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Windows Control: 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:

Character

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: 

Imports System.Drawing
Imports System.Windows.Forms

Module Exercise

    Public Class Starter
        Inherits Form

        Private lblTitle As Label
        Private cbxAcademicDisciplines As ComboBox

        Dim components As System.ComponentModel.Container

        Public Sub New()
            InitializeComponent()
        End Sub

        Public Sub 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)

        End Sub

    End Class

    Function Main() As Integer

        Dim frmStart As Starter = New Starter

        Application.Run(frmStart)

        Return 0
    End Function

End Module

This would produce:

Combo Box

Practical Learning Practical Learning: Introducing Combo Boxes
  1. Start Microsoft Visual Basic and create a new Windows Application named CollegeParkAutoParts1
  2. In the Solution Explorer, right-click Form1.vb and click Rename
  3. Type Central.vb 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 Sub 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)

End Sub

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. Right-click the form and click View Code
  2. In the Class Name combo box, select (Central Events)
  3. In the Method Name combo box, select Load
  4. To create a list of items using the Add() method, implement its Load event as follows:
      
    Private Sub CentralLoad(ByVal sender As Object, 
                             ByVal e As System.EventArgs) 
                             Handles Me.Load
        For i As Integer = Date.Today.Year To 1960 Step -1
            cbxCarYears.Items.Add(i)
        Next
End Sub
  5. 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:
      
    Public Class PartDescription
    Private ID As Long
    Private yr As Integer
    Private mk As String
    Private mdl As String
    Private cat As String
    Private name As String
    Private price As Double

    Public Sub New(ByVal code As Long, ByVal year As Integer, 
                   ByVal make As String, ByVal model As String, 
                   ByVal type As String, ByVal desc As String, 
                   ByVal UPrice As Double)
        ID = code
        yr = year
        mk = make
        mdl = model
        cat = type
        name = desc
        price = UPrice
    End Sub

    Public Property PartNumber() As Long
        Get
            Return ID
        End Get
        Set(ByVal value As Long)
            ID = value
        End Set
    End Property

    Public Property CarYear() As Integer
        Get
            Return yr
        End Get
        Set(ByVal value As Integer)
            yr = value
        End Set
    End Property

    Public Property Make() As String
        Get
            Return mk
        End Get
        Set(ByVal value As String)
            mk = value
        End Set
    End Property

    Public Property Model() As String
        Get
            Return mdl
        End Get
        Set(ByVal value As String)
            mdl = 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 PartName() As String
        Get
            Return name
        End Get
        Set(ByVal value As String)
            name = value
        End Set
    End Property

    Public Property UnitPrice() As Double
        Get
            Return price
        End Get
        Set(ByVal value As Double)
            price = value
        End Set
    End Property

    Public Overrides Function ToString() As String
        Return PartNumber & " " & 
                   CarYear.ToString() & " " & 
                   Make & " " & 
                   Model & " " & 
                   Category & " " & 
                   PartName & " " & 
                   UnitPrice
    End Function
End Class
  4. Access the Central.vb file and change the Load event as follows:
      
    Private parts(55) As PartDescription

Private Sub CentralLoad(ByVal sender As Object, 
                             ByVal e As System.EventArgs) 
                             Handles Me.Load
        For i As Integer = Date.Today.Year To 1960 Step -1
            cbxCarYears.Items.Add(i)
        Next

