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Combo Boxes
 

Introduction to Combo Boxes
 

Description

A combo box is a list of items that the user can select from. Like a list box, a combo box is usually made of a list of strings. Unlike a list box, a combo box saves space by using just as much room as a text box control. To show that it is holding a list, a combo box displays a down-pointing arrow on the right side of its text box. In the following screenshot, the Font Effects property page of the Character dialog box of OpenOffice.org presents the Underlining, the Color, the Effects, the Strikethrough, the Relief, and the Font Color combo boxes:

The Font Effects tab of the Character dialog box displays various combo boxes

Because a combo box does not (permanently) display its list like a list box, to show its content, the user can click the arrow button. Here is an example:

Creating a Combo Box

To support combo boxes, the .NET Framework provides a class named ComboBox. At design time, to add a combo box to your application, from the Common Controls section of the Toolbox, you can click the ComboBox button ComboBox and click the form or a container. Like ListBox, the ComboBox class is derived from the ListControl class. Therefore, to programmatically create a combo box, declare a variable of type ComboBox, allocate its memory with the new operator and add it to the Controls collection of its container. Here is an example: 

using System;
using System.Drawing;
using System.Windows.Forms;

public class Exercise : System.Windows.Forms.Form
{
    Label lblTitle;
    ComboBox cbxAcademicDisciplines;

    public Exercise()
    {
        InitializeComponent();
    }

    private void InitializeComponent()
    {
        lblTitle = new Label();
        lblTitle.Text = "Academic Disciplines";
        lblTitle.Location = new Point(12, 12);
        lblTitle.AutoSize = true;
        Controls.Add(lblTitle);

        cbxAcademicDisciplines = new ComboBox();
        cbxAcademicDisciplines.Location = new Point(12, 32);

        Controls.Add(cbxAcademicDisciplines);
    }
}

public class Program
{
    static int Main()
    {
        System.Windows.Forms.Application.Run(new Exercise());
        return 0;
    }
}

This would produce:

Combo Box

Practical Learning Practical Learning: Introducing Combo Boxes
  1. Start Microsoft Visual C# and create a new Windows Application named CollegeParkAutoParts1
  2. In the Solution Explorer, right-click Form1.cs and click Rename
  3. Type Central.cs and press Enter
  4. Design the form as follows:
     
    College Park Auto Parts - Form Design
    Control Text Name Other Properties
    GroupBox GroupBox Parts Selection    
    Label Label Year   Font: Microsoft Sans Serif, 8.25pt, style=Bold
    Label Label Make   Microsoft Sans Serif, 8.25pt, style=Bold
    Label Label Model   Microsoft Sans Serif, 8.25pt, style=Bold
    Label Label Category   Microsoft Sans Serif, 8.25pt, style=Bold
    ComboBox ComboBox   cbxCarYears  
    ComboBox ComboBox   cbxMakes  
    ComboBox ComboBox   cbxModels  
    ComboBox ComboBox   cbxCategories  
    Label Label Available Parts   Font: Microsoft Sans Serif, 9.75pt, style=Bold
    ListBox ListBox   lbxPartNumbers  
    ListBox ListBox   lbxPartNames  
    ListBox ListBox   lbxUnitPrices  
    GroupBox GroupBox Customer Selected Items    
    Label Label Part #    
    Label Label Part Name    
    Label Label Unit Price    
    Label Label Qty    
    Label Label Sub Total    
    TextBox TextBox   txtPartNumber1  
    TextBox TextBox   txtPartName1  
    TextBox TextBox 0.00 txtUnitPrice1 TextAlign: Right
    TextBox TextBox 0 txtQuantity1 TextAlign: Right
    TextBox TextBox 0.00 txtSubTotal1 TextAlign: Right
    Button Button Remove btnRemove1  
    TextBox TextBox   txtPartNumber2  
    TextBox TextBox   txtPartName2  
    TextBox TextBox 0.00 txtUnitPrice2 TextAlign: Right
    TextBox TextBox 0 txtQuantity2 TextAlign: Right
    TextBox TextBox 0.00 txtSubTotal2 TextAlign: Right
    Button Button Remove btnRemove2  
    TextBox TextBox   txtPartNumber3  
    TextBox TextBox   txtPartName3  
    TextBox TextBox 0.00 txtUnitPrice3 TextAlign: Right
    TextBox TextBox 0 txtQuantity3 TextAlign: Right
    TextBox TextBox 0.00 txtSubTotal3 TextAlign: Right
    Button Button Remove btnRemove3  
    TextBox TextBox   txtPartNumber4  
    TextBox TextBox   txtPartName4  
    TextBox TextBox 0.00 txtUnitPrice4 TextAlign: Right
    TextBox TextBox 0 txtQuantity4 TextAlign: Right
    TextBox TextBox 0.00 txtSubTotal4 TextAlign: Right
    Button Button Remove btnRemove4  
    TextBox TextBox   txtPartNumber5  
    TextBox TextBox   txtPartName5  
    TextBox TextBox 0.00 txtUnitPrice5 TextAlign: Right
    TextBox TextBox 0 txtQuantity5 TextAlign: Right
    TextBox TextBox 0.00 txtSubTotal5 TextAlign: Right
    Button Button Remove btnRemove5  
    TextBox TextBox   txtPartNumber6  
    TextBox TextBox   txtPartName6  
    TextBox TextBox 0.00 txtUnitPrice6 TextAlign: Right
    TextBox TextBox 0 txtQuantity6 TextAlign: Right
    TextBox TextBox 0.00 txtSubTotal6 TextAlign: Right
    Button Button Remove btnRemove6  
    Label Label Total Order    
    TextBox TextBox 0.00 txtTotalOrder  
    Button Button Close btnClose  
  5. Execute the application to test it
  6. Close the form and return to your programming environment

Creating and Selecting Items
 

Introduction

Like all visual controls, a combo box shares all the basic characteristics of other graphic control: the name, the location, the size, the ability to be enabled or disabled, the ability to hide or show it, the ability to dock or anchor, etc.

Creating the List of Items

Probably the most important aspect of a combo box is the list of items it holds. Like the list box and all other list-based controls, the list of a combo box is held by the Items property. You create this list using the exact same techniques we reviewed for the list box. This means that you can use the String Collection Editor.

