdrCustomerOrder.ReadLine() txtUnitPrice5.Text = rdrCustomerOrder.ReadLine() txtQuantity5.Text = rdrCustomerOrder.ReadLine() Item5Leave(sender, e) txtItemNumber6.Text = rdrCustomerOrder.ReadLine() txtDescription6.Text = rdrCustomerOrder.ReadLine() txtUnitPrice6.Text = rdrCustomerOrder.ReadLine() txtQuantity6.Text = rdrCustomerOrder.ReadLine() Item6Leave(sender, e) txtItemsTotal.Text = rdrCustomerOrder.ReadLine() txtTaxRate.Text = rdrCustomerOrder.ReadLine() CalculateOrder() IsNewCustomerOrder = False Finally rdrCustomerOrder.Close() End Try Catch Exc As FileNotFoundException MsgBox("There is no customer order with that receipt number") End Try End If End Sub

32. In the Class Name combo box, select (MusicStore Events)
33. In the Method Name combo box, select Load and implement the event as follows:
     

    Private Sub MusicStore_Load(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles Me.Load btnNewCustomerOrder_Click(sender, e) LoadMusicStore() IsNewCustomerOrder = True End Sub

34. In the Class Name combo box, select btnClose
35. Double-click the Close and implement its Click event as follows:
     

    Private Sub btnClose_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnClose.Click End End Sub

36. Execute the application to test it
37. Access the Item Editor dialog box and create a few items as follows (let the computer create the item numbers):
     

    Category Type Item Name Unit Price Picture Guitars Electric Gibson Les Paul Vintage Mahogany Electric Guitar 745.95 Picture Bass Electric 4-String Epiphone Thunderbird IV Bass 325.85 Picture Keyboards Synthesizers Alesis QS8.2 88 Key Synthesizer 825.50 Picture Guitars Acoustic Gretsch Guitars G100 Synchromatic Archtop Acoustic Guitar 595.95 Picture Drums Drum Set Pulse Pro 5-Piece Drum Set with Cymbals 395.95 Picture Keyboards Pianos Roland RD-700SX Digital Piano 2195.00   Accessories Cables Mogami Gold AES/EBU Interconnect Cable with Neutrik XLR 45.85   Guitars Acoustic-Electric Ibanez V Series V70CE Dreadnought Cutaway Acoustic-Electric Guitar 225.50   Guitars Electric Schecter C-1 Hellraiser Electric Guitar 650.00 Picture Keyboards Synthesizers Roland V Synth GT Elastic Audio Synthesizer Keyboard 2895.50 Picture Bass Electric 5-String Fender Jazz Bass 24 V 5-String Bass Guitar 825.50 Picture Guitars Electric Fender Standard Stratocaster Left-Handed Electric Guitar 425.85 Picture Recording Microphone MXL V63M Studio Condenser Microphone 72.95 Picture Guitars Acoustic Yamaha FD01S Acoustic Folk Guitar 185.95   Book/CD/DVD Instructional Hal Leonard Amazing Phrasing - Alto Sax (Book/CD) 18.00   Guitars Classical & Nylon Alvarez Artist Series AC60S Classical Acoustic Guitar 275.95 Picture Guitars Acoustic Washburn D100DL Acoustic Guitar 150.50 Picture Drums Electronic Percussion Boss DR-670 Dr. Rhythm Drum Machine 275.85 Picture Recording Microphone Shure SM58 Mic 95.95 Picture Accessories Cables Live Wire HPE325 Headphone and Extension Cable 10.95 Picture Bass Acoustic Fretted Ibanez AEB10E Acoustic-Electric Bass Guitar with Onboard Tuner 350.00 Picture Drums World Percussion Latin Percussion Conga 3-Pack with Bongos 595.95 Picture Keyboards Synthesizers Roland JUNO-D 61-Key Synthesizer 595.95 Picture Drums World Percussion Latin Percussion Aspire Conga Set with Bongos and Stand 425.50 Picture Recording Microphone AKG Perception 200 Condenser Microphone 160.00 Picture

38. Create a few customers' orders and click Save then New Order each time. Here are examples:
     
39. Close the form and return to your programming environment

One of the most valuables features of Visual Basic is to use the For Each loop to enumerate the members of a collection. To make this possible, you must implement the IEnumerator interface in your collection class. Following the rules of interface implementation, you must override the members of IEnumerator. 

One of the routine operations you can perform on a list is to find out whether it contains a certain value. To assist you with this operation, the System.Collections.IList interface is equipped with a method named Contains. Its syntax is:

Function Contains(value As Object) As Boolean

This method takes as argument the value to look for. If the value is found in the list, the method returns True. If no value is found in the collection, this method returns False.

Here is an example of implementing this method:

Public Function Contains(ByVal value As Object) As Boolean _
                Implements System.Collections.IList.Contains
    Dim i As Integer

    For i = 0 To Count - 1
        If Objects(i) = value Then
            Return True
            i = i + 1
        End If
    Next

    Return False
End Function

This method calls the Equals() method of the objects that make up the list to find out whether the value argument exists in the collection. If this method produces a wrong result, especially if you are using your own class to represent the item, you may have to override your own Equals() method.