        parts(0) = New PartDescription(447093, 2002, "Ford", 
                "Escort SE L4 2.0", "Engine Electrical", 
                "Alternator 75amp  Remanufactured w/ 75 Amp", 
                205.05)
        parts(1) = New PartDescription(203815, 2006, "Dodge", 
            "Caravan SE L4 2.4", "Cooling System", 
            "Radiator Cap", 6.65)
        parts(2) = New PartDescription(293047, 2000, "Toyota", 
            "RAV4 2WD/4-DOOR", "Cooling System", 
            "Thermostat Gasket", 4.95)
        parts(3) = New PartDescription(990468, 2002, "Honda", 
            "Civic 1.7 EX 4DR", "Exhaust", 
            "Bolt & Spring Kit (Manifold outlet, Muffler Inlet)", 
            85.75)
        parts(4) = New PartDescription(304158, 1996, "Buick", 
            "Regal Custom V6 3.8", "Fuel Injection", 
            "Fuel Injector", 82.75)
        parts(5) = New PartDescription(807245, 2004, "Acura", 
            "MDX 3.5 4WD", "Driveshaft & Axle", 
            "CV Boot Clamp 7 x 750mm 1 Large + 1 Small Clamp", 
            1.6)
        parts(6) = New PartDescription(203485, 2001, "Ford", 
            "Taurus LX V6 3.0", "Fuel Injection", 
            "Oxygen Sensor OE Style 4Wire Front 2 Required", 
            52.65)
        parts(7) = New PartDescription(248759, 1999, "Jeep", 
            "Wrangler Sahara", "Air Intake", 
            "Air Filter AirSoft Panel", 7.95)
        parts(8) = New PartDescription(202848, 1998, "Honda", 
            "Accord 2.3 LX 4DR", "Air Intake", 
            "Air Filter", 12.55)
        parts(10) = New PartDescription(932759, 2006, "Kia", 
            "Rio 1.6DOHC16V 4-DR", "Cooling System", 
            "Thermostat", 14.45)
        parts(11) = New PartDescription(304975, 2000, "Honda", 
                "Civic 1.6 EX 4DR", "Suspension", 
                "Ball Joint Front Lower 2 per car", 40.55)
        parts(12) = New PartDescription(208450, 2003, "Chevrolet", 
            "Monte Carlo LS V6 3.4", "Fuel Injection", 
            "Oxygen Sensor OE connector Rear", 65.55)
        parts(13) = New PartDescription(209480, 2002, "Ford", 
            "Focus SE DOHC L4 2.0", "Steering", 
            "Steering Rack Remanufactured", 170.85)
        parts(9) = New PartDescription(203495, 2004, "Honda", 
            "Civic 1.7 EX 4DR", "Climate Control", 
            "A/C Clutch OE compressor = Sanden", 184.95)
        parts(14) = New PartDescription(203480, 2007, "Toyota", 
            "Corolla", "Air Intake", "Air Filter", 12.65)
        parts(15) = New PartDescription(109379, 2005, "Volvo", 
            "S40 2.5L T5 AWD", "Fuel Delivery", 
            "Fuel Filter Early Design Outer Diameter = 55mm", 
            30.95)
        parts(16) = New PartDescription(935794, 2002, "Ford", 
            "Escape XLS 4WD", "Brake", 
            "Brake Caliper Remanufactured Front Right", 
            65.55)
        parts(17) = New PartDescription(203485, 2006, "BMW", 
            "325i", "Climate Control", 
            "AC High Pressure Side Switch", 49.95)
        parts(18) = New PartDescription(204875, 1996, "Chevrolet", 
                "Monte Carlo Z34 V6 3.4", "Fuel Delivery", 
                "Fuel Filter", 8.05)
        parts(19) = New PartDescription(937485, 2007, "Toyota", 
                "Camry V6", "Air Intake", "Air Filter", 12.95)
        parts(20) = New PartDescription(294759, 2001, "Ford", 
            "Escape XLT 4WD", "Air Intake", 
            "Air Filter Panel", 7.25)
        parts(21) = New PartDescription(297495, 2003, "Honda", 
            "Civic 1.7 EX 4DR", "Brake", 
            "Brake Caliper Reman w/ ProAct Pads Front Right", 
            82.55)
        parts(22) = New PartDescription(794735, 2006, "BMW", 
            "325i", "Climate Control", 
            "Cabin Air/Pollen Filter With Activated Carbon", 
            28.05)
        parts(23) = New PartDescription(937485, 2007, "Toyota", 
            "Corolla", "Body Electrical", 
            "Halogen  SilverStar 12V 65W inner-high beam", 
            22.85)
        parts(24) = New PartDescription(492745, 2005, "Ford", 
            "Focus ZX3 L4 2.0", "Air Intake", 
            "Fuel Injection Perf Kit", 342.95)
        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", 
            1.6)
        parts(26) = New PartDescription(293749, 2004, "Acura", 
            "MDX 3.5 4WD", "Driveshaft & Axle", 
            "Axle Nut 24mm x 15 rear ", 
            2.35)
        parts(27) = New PartDescription(920495, 2006, "BMW", 
            "325i", "Climate Control", 
            "Adjustable Telescoping Mirror", 7.95)
        parts(28) = New PartDescription(204075, 2004, "Acura", 