The Items property of the ComboBox class is created from the nested ObjectCollection class. This class has the same functionality and features as its counterpart of the list box. This means that, to programmatically create of items, you can (continuously) call the ObjectCollection.Add() method. Here is an example: 

public class Exercise : System.Windows.Forms.Form
{
    Label lblTitle;
    ComboBox cbxAcademicDisciplines;

    public Exercise()
    {
        InitializeComponent();
    }

    private void InitializeComponent()
    {
        lblTitle = new Label();
        lblTitle.Text = "Academic Disciplines";
        lblTitle.Location = new Point(12, 12);
        lblTitle.AutoSize = true;
        Controls.Add(lblTitle);

        cbxAcademicDisciplines = new ComboBox();
        cbxAcademicDisciplines.Location = new Point(12, 32);
        cbxAcademicDisciplines.Items.Add("Natural sciences");
        cbxAcademicDisciplines.Items.Add("Mathematics and Computer sciences");
        cbxAcademicDisciplines.Items.Add("Social sciences");
        cbxAcademicDisciplines.Items.Add("Humanities");
        cbxAcademicDisciplines.Items.Add("Professions and Applied sciences");

        Controls.Add(cbxAcademicDisciplines);
    }
}

This would produce:

Combo Box

To add an array of items, you can call the AddRange() method. To insert an item somewhere inside the list, you can call the Insert() method.

If you want the list of items to be sorted, you can change the value of the Sorted property in the Properties window from False (the default) to True. To sort a list programmatically, you can assign a true value to the Sorted property. You can un-sort the list by changing the value of the Sorted property. This property works exactly like its equivalent in the ListBox control.

Practical Learning Practical Learning: Using the ObjectCollection Class
  1. To create a list of items using the Add() method, double-click an unoccupied area of the form and implement its Load event as follows:
      
    private void Central_Load(object sender, EventArgs e)
{
        for (int i = DateTime.Now.Year; i >= 1960; i--)
                this.cbxCarYears.Items.Add(i);
}
  2. Save the form

Selecting an Item

To select an item from the list, the user can click it. To programmatically select an item, you can assign a string to the Text property of a DropDown or a Simple combo box. Probably the best way to select an item is to specify its index. The items of a combo box are stored in a zero-based list. To select an item, you can assign its position to the SelectedIndex property. In the same way, to find out what item is selected, you can get the value of the SelectedIndex property.

Instead of using of using the index of an item, to select an item using its identity or name, you can use the SelectedItem property. To select an item by its name, assign it to the SelectedItem property.

Practical Learning Practical Learning: Using the ObjectCollection Class
  1. On the main menu, click Project -> Add Class...
  2. Set the name to PartDescription and press Enter
  3. To create a class that can holds a structured item of a list, change the class as follows:
      
    using System;

namespace CollegeParkAutoParts1
{
    public class PartDescription
    {
        private long ID;
        private int yr;
        private string mk;
        private string mdl;
        private string cat;
        private string name;
        private decimal price;

        public PartDescription()
        {
            this.ID = 0;
            this.yr = 1960;
            this.mk = "";
            this.mdl = "";
            this.name = "Unknown";
            this.price = 0.00M;
        }

        public PartDescription(long code, int year, string make,
                               string model, string type,
                               string desc, decimal UPrice)
        {
            this.ID = code ;
            this.yr = year;
            this.mk = make;
            this.mdl = model;
            this.cat = type;
            this.name = desc;
            this.price = UPrice;
        }

        public long PartNumber
        {
            get { return ID; }
            set { ID = value; }
        }

        public int CarYear
        {
            get { return yr; }
            set { yr = value; }
        }

        public string Make
        {
            get { return mk; }
            set { mk = value; }
        }

        public string Model
        {
            get { return mdl; }
            set { mdl = value; }
        }

        public string Category
        {
            get { return cat ; }
            set { cat = value; }
        }

        public string PartName
        {
            get { return name; }
            set { name = value; }
        }

        public decimal UnitPrice
        {
            get { return (price < 0) ? 0.00M : price; }
            set { price = value; }
        }

        public override string ToString()
        {
            return this.PartNumber + " " +
                   this.CarYear.ToString() + " " +
                   this.Make + " " +
                   this.Model + " " +
                   this.Category + " " +
                   this.PartName + " " +
                   this.UnitPrice;
        }
    }
}
  4. Access the Central.cs code file and change it as follows:
      
    using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace CollegeParkAutoParts1
{
    public partial class Central : Form
    {
        PartDescription[] parts = new PartDescription[56];

        public Central()
        {
            InitializeComponent();
        }

        private void Central_Load(object sender, EventArgs e)
        {
            for (int i = DateTime.Now.Year; i >= 1960; i--)
                this.cbxCarYears.Items.Add(i);