The System.Collections.IList.Contains() method is used to check whether a particular value (already) exists in the collection. If you know that a certain item exists in the collection but you do not know its index inside the list, the IList interface can assist you through a method named IndexOf. Its syntax is:

Function IndexOf(value As Object) As Integer

This method takes as argument the value to look for in the list. If the value is found in the collection, the method returns its index. If there is no value defined like that, the method returns -1. Here is an example of implementing this method:

Public Function IndexOf(ByVal value As Object) As Integer _
                    Implements System.Collections.IList.IndexOf

        Dim i As Integer

        For i = 0 To Count - 1
            If Objects(i) = value Then
                Return 1
                i = i + 1
            End If
        Next

        Return -1
End Function

This method calls the Equals() method of the objects that make up the collection to find out whether the value argument exists in the list. If this method produces a wrong result, especially if you are using your own class to represent the value, you may have to override your own Equals() method.

If a value is not necessary in your list, you can delete it. Probably the simplest way to delete a value is to specify its position in the list. To support this operation, both the System.Collections.IList and the System.Collections.Generic.IList interfaces are equipped with a method named RemoveAt. The syntax of the RemoveAt() method is is the same for both interfaces and it is:

Sub RemoveAt(index As Integer)

This method takes as argument the index of the value to be removed. Here is an example:

Public Sub RemoveAt(ByVal index As Integer) _
            Implements System.Collections.IList.RemoveAt
        Dim i As Integer

        If (index >= 0) And (index < Count) Then
            For i = index To Count - 1

                Objects(i) = Objects(i + 1)
                Counter = Counter - 1
            Next
        End If
End Sub

The problem with deleting a value based on its index is that, if you are not sure, you could delete the wrong value. An alternative is to first precisely define the value you want to get rid of, and then hand the value itself to the compiler that would remove it. To support this approach, the System.Collections.IList interface is equipped with a method named Remove() and whose syntax is:

Sub Remove(value As Object)

This method takes as argument the value to be deleted. This means that the compiler will first look for the value in the list. If it finds that value, it removes it. If there is no value like that, nothing happens (the compiler does not throw an exception. Here is an example of implementing this method:

Public Sub Remove(ByVal value As Object) _
            Implements System.Collections.IList.Remove
        RemoveAt(IndexOf(value))
End Sub

To remove all value from a list at once, you can implement Clear() method of the System.Collections.IList interface. Its syntax is:

Sub Clear

Here is an example of implementing it:

Public Class BookCollection
    Implements IList

    Private Counter As Integer
    Private Objects(10) As Object

    Public Sub New()
        Counter = 0
    End Sub
      
    Public Sub CopyTo(ByVal array As System.Array, ByVal index As Integer) _
                            Implements System.Collections.ICollection.CopyTo

    End Sub

    Public ReadOnly Property Count() As Integer _
            Implements System.Collections.ICollection.Count
        Get
            Return Counter
        End Get
    End Property

    Public ReadOnly Property IsSynchronized() As Boolean _
            Implements System.Collections.ICollection.IsSynchronized
        Get
            Return False
        End Get
    End Property

    Public ReadOnly Property SyncRoot() As Object _
            Implements System.Collections.ICollection.SyncRoot
        Get
            Return Me
        End Get
    End Property

    Public Function GetEnumerator() As System.Collections.IEnumerator _
                Implements System.Collections.IEnumerable.GetEnumerator
        Return Nothing
    End Function

    Public Function Add(ByVal value As Object) As Integer _
                    Implements System.Collections.IList.Add
        ' Check whether there is still room in
        ' the array to add a new item
        If Counter < Objects.Length Then
            ' Since there is room, put the new item to the end
            Objects(Counter) = value
            ' increase the number of items
            Counter = Counter + 1
            ' Return the index of the item that was added
            Return Counter - 1
            ' Since the item could not be added, return a negative index
        Else
            Return -1
        End If
    End Function

    Public Sub Clear() Implements System.Collections.IList.Clear
        Counter = 0
    End Sub

    Public Function Contains(ByVal value As Object) As Boolean _
                    Implements System.Collections.IList.Contains
        Dim i As Integer

        For i = 0 To Count - 1
            If Objects(i) = value Then
                Return True
                i = i + 1
            End If
        Next

        Return False
    End Function

    Public Function IndexOf(ByVal value As Object) As Integer _
                    Implements System.Collections.IList.IndexOf

        Dim i As Integer

        For i = 0 To Count - 1
            If Objects(i) = value Then
                Return 1
                i = i + 1
            End If
        Next

        Return -1
    End Function

    Public Sub Insert(ByVal index As Integer, ByVal value As Object) _
                            Implements System.Collections.IList.Insert

    End Sub

    Public ReadOnly Property IsFixedSize() As Boolean _
            Implements System.Collections.IList.IsFixedSize
        Get
            Return False
        End Get
    End Property

    Public ReadOnly Property IsReadOnly() As Boolean _
            Implements System.Collections.IList.IsReadOnly
        Get
            Return False
        End Get
    End Property

    Default Public Property Item(ByVal index As Integer) As Object _
                            Implements System.Collections.IList.Item
        Get
            Return Objects(index)
        End Get
        Set(ByVal value As Object)
            Objects(index) = value
        End Set
    End Property

    Public Sub Remove(ByVal value As Object) _
            Implements System.Collections.IList.Remove
        RemoveAt(IndexOf(value))
    End Sub

    Public Sub RemoveAt(ByVal index As Integer) _
            Implements System.Collections.IList.RemoveAt
        Dim i As Integer

        If (index >= 0) And (index < Count) Then
            For i = index To Count - 1

                Objects(i) = Objects(i + 1)
                Counter = Counter - 1
            Next
        End If
    End Sub
End Class