            "MDX 3.5 4WD", "Driveshaft & Axle", 
            "Wheel Bearing Rear 1 per wheel", 70.15)
        parts(29) = New PartDescription(979304, 2000, "Toyota", 
                "RAV4 2WD/4-DOOR", "Cooling System", 
                "Thermostat Housing", 20.95)
        parts(30) = New PartDescription(300456, 2004, "Acura", 
            "MDX 3.5 4WD", "Driveshaft & Axle", 
            "Wheel Bearing Front 1 per wheel", 66.65)
        parts(31) = New PartDescription(404860, 2001, "Ford", 
            "Taurus LX V6 3.0", "Suspension", 
            "Shock Absorber GR2 Rear Wagon only", 39.4)
        parts(32) = New PartDescription(585688, 2007, "Buick", 
                "Lacrosse CXS V6 3.6", "Brake", 
                "Climate Control", 10.65)
        parts(33) = New PartDescription(739759, 2001, "Ford", 
            "Taurus LX V6 3.0", "Suspension", 
            "Shock Absorber GasaJust Rear Wagon only", 30.95)
        parts(34) = New PartDescription(927495, 2005, "Volvo", 
                "S40 2.5L T5 AWD", "Engine Mechanical", 
                "Timing Belt Idler Pulley Original Equipment INA", 
                65.55)
        parts(40) = New PartDescription(979374, 2000, "Toyota", 
            "RAV4 2WD/4-DOOR", "Cooling System", 
            "Thermostat Gasket", 4.95)
        parts(35) = New PartDescription(542347, 2007, "Buick", 
            "Lacrosse CXS V6 3.6", "Brake", 
            "Brake Pad Set ProACT Ceramic w/Shims Front", 80.05)
        parts(36) = New PartDescription(683064, 2000, "Toyota", 
            "RAV4 2WD/4-DOOR", "Cooling System", 
            "Radiator Hose Upper", 103.75)
        parts(37) = New PartDescription(248759, 1999, "Jeep", 
            "Wrangler Sahara", "Air Intake", 
            "Air Filter", 50.95)
        parts(38) = New PartDescription(973974, 2007, "Toyota", 
            "Corolla", "Air Intake", 
            "Air Mass Meter W/o Housing Meter/sensor only", 
            134.95)
        parts(39) = New PartDescription(285800, 2001, "Ford", 
                "Escape XLT 4WD", "Transmission", "AT Filter", 34.95)
        parts(41) = New PartDescription(207495, 2007, "Toyota", 
            "Corolla", "Body Electrical", 
            "Headlight Bulb 12V 65W inner-high beam", 9.35)
        parts(42) = New PartDescription(566676, 2000, "Toyota", 
            "RAV4 2WD/4-DOOR", "Cooling System", 
            "Auxiliary Fan Switch", 42.95)
        parts(43) = New PartDescription(304950, 2007, "Toyota", 
            "Corolla", "Body Electrical", 
            "Headlight Bulb 12V 51W outer", 7.85)
        parts(44) = New PartDescription(797394, 2000, "Toyota", 
            "RAV4 2WD/4-DOOR", "Cooling System", 
            "Water Flange Gasket", 0.85)
        parts(45) = New PartDescription(910203, 2007, "Buick", 
            "Lacrosse CXS V6 3.6", "Suspension", 
            "Strut Mount Inc Sleeve Rear Right", 80.85)
        parts(46) = New PartDescription(790794, 2000, "Toyota", 
            "RAV4 2WD/4-DOOR", "Cooling System", 
            "Radiator Hose Lower", 9.45)
        parts(47) = New PartDescription(970394, 2007, "Buick", 
            "Lacrosse CXS V6 3.6", "Suspension", 
            "Coil Spring Insulator Front Lower", 14.55)
        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", 
                26.95)
        parts(49) = New PartDescription(209704, 2007, "Toyota", 
            "Corolla", "Body Electrical", 
            "Wiper Blade Excel+ Front Right", 7.25)
        parts(50) = New PartDescription(200368, 2000, "Toyota", 
            "RAV4 2WD/4-DOOR", "Cooling System", 
            "Radiator Drain Plug incl gasket", 3.15)
        parts(51) = New PartDescription(200970, 2005, "Volvo", 
            "S40 2.5L T5 AWD", "Engine Mechanical", 
            "Reference Sensor Flywheel Engine Speed", 62.05)
        parts(52) = New PartDescription(542347, 2007, "Buick", 
            "Lacrosse CXS V6 3.6", "Air Intake", 
            "Air Filter", 50.25)
        parts(53) = New PartDescription(927045, 2001, "Ford", 
                "Escape XLT 4WD", "Air Intake", 
                "Air Filter", 62.95)
        parts(54) = New PartDescription(990659, 2000, "Toyota", 
            "RAV4 2WD/4-DOOR", "Cooling System", 
            "Radiator OE Plastic tank", 136.85)
        parts(55) = New PartDescription(440574, 2007, "Buick", 
            "Lacrosse CXS V6 3.6", "Suspension", 
            "Strut Mount Inc Sleeve Rear Left", 80.8)
    End Sub
  5. In the Class Name combo box, select cbxCarYears
  6. In the Method Name combo box, select SelectedIndexChanged and implement the event as follows:
      