            parts[0] = new PartDescription(447093, 2002, "Ford",
                "Escort SE L4 2.0", "Engine Electrical",
                "Alternator 75amp  Remanufactured; w/ 75 Amp",
                205.05M);
            parts[1] = new PartDescription(203815, 2006, "Dodge",
                "Caravan SE L4 2.4", "Cooling System",
                "Radiator Cap", 6.65M);
            parts[2] = new PartDescription(293047, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Thermostat Gasket", 4.95M);
            parts[3] = new PartDescription(990468, 2002, "Honda",
                "Civic 1.7 EX 4DR", "Exhaust",
                "Bolt & Spring Kit (Manifold outlet, Muffler Inlet)",
                85.75M);
            parts[4] = new PartDescription(304158, 1996, "Buick",
                "Regal Custom V6 3.8", "Fuel Injection",
                "Fuel Injector", 82.75M);
            parts[5] = new PartDescription(807245, 2004, "Acura",
                "MDX 3.5 4WD", "Driveshaft & Axle",
                "CV Boot Clamp 7 x 750mm; 1 Large + 1 Small Clamp",
                1.60M);
            parts[6] = new PartDescription(203485, 2001, "Ford",
                "Taurus LX V6 3.0", "Fuel Injection",
                "Oxygen Sensor OE Style 4Wire; Front; 2 Required",
                52.65M);
            parts[7] = new PartDescription(248759, 1999, "Jeep",
                "Wrangler Sahara", "Air Intake",
                "Air Filter AirSoft Panel", 7.95M);
            parts[8] = new PartDescription(202848, 1998, "Honda",
                "Accord 2.3 LX 4DR", "Air Intake",
                "Air Filter", 12.55M);
            parts[10] = new PartDescription(932759, 2006, "Kia",
                "Rio 1.6DOHC16V 4-DR", "Cooling System",
                "Thermostat", 14.45M);
            parts[11] = new PartDescription(304975, 2000, "Honda",
                "Civic 1.6 EX 4DR", "Suspension",
                "Ball Joint; Front Lower; 2 per car", 40.55M);
            parts[12] = new PartDescription(208450, 2003, "Chevrolet",
                "Monte Carlo LS V6 3.4", "Fuel Injection",
                "Oxygen Sensor OE connector; Rear", 65.55M);
            parts[13] = new PartDescription(209480, 2002, "Ford",
                "Focus SE DOHC L4 2.0", "Steering",
                "Steering Rack Remanufactured", 170.85M);
            parts[9] = new PartDescription(203495, 2004, "Honda",
                "Civic 1.7 EX 4DR", "Climate Control",
                "A/C Clutch; OE compressor = Sanden", 184.95M);
            parts[14] = new PartDescription(203480, 2007, "Toyota",
                "Corolla", "Air Intake",
                "Air Filter", 12.65M);
            parts[15] = new PartDescription(109379, 2005, "Volvo",
                "S40 2.5L T5 AWD", "Fuel Delivery",
                "Fuel Filter; Early Design; Outer Diameter = 55mm",
                30.95M);
            parts[16] = new PartDescription(935794, 2002, "Ford",
                "Escape XLS 4WD", "Brake",
                "Brake Caliper Remanufactured; Front Right",
                65.55M);
            parts[17] = new PartDescription(203485, 2006, "BMW",
                "325i", "Climate Control",
                "AC High Pressure Side Switch",
                49.95M);
            parts[18] = new PartDescription(204875, 1996, "Chevrolet",
                "Monte Carlo Z34 V6 3.4", "Fuel Delivery",
                "Fuel Filter", 8.05M);
            parts[19] = new PartDescription(937485, 2007, "Toyota",
                "Camry V6", "Air Intake", "Air Filter", 12.95M);
            parts[20] = new PartDescription(294759, 2001, "Ford",
                "Escape XLT 4WD", "Air Intake",
                "Air Filter Panel", 7.25M);
            parts[21] = new PartDescription(297495, 2003, "Honda",
                "Civic 1.7 EX 4DR", "Brake",
                "Brake Caliper Reman; w/ ProAct Pads; Front Right",
                82.55M);
            parts[22] = new PartDescription(794735, 2006, "BMW",
                "325i", "Climate Control",
                "Cabin Air/Pollen Filter; With Activated Carbon",
                28.05M);
            parts[23] = new PartDescription(937485, 2007, "Toyota",
                "Corolla", "Body Electrical",
                "Halogen  SilverStar; 12V 65W; inner-high beam",
                22.85M);
            parts[24] = new PartDescription(492745, 2005, "Ford",
                "Focus ZX3 L4 2.0", "Air Intake",
                "Fuel Injection Perf Kit", 342.95M);
            parts[25] = new PartDescription(937005, 2004, "Acura",
                "MDX 3.5 4WD", "Driveshaft & Axle",
                "CV Boot Clamp 7 x 750mm; For Large End of Boot; inner boot",
                1.60M);
            parts[26] = new PartDescription(293749, 2004, "Acura",
                "MDX 3.5 4WD", "Driveshaft & Axle",
                "Axle Nut 24mm x 1;5; rear ",
                2.35M);
            parts[27] = new PartDescription(920495, 2006, "BMW",
                "325i", "Climate Control",
                "Adjustable Telescoping Mirror", 7.95M);
            parts[28] = new PartDescription(204075, 2004, "Acura",
                "MDX 3.5 4WD", "Driveshaft & Axle",
                "Wheel Bearing; Rear; 1 per wheel",
                70.15M);
            parts[29] = new PartDescription(979304, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Thermostat Housing", 20.95M);
            parts[30] = new PartDescription(300456, 2004, "Acura",
                "MDX 3.5 4WD", "Driveshaft & Axle",
                "Wheel Bearing; Front; 1 per wheel",
                66.65M);
            parts[31] = new PartDescription(404860, 2001, "Ford",
                "Taurus LX V6 3.0", "Suspension",
                "Shock Absorber GR2; Rear; Wagon only",
                39.40M);
            parts[32] = new PartDescription(585688, 2007, "Buick",
                "Lacrosse CXS V6 3.6", "Brake",
                "Climate Control", 10.65M);
            parts[33] = new PartDescription(739759, 2001, "Ford",
                "Taurus LX V6 3.0", "Suspension",
                "Shock Absorber GasaJust; Rear; Wagon only",
                30.95M);
            parts[34] = new PartDescription(927495, 2005, "Volvo",
                "S40 2.5L T5 AWD", "Engine Mechanical",
                "Timing Belt Idler Pulley Original Equipment INA",
                65.55M);
            parts[40] = new PartDescription(979374, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Thermostat Gasket", 4.95M);
            parts[35] = new PartDescription(542347, 2007, "Buick",
                "Lacrosse CXS V6 3.6", "Brake",
                "Brake Pad Set ProACT Ceramic w/Shims; Front", 80.05M);
            parts[36] = new PartDescription(683064, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Radiator Hose; Upper", 103.75M);
            parts[37] = new PartDescription(248759, 1999, "Jeep",
                "Wrangler Sahara", "Air Intake",
                "Air Filter", 50.95M);
            parts[38] = new PartDescription(973974, 2007, "Toyota",
                "Corolla", "Air Intake",
                "Air Mass Meter; W/o Housing; Meter/sensor only",
                134.95M);
            parts[39] = new PartDescription(285800, 2001, "Ford",
                "Escape XLT 4WD", "Transmission",
                "AT Filter", 34.95M);
            parts[41] = new PartDescription(207495, 2007, "Toyota",
                "Corolla", "Body Electrical",
                "Headlight Bulb; 12V 65W; inner-high beam", 9.35M);
            parts[42] = new PartDescription(566676, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Auxiliary Fan Switch", 42.95M);
            parts[43] = new PartDescription(304950, 2007, "Toyota",
                "Corolla", "Body Electrical",
                "Headlight Bulb; 12V 51W; outer", 7.85M);
            parts[44] = new PartDescription(797394, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Water Flange Gasket", 0.85M);
            parts[45] = new PartDescription(910203, 2007, "Buick",
                "Lacrosse CXS V6 3.6", "Suspension",
                "Strut Mount Inc; Sleeve; Rear Right", 80.85M);
            parts[46] = new PartDescription(790794, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Radiator Hose; Lower", 9.45M);
            parts[47] = new PartDescription(970394, 2007, "Buick",
                "Lacrosse CXS V6 3.6", "Suspension",
                "Coil Spring Insulator; Front Lower",
                14.55M);
            parts[48] = new PartDescription(290840, 2005, "Volvo",
                "S40 2.5L T5 AWD", "Engine Mechanical",
                "Rod Bearing Set 1 per Rod; Standard; Reqs. 5-per Engine",
                26.95M);
            parts[49] = new PartDescription(209704, 2007, "Toyota",
                "Corolla", "Body Electrical",
                "Wiper Blade Excel+; Front Right", 7.25M);
            parts[50] = new PartDescription(200368, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Radiator Drain Plug incl; gasket", 3.15M);
            parts[51] = new PartDescription(200970, 2005, "Volvo",
                "S40 2.5L T5 AWD", "Engine Mechanical",
                "Reference Sensor; Flywheel Engine Speed",
                62.05M);
            parts[52] = new PartDescription(542347, 2007, "Buick",
                "Lacrosse CXS V6 3.6", "Air Intake",
                "Air Filter", 50.25M);
            parts[53] = new PartDescription(927045, 2001, "Ford",
                "Escape XLT 4WD", "Air Intake",
                "Air Filter", 62.95M);
            parts[54] = new PartDescription(990659, 2000, "Toyota",
                "RAV4 2WD/4-DOOR", "Cooling System",
                "Radiator OE Plastic tank", 136.85M);
            parts[55] = new PartDescription(440574, 2007, "Buick",
                "Lacrosse CXS V6 3.6", "Suspension",
                "Strut Mount Inc; Sleeve; Rear Left",
                80.80M);
        }
    }
}
  5. Return to the form and double-click the Year combo box
  6. To display the list of car makes when the user selects a year, implement the event as follows:
      