    Private Sub cbxCarYearsSelectedIndexChanged(ByVal sender As Object, 
                                             ByVal e As System.EventArgs) 
                                   Handles cbxCarYears.SelectedIndexChanged
        cbxMakes.Text = ""
        cbxMakes.Items.Clear()
        cbxModels.Text = ""
        cbxModels.Items.Clear()
        cbxCategories.Text = ""
        cbxCategories.Items.Clear()

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

        For Each part As PartDescription In parts
            If part.CarYear = CInt(cbxCarYears.Text) Then
                If cbxMakes.FindString(part.Make) = -1 Then
                    cbxMakes.Items.Add(part.Make)
                End If
            End If
        Next
End Sub
  7. In the Class Name combo box, select cbxMakes
  8. In the Method Name combo box, select SelectedIndexChanged and implement the event as follows:
      
    Private Sub cbxMakesSelectedIndexChanged(ByVal sender As Object, 
                                          ByVal e As System.EventArgs) 
                                      Handles cbxMakes.SelectedIndexChanged
        cbxModels.Text = ""
        cbxModels.Items.Clear()
        cbxCategories.Text = ""
        cbxCategories.Items.Clear()

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

        For Each part As PartDescription In parts
            If (part.CarYear = CInt(cbxCarYears.Text) And 
                    (part.Make = cbxMakes.Text)) Then
                If cbxModels.FindString(part.Model) = -1 Then
                    cbxModels.Items.Add(part.Model)
                End If
            End If
        Next
End Sub
  9. In the Class Name combo box, select cbxModels
  10. In the Method Name combo box, select SelectedIndexChanged and implement the event as follows:
      
    Private Sub cbxModelsSelectedIndexChanged(ByVal sender As Object, 
                                           ByVal e As System.EventArgs) 
                                       Handles cbxModels.SelectedIndexChanged
        lbxPartNumbers.Items.Clear()
        lbxPartNames.Items.Clear()
        lbxUnitPrices.Items.Clear()

        For Each part As PartDescription In parts
            If (part.CarYear = CInt(cbxCarYears.Text)) And 
                (part.Make = cbxMakes.Text) And 
                (part.Model = cbxModels.Text) Then
                If cbxCategories.FindString(part.Category) = -1 Then
                    cbxCategories.Items.Add(part.Category)
                End If
            End If
        Next
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 cbxCategoriesSelectedIndexChanged(ByVal sender As Object, 
                                               ByVal e As System.EventArgs) 
                                   Handles cbxCategories.SelectedIndexChanged
        lbxPartNumbers.Items.Clear()
        lbxPartNames.Items.Clear()
        lbxUnitPrices.Items.Clear()