    private void cbxCarYears_SelectedIndexChanged(object sender, EventArgs e)
{
        cbxMakes.Text = "";
        cbxMakes.Items.Clear();
        cbxModels.Text = "";
        cbxModels.Items.Clear();
        cbxCategories.Text = "";
        cbxCategories.Items.Clear();

        lbxPartNumbers.Items.Clear();
        lbxPartNames.Items.Clear();
        lbxUnitPrices.Items.Clear();

        foreach (PartDescription part in parts)
                if (part.CarYear == int.Parse(cbxCarYears.Text))
                    if( cbxMakes.FindString(part.Make) == -1 )
                        cbxMakes.Items.Add(part.Make);
}
  7. Return to the form and double-click the Make combo box
  8. To display the list of car models when the user has selected a year and a make, implement the event as follows:
      
    private void cbxMakes_SelectedIndexChanged(object sender, EventArgs e)
{
        cbxModels.Text = "";
        cbxModels.Items.Clear();
        cbxCategories.Text = "";
        cbxCategories.Items.Clear();

        lbxPartNumbers.Items.Clear();
        lbxPartNames.Items.Clear();
        lbxUnitPrices.Items.Clear();

        foreach (PartDescription part in parts)
                if( (part.CarYear == int.Parse(cbxCarYears.Text)) &&
                    (part.Make == cbxMakes.Text) )
                    if (cbxModels.FindString(part.Model) == -1)
                        cbxModels.Items.Add(part.Model);
}
  9. Return to the form and double-click the Model combo box
  10. To display the list of categories after the user has selected the year, the make, and the model, implement the event as follows:
      
    private void cbxModels_SelectedIndexChanged(object sender, EventArgs e)
{
        lbxPartNumbers.Items.Clear();
        lbxPartNames.Items.Clear();
        lbxUnitPrices.Items.Clear();

        foreach (PartDescription part in parts)
                if ((part.CarYear == int.Parse(cbxCarYears.Text)) &&
                    (part.Make == cbxMakes.Text) &&
                    (part.Model == cbxModels.Text) )
                    if (cbxCategories.FindString(part.Category) == -1)
                        cbxCategories.Items.Add(part.Category);
}
  11. Return to the form and double-click the Category combo box
  12. To display the list of available parts, implement the event and define a new method  as follows:
      
    private void cbxCategories_SelectedIndexChanged(object sender, EventArgs e)
{
        lbxPartNumbers.Items.Clear();
        lbxPartNames.Items.Clear();
        lbxUnitPrices.Items.Clear();

        foreach (PartDescription part in parts)
            if ((part.CarYear == int.Parse(cbxCarYears.Text)) &&
                    (part.Make == cbxMakes.Text) &&
                    (part.Model == cbxModels.Text) &&
                    (part.Category == cbxCategories.Text))
            {
                    lbxPartNumbers.Items.Add(part.PartNumber.ToString());
                    lbxPartNames.Items.Add(part.PartName);
                    lbxUnitPrices.Items.Add(part.UnitPrice.ToString());
            }
}
  13. Return to the form and double-click the lbxPartNumbers list box
  14. When the user clicks an item from the Part Numbers list box, to make sure the corresponding item is selected in the other list boxes, implement the event as follows:
      
    private void lbxPartNumbers_SelectedIndexChanged(object sender, EventArgs e)
{
        lbxUnitPrices.SelectedIndex =
                lbxPartNames.SelectedIndex =
                lbxPartNumbers.SelectedIndex;
}
  15. Return to the form and double-click the lbxPartNames list box
  16. Implement the event as follows:
      
    private void lbxPartNames_SelectedIndexChanged(object sender, EventArgs e)
{
        lbxPartNumbers.SelectedIndex =
                lbxUnitPrices.SelectedIndex =
                lbxPartNames.SelectedIndex;
}
  17. Return to the form and double-click the lbxUnitPrices list box
  18. Implement the event as follows:
      
    private void lbxUnitPrices_SelectedIndexChanged(object sender, EventArgs e)
{
            lbxPartNames.SelectedIndex =
                lbxPartNumbers.SelectedIndex =
                lbxUnitPrices.SelectedIndex;
}
  19. Return to the form and click (once) the lbxPartNumbers list combo box
  20. In the Properties window, click the Events button and, in the Events section, double-click DoubleClick
  21. Implement the event as follows:
      
    internal void CalculateOrder()
{
        // Calculate the current total order and update the order
        decimal subTotal1 = 0.00M, subTotal2 = 0.00M, subTotal3 = 0.00M,
                    subTotal4 = 0.00M, subTotal5 = 0.00M, subTotal6 = 0.00M;
        decimal orderTotal = 0.00M;