        For Each part As PartDescription In parts
            If (part.CarYear = CInt(cbxCarYears.Text) And 
                    (part.Make = cbxMakes.Text) And 
                    (part.Model = cbxModels.Text) And 
                    (part.Category = cbxCategories.Text)) Then
                lbxPartNumbers.Items.Add(CStr(part.PartNumber))
                lbxPartNames.Items.Add(part.PartName)
                lbxUnitPrices.Items.Add(CStr(part.UnitPrice))
            End If
        Next
End Sub
  13. In the Class Name combo box, select lbxPartNumbers
  14. In the Method Name combo box, select SelectedIndexChanged
  15. 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 Sub lbxPartNumbersSelectedIndexChanged(ByVal sender As Object, 
                                            ByVal e As System.EventArgs) 
                                Handles lbxPartNumbers.SelectedIndexChanged
        lbxUnitPrices.SelectedIndex = lbxPartNumbers.SelectedIndex
        lbxPartNames.SelectedIndex = lbxUnitPrices.SelectedIndex

End Sub
  16. In the Class Name combo box, select lbxPartNames
  17. In the Method Name combo box, select SelectedIndexChanged and implement the event as follows:
      
    Private Sub lbxPartNamesSelectedIndexChanged(ByVal sender As Object, 
                                              ByVal e As System.EventArgs) 
                                  Handles lbxPartNames.SelectedIndexChanged
        lbxPartNumbers.SelectedIndex = lbxPartNames.SelectedIndex
        lbxUnitPrices.SelectedIndex = lbxPartNames.SelectedIndex
End Sub
  18. In the Class Name combo box, select lbxUnitPrices
  19. In the Method Name combo box, select SelectedIndexChanged and implement the event as follows:
      
    Private Sub lbxUnitPricesSelectedIndexChanged(ByVal sender As Object, 
                                           ByVal e As System.EventArgs) 
                                   Handles lbxUnitPrices.SelectedIndexChanged
        lbxPartNames.SelectedIndex = lbxUnitPrices.SelectedIndex
        lbxPartNumbers.SelectedIndex = lbxUnitPrices.SelectedIndex
End Sub
  20. Under the above event, create a procedure as follows:
      
    Private Sub CalculateOrder()
        ' Calculate the current total order and update the order
        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 OrderTotal As Double

        ' Retrieve the value of each sub total
        Try
            SubTotal1 = CDbl(txtSubTotal1.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            subTotal2 = CDbl(txtSubTotal2.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            SubTotal3 = CDbl(txtSubTotal3.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            SubTotal4 = CDbl(txtSubTotal4.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            SubTotal5 = CDbl(txtSubTotal5.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            SubTotal6 = CDbl(txtSubTotal6.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        ' Calculate the total value of the sub totals
        OrderTotal = SubTotal1 + subTotal2 + SubTotal3 + 
                     SubTotal4 + SubTotal5 + SubTotal6

        ' Display the total order in the appropriate text box
        txtTotalOrder.Text = CStr(OrderTotal)
End Sub
  21. In the Class Name combo box, select lbxPartNumbers
  22. In the Method Name combo box, select DoubleClick and implement the event as follows:
      
    Private Sub lbxPartNumbersDoubleClick(ByVal sender As Object, 
                                           ByVal e As System.EventArgs) 
                                           Handles lbxPartNumbers.DoubleClick
        If txtPartNumber1.Text = "" Then
            ' Display the item number in the Part # text box
            txtPartNumber1.Text = CType(lbxPartNumbers.SelectedItem, String)
            ' Display the name of the selected item in
            ' the current Description text box
            txtPartName1.Text = CType(lbxPartNames.SelectedItem, String)
            ' Display the unit price of this item in
            ' the corresponding Unit Price text box
            txtUnitPrice1.Text = CType(lbxUnitPrices.SelectedItem, String)
            ' 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
        ElseIf txtPartNumber2.Text = "" Then
            txtPartNumber2.Text = CType(lbxPartNumbers.SelectedItem, String)
            txtPartName2.Text = CType(lbxPartNames.SelectedItem, String)
            txtUnitPrice2.Text = CType(lbxUnitPrices.SelectedItem, String)

            txtQuantity2.Text = "1"
            txtSubTotal2.Text = txtUnitPrice2.Text
            txtQuantity2.Focus()
        ElseIf txtPartNumber3.Text = "" Then
            txtPartNumber3.Text = CType(lbxPartNumbers.SelectedItem, String)
            txtPartName3.Text = CType(lbxPartNames.SelectedItem, String)
            txtUnitPrice3.Text = CType(lbxUnitPrices.SelectedItem, String)