        // Retrieve the value of each sub total
        try
        {
                subTotal1 = decimal.Parse(this.txtSubTotal1.Text);
        }
        catch (FormatException)
        {
                MessageBox.Show("Invalid Value");
        }

        try
        {
                subTotal2 = decimal.Parse(this.txtSubTotal2.Text);
        }
        catch (FormatException)
        {
                MessageBox.Show("Invalid Value");
        }

        try
        {
                subTotal3 = decimal.Parse(this.txtSubTotal3.Text);
        }
        catch (FormatException)
        {
                MessageBox.Show("Invalid Value");
        }

        try
        {
                subTotal4 = decimal.Parse(this.txtSubTotal4.Text);
        }
        catch (FormatException)
        {
                MessageBox.Show("Invalid Value");
        }

        try
        {
                subTotal5 = decimal.Parse(this.txtSubTotal5.Text);
        }
        catch (FormatException)
        {
                MessageBox.Show("Invalid Value");
        }

        try
        {
                subTotal6 = decimal.Parse(this.txtSubTotal6.Text);
        }
        catch (FormatException)
        {
                MessageBox.Show("Invalid Value");
        }

        // Calculate the total value of the sub totals
        orderTotal = subTotal1 + subTotal2 + subTotal3 +
                             subTotal4 + subTotal5 + subTotal6;

        // Display the total order in the appropriate text box
        this.txtTotalOrder.Text = orderTotal.ToString();
}

private void lbxPartNumbers_DoubleClick(object sender, EventArgs e)
{
        if (txtPartNumber1.Text == "")
        {
                // Display the item number in the Part # text box
                txtPartNumber1.Text = (string)lbxPartNumbers.SelectedItem;
                // Display the name of the selected item in
                // the current Description text box
                txtPartName1.Text = (string)lbxPartNames.SelectedItem;
                // Display the unit price of this item in
                // the corresponding Unit Price text box
                txtUnitPrice1.Text = (string)lbxUnitPrices.SelectedItem;
                // Since an item was selected, set its quantity to 1
                txtQuantity1.Text = "1";
                // Calculate the sub total of the current item item
                txtSubTotal1.Text = txtUnitPrice1.Text;
                // Give focus to the Qty text box of the current item
                txtQuantity1.Focus();
        }// If the previous Part # text box is not empty, then use the next one
        else if (txtPartNumber2.Text == "")
        {
                txtPartNumber2.Text = (string)lbxPartNumbers.SelectedItem;
                txtPartName2.Text = (string)lbxPartNames.SelectedItem;
                txtUnitPrice2.Text = (string)lbxUnitPrices.SelectedItem;

                txtQuantity2.Text = "1";
                txtSubTotal2.Text = txtUnitPrice2.Text;
                txtQuantity2.Focus();
        }
        else if (txtPartNumber3.Text == "")
        {
                txtPartNumber3.Text = (string)lbxPartNumbers.SelectedItem;
                txtPartName3.Text = (string)lbxPartNames.SelectedItem;
                txtUnitPrice3.Text = (string)lbxUnitPrices.SelectedItem;

                txtQuantity3.Text = "1";
                txtSubTotal3.Text = txtUnitPrice3.Text;
                txtQuantity3.Focus();
        }
        else if (txtPartNumber4.Text == "")
        {
                txtPartNumber4.Text = (string)lbxPartNumbers.SelectedItem;
                txtPartName4.Text = (string)lbxPartNames.SelectedItem;
                txtUnitPrice4.Text = (string)lbxUnitPrices.SelectedItem;

                txtQuantity4.Text = "1";
                txtSubTotal4.Text = txtUnitPrice4.Text;
                txtQuantity4.Focus();
        }
        else if (txtPartNumber5.Text == "")
        {
                txtPartNumber5.Text = (string)lbxPartNumbers.SelectedItem;
                txtPartName5.Text = (string)lbxPartNames.SelectedItem;
                txtUnitPrice5.Text = (string)lbxUnitPrices.SelectedItem;

                txtQuantity5.Text = "1";
                txtSubTotal5.Text = txtUnitPrice5.Text;
                txtQuantity5.Focus();
        }
        else if (txtPartNumber6.Text == "")
        {
                txtPartNumber6.Text = (string)lbxPartNumbers.SelectedItem;
                txtPartName6.Text = (string)lbxPartNames.SelectedItem;
                txtUnitPrice6.Text = (string)lbxUnitPrices.SelectedItem;

                txtQuantity6.Text = "1";
                txtSubTotal6.Text = txtUnitPrice6.Text;
                txtQuantity6.Focus();
        } // If all Part # text boxes are filled, don't do anything
        else
                return;

        CalculateOrder();
}
  22. Return to the form and click (once) the lbxPartNames list box
  23. In the Events section of the Properties window, double-click Double-Click
  24. Implement the event as follows:
      
    private void lbxPartNames_DoubleClick(object sender, EventArgs e)
{
        // This event should have been fired already
        // But we call it just to be on the safe side
        lbxPartNames_SelectedIndexChanged(sender, e);
        // Now behave as if the Part Numbers list box was double-clicked
        lbxPartNumbers_DoubleClick(sender, e);
}
  25. Return to the form and click (once) the lbxUnitPrices list box
  26. In the Events section of the Properties window, double-click Double-Click
  27. Implement the event as follows:
      
    private void lbxUnitPrices_DoubleClick(object sender, EventArgs e)
{
        lbxUnitPrices_SelectedIndexChanged(sender, e);
        lbxPartNumbers_DoubleClick(sender, e);
}
  28. Display the form and click the first text box under Qty
  29. In the Properties window and in the Events section, double-click the Leave field
  30. Implement the event as follows:
      
    private void txtQuantity1_Leave(object sender, EventArgs e)
{
            int qty = 0;
            decimal unitPrice = 0.00M, subTotal = 0.00M;

            // Get the quantity of the current item
            try
            {
                qty = int.Parse(this.txtQuantity1.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            // Get the unit price of the current item
            try
            {
                unitPrice = decimal.Parse(this.txtUnitPrice1.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            // Calculate the current sub total
            subTotal = qty * unitPrice;

            // Display the new sub total in the corresponding text box
            this.txtSubTotal1.Text = subTotal.ToString();
            // Update the order
            CalculateOrder();
}
  31. Return to the form and click the second text box under Qty
  32. In the Events section of the Properties window, double-click the Leave field and implement the event as follows:
      
    private void txtQuantity2_Leave(object sender, EventArgs e)
{
            int qty = 0;
            decimal unitPrice = 0.00M, subTotal = 0.00M;

            try
            {
                qty = int.Parse(this.txtQuantity2.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            try
            {
                unitPrice = decimal.Parse(this.txtUnitPrice2.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            subTotal = qty * unitPrice;
            this.txtSubTotal2.Text = subTotal.ToString();
            
            CalculateOrder();
}
  33. Return to the form and click the third text box under Qty
  34. In the Events section of the Properties window, double-click the Leave field and implement the event as follows:
      
    private void txtQuantity3_Leave(object sender, EventArgs e)
{
            int qty = 0;
            decimal unitPrice = 0.00M, subTotal = 0.00M;

            try
            {
                qty = int.Parse(this.txtQuantity3.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            try
            {
                unitPrice = decimal.Parse(this.txtUnitPrice3.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            subTotal = qty * unitPrice;
            this.txtSubTotal3.Text = subTotal.ToString();

            CalculateOrder();
}
  35. Return to the form and click the fourth text box under Qty
  36. In the Events section of the Properties window, double-click the Leave field and implement the event as follows:
      
    private void txtQuantity4_Leave(object sender, EventArgs e)
{
            int qty = 0;
            decimal unitPrice = 0.00M, subTotal = 0.00M;

            try
            {
                qty = int.Parse(this.txtQuantity4.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            try
            {
                unitPrice = decimal.Parse(this.txtUnitPrice4.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            subTotal = qty * unitPrice;
            this.txtSubTotal4.Text = subTotal.ToString();

            CalculateOrder();
}
  37. Return to the form and click the fifth text box under Qty
  38. In the Events section of the Properties window, double-click the Leave field and implement the event as follows:
      
    private void txtQuantity5_Leave(object sender, EventArgs e)
{
            int qty = 0;
            decimal unitPrice = 0.00M, subTotal = 0.00M;

            try
            {
                qty = int.Parse(this.txtQuantity5.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            try
            {
                unitPrice = decimal.Parse(this.txtUnitPrice5.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            subTotal = qty * unitPrice;
            this.txtSubTotal5.Text = subTotal.ToString();

            CalculateOrder();
}
  39. Return to the form and click the sixth text box under Qty
  40. In the Events section of the Properties window, double-click the Leave field and implement the event as follows:
      
    private void txtQuantity6_Leave(object sender, EventArgs e)
{
            int qty = 0;
            decimal unitPrice = 0.00M, subTotal = 0.00M;

            try
            {
                qty = int.Parse(this.txtQuantity6.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            try
            {
                unitPrice = decimal.Parse(this.txtUnitPrice6.Text);
            }
            catch (FormatException)
            {
                MessageBox.Show("Invalid Value");
            }

            subTotal = qty * unitPrice;
            this.txtSubTotal6.Text = subTotal.ToString();

            CalculateOrder();
}
  41. Return to the form
  42. Double-click the first Remove button and implement its event as follows:
      
    private void btnRemove1_Click(object sender, EventArgs e)
{
        txtPartNumber1.Text = "";
        txtPartName1.Text = "";
        txtUnitPrice1.Text = "0.00";
        txtQuantity1.Text = "0";
        txtSubTotal1.Text = "0.00";
        CalculateOrder();
}
  43. Return to the form
  44. Double-click the second Remove button and implement its event as follows:
      
    private void btnRemove2_Click(object sender, EventArgs e)
{
        txtPartNumber2.Text = "";
        txtPartName2.Text = "";
        txtUnitPrice2.Text = "0.00";
        txtQuantity2.Text = "0";
        txtSubTotal2.Text = "0.00";
        CalculateOrder();
}
  45. Return to the form
  46. Double-click the third Remove button and implement its event as follows:
      
    private void btnRemove3_Click(object sender, EventArgs e)
{
        txtPartNumber3.Text = "";
        txtPartName3.Text = "";
        txtUnitPrice3.Text = "0.00";
        txtQuantity3.Text = "0";
        txtSubTotal3.Text = "0.00";
        CalculateOrder();
}
  47. Return to the form
  48. Double-click the fourth Remove button and implement its event as follows:
      
    private void btnRemove4_Click(object sender, EventArgs e)
{
        txtPartNumber4.Text = "";
        txtPartName4.Text = "";
        txtUnitPrice4.Text = "0.00";
        txtQuantity4.Text = "0";
        txtSubTotal4.Text = "0.00";
        CalculateOrder();
}
  49. Return to the form
  50. Double-click the fifth Remove button and implement its event as follows:
      
    private void btnRemove5_Click(object sender, EventArgs e)
{
        txtPartNumber5.Text = "";
        txtPartName5.Text = "";
        txtUnitPrice5.Text = "0.00";
        txtQuantity5.Text = "0";
        txtSubTotal5.Text = "0.00";
        CalculateOrder();
}
  51. Return to the form
  52. Double-click the sixth Remove button and implement its event as follows:
      
    private void btnRemove6_Click(object sender, EventArgs e)
{
        txtPartNumber6.Text = "";
        txtPartName6.Text = "";
        txtUnitPrice6.Text = "0.00";
        txtQuantity6.Text = "0";
        txtSubTotal6.Text = "0.00";
        CalculateOrder();
}
  53. Return to the form
  54. Double-click the Close button and implement its Click event as follows:
      
    private void btnClose_Click(object sender, EventArgs e)
{
        Close();
}
  55. Execute the application to test it
     
    College Park Auto Parts - Form Result
  56. After using it, close the form

Finding a String in the Combo Box

Instead of simply selecting an item from a combo box, the user may want to find out if a certain string exists in the  list. To support this operation, the ComboBox class is equipped with a method named FindString that is overloaded with two versions. One of the syntaxes of this method is: 

public int FindString(string s);

This method takes as argument the string to find in the combo box. If the item is found in the list, the method returns its position. If the list does not have that string, the method return -1. The above syntax of the method would look through the whole list. If you want the search to start at a specific index, you can use the following version of the FindString() method: 

public int FindString(string s, int startIndex);

This version takes as the first argument a string. Instead of start looking for it from the beginning of the list, this method starts at the index specified by the startIndex value.

The FindString() method performs its operation without regards to case. This means that it would perform the same search for BlindMan, Blindman, blindMan, or BLINDMAN and would produce the same result for them. If you want the case of the characters to be taken into consideration, use the FindStringExact() method that also is overloaded with two versions. The syntax of the first version is: 

public int FindStringExact(string s);

This method proceeds like the FindString() method by starting to look for the string from the beginning of the list. If you want to specify from where to start looking for the string, you should use the following version: 

public int FindStringExact(string s, int startIndex);

The Styles of a Combo Box
 

The Flat Styles

Like most graphical controls, a combo box appears as a 3-D object with raised borders. As an alternative, you can display it as a flat object. To assist you with this choice, the ComboBox class provides the FlatStyle property. The FlatStyle property is based on the FlatStyle enumeration. Its members are:

  • Standard: This is the default value of the property. It makes the control appear with raised borders:
      
    private void InitializeComponent()
{
        SuspendLayout();
        lblTitle = new Label();
        lblTitle.Text = "Academic Disciplines";
        lblTitle.Location = new Point(12, 12);
        lblTitle.AutoSize = true;
        Controls.Add(lblTitle);

        cbxAcademicDisciplines = new ComboBox();
        cbxAcademicDisciplines.Location = new Point(12, 32);
        cbxAcademicDisciplines.Width = 232;
 
        cbxAcademicDisciplines.Items.Add("Natural Sciences");
        cbxAcademicDisciplines.Items.Add("Mathematics and Computer Sciences");
        cbxAcademicDisciplines.Items.Add("Social Sciences");
        cbxAcademicDisciplines.Items.Add("Humanities");
        cbxAcademicDisciplines.Items.Add("Professions and Applied Sciences");

        cbxAcademicDisciplines.FlatStyle = FlatStyle.Standard;

        Controls.Add(cbxAcademicDisciplines);
}
  • Popup: The control will appear flat with a surrounding gray line:
      
    cbxAcademicDisciplines.FlatStyle = FlatStyle.Popup;
    Popup Combo Box
  • Flat: The control appears flat with a white surroundiong border:
      
    cbxAcademicDisciplines.FlatStyle = FlatStyle.Flat;
  • System: The user's operating system (and theme, if any) will determine how the control must appear

The Drop Down Style

In our introduction to the combo box, we saw that it appeared like a text box with a down-pointing button on its right side. In reality, that was the description of just one type of combo box. There are three styles of combo boxes, although all allow the user to make only one selection. These styles are controlled by the DropDownStyle property, which is based on the ComboBoxStyle enumeration.

One of the types of combo boxes is referred to as Drop Down and is created by setting the DropDownStyle property to DropDown. Here is an example:

private void InitializeComponent()
{
        lblTitle = new Label();
        lblTitle.Text = "Academic Disciplines";
        lblTitle.Location = new Point(12, 12);
        lblTitle.AutoSize = true;
        Controls.Add(lblTitle);

        cbxAcademicDisciplines = new ComboBox();
        cbxAcademicDisciplines.Location = new Point(12, 32);
        cbxAcademicDisciplines.Items.Add("Natural sciences");
        cbxAcademicDisciplines.Items.Add("Mathematics and Computer sciences");
        cbxAcademicDisciplines.Items.Add("Social sciences");
        cbxAcademicDisciplines.Items.Add("Humanities");
        cbxAcademicDisciplines.Items.Add("Professions and Applied sciences");

        cbxAcademicDisciplines.DropDownStyle = ComboBoxStyle.DropDown;

        Controls.Add(cbxAcademicDisciplines);
}

This type is made of a text box on the left side and a down-pointing arrowed button on the right side. Depending on how the control was created, when it comes up, it may not display anything:

A Drop Down Combo Box

Normally, if you want a DropDown style of combo box to display a string when the control comes up, you can either enter a value in the Text property or assign a string to the ComboBox.Text property. Here is an example:

private void InitializeComponent()
{
        lblTitle = new Label();
        lblTitle.Text = "Academic Disciplines";
        lblTitle.Location = new Point(12, 12);
        lblTitle.AutoSize = true;
        Controls.Add(lblTitle);

        cbxAcademicDisciplines = new ComboBox();
        cbxAcademicDisciplines.Location = new Point(12, 32);
        cbxAcademicDisciplines.Items.Add("Natural sciences");
        cbxAcademicDisciplines.Items.Add("Mathematics and Computer sciences");
        cbxAcademicDisciplines.Items.Add("Social sciences");
        cbxAcademicDisciplines.Items.Add("Humanities");
        cbxAcademicDisciplines.Items.Add("Professions and Applied sciences");

        cbxAcademicDisciplines.DropDownStyle = ComboBoxStyle.DropDown;
        cbxAcademicDisciplines.Text = "Social sciences";

        Controls.Add(cbxAcademicDisciplines);
}

This would produce:

The Text of a Combo Box

The string you give to the Text property does not have to be one of the items of the list. 

To use the combo box, the user can click its down pointing arrow. At any time, to find out whether the list is displaying, you can check the value of the DroppedDown Boolean property. In the same way, to drop the list, you can programmatically set the combo box' DroppedDown property to true.

Once the list is displaying, if the user clicks that arrow, a list would appear (or expand). If the string assigned to the Text property is one of the items in the list, it would display in the text box side of the control and it would be selected in the list. Here is an example:

private void InitializeComponent()
{
        lblTitle = new Label();
        lblTitle.Text = "Academic Disciplines";
        lblTitle.Location = new Point(12, 12);
        lblTitle.AutoSize = true;
        Controls.Add(lblTitle);

        cbxAcademicDisciplines = new ComboBox();
        cbxAcademicDisciplines.Location = new Point(12, 32);
        cbxAcademicDisciplines.Items.Add("Natural Sciences");
        cbxAcademicDisciplines.Items.Add("Mathematics and Computer Sciences");
        cbxAcademicDisciplines.Items.Add("Social Sciences");
        cbxAcademicDisciplines.Items.Add("Humanities");
        cbxAcademicDisciplines.Items.Add("Professions and Applied Sciences");

        cbxAcademicDisciplines.DropDownStyle = ComboBoxStyle.DropDown;
        cbxAcademicDisciplines.Text = "Social Sciences";

        Controls.Add(cbxAcademicDisciplines);
}

This would produce:

Using a DropDown combo box

If the string assigned to the Text property is not one of the items in the list, it would still appear selected in the text box side of the control:

Here is an example:

private void InitializeComponent()
{
        lblTitle = new Label();
        lblTitle.Text = "Academic Disciplines";
        lblTitle.Location = new Point(12, 12);
        lblTitle.AutoSize = true;
        Controls.Add(lblTitle);

        cbxAcademicDisciplines = new ComboBox();
        cbxAcademicDisciplines.Location = new Point(12, 32);
        cbxAcademicDisciplines.Items.Add("Natural Sciences");
        cbxAcademicDisciplines.Items.Add("Mathematics and Computer Sciences");
        cbxAcademicDisciplines.Items.Add("Social Sciences");
        cbxAcademicDisciplines.Items.Add("Humanities");
        cbxAcademicDisciplines.Items.Add("Professions and Applied Sciences");

        cbxAcademicDisciplines.DropDownStyle = ComboBoxStyle.DropDown;

        cbxAcademicDisciplines.Text = "Arts & Sciences";

        Controls.Add(cbxAcademicDisciplines);
}

This would produce:

When the list displays, either because the user clicked the arrow button, pressed Alt + the down arrow key or because you decided to display it, the control fires a DropDown event, which is of type EventArgs.

If the user sees an item that he or she wants or was asked to select, he or she can click it. After an item has been clicked, two things happen: 1. the list retracts (or collapses) like a plastic; 2. the item that was clicked fills the text part and becomes the new selection:

Selecting an existing item in a DropDown combo box After selecting an item from a DropDown combo box

On the other hand, after displaying the list, if the user doesn't want to select anything from the list, he or she can click the arrow again or click anywhere away from the list. The list would collapse and the text part would get back to the previous text.

One of the major characteristics of a DropDown style of combo box, as compared to the type we will see next, is that, if the user knows for sure that the item he or she is looking for is in the list, he can first delete the string in the text box part of the control, then start typing. For example, if the list contains a string such as Social Sciences , the user can delete the text part, and start typing so. If there is only one item that starts with s, the user can then click the arrow twice and the item would be selected. Imagine the list contains such items as Jules and Julienne, if the user types the first common letters of these item and double-click the arrow, the first item that has these letters would be selected. This means that, if the user wants to other item to be selected, he or she should type the letters beyond the common ones. In the case of Jules and Julienne, if the user wants Julienne to be selected from an incomplete string, he or she can type juli and click the arrowed button twice.

The Drop Down List

Another style of combo box is gotten by setting the DropDownStyle to DropDownList. This type also is made of a text box on the left and a down-pointing arrowed button on the right side. It also may appear empty when it comes up, depending on how it was created. The biggest difference between a DropDown combo box and a DropDownList combo box is that, with the drop down list, the user can only select from the list: he or she cannot type anything in the text box part of the control.

Once again, to use the control, the user can click its arrow, which causes the list to display. The user can also display the list using the keyboard by pressing Alt + down arrow key after giving focus to the control.

The Simple Combo Box

The last type of combo box is called a simple combo box and is gotten by setting the DropDownStyle to Simple. After setting this value, you must heighten the control to get the desired size. This type of combo box is also made of two parts but they are distinct. The top section of the combo box displays a text box. Immediately under the text box, there is a list box. The following is the Character dialog box of OpenOffice.org. Its Font property page is equipped with the Font, the Typeface, and the Size combo boxes that are of a Simple style:

The Simple Combo Box

Notice that the control doesn't display a down-arrow pointing button on the right side of the selected item since the list is available already. To use this combo box, the user can examine the list part. If he or she sees the desired item, he can click it. When an item is clicked, it becomes the string of the top text part. If the user clicks a different item, it would become the new selection, replacing the one that was in the text part. Although this appears as a list box, the user cannot select more than one item.

The most regularly used combo boxes are made of text items. You can also create a combo box that displays colors or pictures. To create such a combo box, you start by changing the value of the DrawMode property that is set to Normal by default. If you want to display items that are not just regular text, you can set this property to either OwnerDrawFixed, which would make all items have the same height, or OwnerDrawVariable, which allows different items to have different sizes.

If the combo box has a DropDownStyle other than Simple, there is typically a fixed number of items that display when the user clicks the control’s arrow. You can control the number of items that displays using the MaxDropDownItems property. By default, this is set to 8. If the list contains a number of items less than the MaxDropDownItems integer value, all of the items would display fine. If the list contains more than the MaxDropDownItems number of items, when the user clicks the arrow, a vertical scroll box would appear. The control would display MaxDropDownItems number of items; to reveal more, the user would have to scroll in the list.

Automatic List Creation
 

Using an External List
 

In the next previous sections, we saw how to create a list of items. The .NET Framework provides an alternative. Instead of creating a list from scratch, you can use one that exists already. For example, you can use a list of recently accessed web sites or custom list of your own. To assist you with this, the ComboBox class provides with three techniques.

To specify an external list of items to use for the combo box, you have two options. You can use the AutoCompleteSource property, that is based on the AutoCompleteSource enumeration. The members of this enumeration are: None, RecentlyUsedList, FileSystem, FileSystemDirectories, HistoryList, ListItems, AllSystemSources, AllUrl, and CustomSource. Imagine that you want to use the list of web pages you had visited lately. To use that list, you can specify the AutoCompleteSource as HistoryList.

After specifying the source of the list, use the AutoCompleteMode property to specify how the combo box (or rather the text box side of the control) will assist the user. This property is based on the AutoCompleteMode enumeration that has four members. None is the default value. Imagine you had set the value of the AutoCompleteSource property as HistoryList. If you specify AutoCompleteMode as:

  • Suggest: In the text box part of the combo box, the user can click and start typing. A list of closely-matched items would display:
     


    In this case, as soon as the user types h, a list of URLs that start with h (for http) would come up. Once the user sees the desired item, he or she can then click that item to select it. Since there are many items, to continuously narrow the list, the user can keep typing until the desired item comes up
  • Append: In the text box part, the user can start typing. The control would then start looking for the closest matches and try to complete the user's entry with those available. Here is an example:
     

    First the user types h and http:// comes up as the first closest match. Then, the user specifies that the address starts with m and the compiler suggests, in alphabetical order, the closest URL with that. Then, the user types ms and finds out that msdn2 is available

  • SuggestAppend: This is a combination of the previous two options. When the control comes up, the user can start typing. The control would then display the list of items that start with what the user typed and it would display the starting closest match
     

    The user can continue typing. If the desired item appears in the list, the user can select it. Otherwise, as the user is typing, the closest match displays in the text box part of the control
     

    Combo Box Combo Box

Using a Custom List
 

Instead of using an external list, you can create your own. To do this, use the AutoCompleteCustomSource property. At design time, to create a list of strings, access the Properties window for the text box. In the Properties window, click the ellipsis button of the AutoCompleteCustomSource field to open the String Collection Editor. Enter the strings separated by a hard Return, and click OK. You can also programmatically create the list. To assist you, the .NET Framework provides a class named AutoCompleteStringCollection. The AutoCompleteStringCollection class implements the IList, the ICollection, and the IEnumerable interfaces.

After creating the custom list, to let the combo box use it, set the AutoCompleteMode property to CustomSource.
 
 
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