            txtQuantity3.Text = "1"
            txtSubTotal3.Text = txtUnitPrice3.Text
            txtQuantity3.Focus()
        ElseIf txtPartNumber4.Text = "" Then
            txtPartNumber4.Text = CType(lbxPartNumbers.SelectedItem, String)
            txtPartName4.Text = CType(lbxPartNames.SelectedItem, String)
            txtUnitPrice4.Text = CType(lbxUnitPrices.SelectedItem, String)

            txtQuantity4.Text = "1"
            txtSubTotal4.Text = txtUnitPrice4.Text
            txtQuantity4.Focus()
        ElseIf txtPartNumber5.Text = "" Then
            txtPartNumber5.Text = CType(lbxPartNumbers.SelectedItem, String)
            txtPartName5.Text = CType(lbxPartNames.SelectedItem, String)
            txtUnitPrice5.Text = CType(lbxUnitPrices.SelectedItem, String)

            txtQuantity5.Text = "1"
            txtSubTotal5.Text = txtUnitPrice5.Text
            txtQuantity5.Focus()
        ElseIf txtPartNumber6.Text = "" Then
            txtPartNumber6.Text = CType(lbxPartNumbers.SelectedItem, String)
            txtPartName6.Text = CType(lbxPartNames.SelectedItem, String)
            txtUnitPrice6.Text = CType(lbxUnitPrices.SelectedItem, String)

            txtQuantity6.Text = "1"
            txtSubTotal6.Text = txtUnitPrice6.Text
            txtQuantity6.Focus()
            ' If all Part # text boxes are filled, don't do anything
        Else
            Exit Sub
        End If

        CalculateOrder()
End Sub
  23. In the Class Name combo box, select lbxPartNames
  24. In the Method Name combo box, select DoubleClick and implement the event as follows:
      
    Private Sub lbxPartNamesDoubleClick(ByVal sender As Object, 
                                         ByVal e As System.EventArgs) 
                                         Handles lbxPartNames.DoubleClick
        ' This event should have been fired already
        ' But we call it just to be on the safe side
        lbxPartNamesSelectedIndexChanged(sender, e)
        ' Now behave as if the Part Numbers list box was double-clicked
        lbxPartNumbersDoubleClick(sender, e)
End Sub
  25. In the Class Name combo box, select lbxUnitPrices
  26. In the Method Name combo box, select Double-Click and implement the event as follows:
      
    Private Sub lbxUnitPricesSelectedIndexChanged(ByVal sender As Object, 
                                           ByVal e As System.EventArgs) 
                               Handles lbxUnitPrices.SelectedIndexChanged
        lbxPartNames.SelectedIndex = lbxUnitPrices.SelectedIndex
        lbxPartNumbers.SelectedIndex = lbxUnitPrices.SelectedIndex
End Sub
  27. In the Class Name combo box, select txtQuantity1
  28. In the Method Name combo box, select Leave and implement the event as follows:
      
    Private Sub txtQuantity1Leave(ByVal sender As Object, 
                                   ByVal e As System.EventArgs) 
                                   Handles txtQuantity1.Leave
        Dim Quantity As Integer
        Dim UnitPrice As Double, SubTotal As Double

        ' Get the quantity of the current item
        Try
            Quantity = CInt(txtQuantity1.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        ' Get the unit price of the current item
        Try
            UnitPrice = CDbl(txtUnitPrice1.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        ' Calculate the current sub total
        SubTotal = Quantity * UnitPrice

        ' Display the new sub total in the corresponding text box
        txtSubTotal1.Text = CStr(SubTotal)
        ' Update the order
        CalculateOrder()
End Sub
  29. In the Class Name combo box, select txtQuantity2
  30. In the Method Name combo box, select Leave and implement the event as follows:
      
    Private Sub txtQuantity2Leave(ByVal sender As Object, 
                                   ByVal e As System.EventArgs) 
                                   Handles txtQuantity2.Leave
        Dim Quantity As Integer
        Dim UnitPrice As Double, SubTotal As Double

        Try
            Quantity = CInt(txtQuantity2.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            UnitPrice = CDbl(txtUnitPrice2.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        SubTotal = Quantity * UnitPrice
        txtSubTotal2.Text = CStr(SubTotal)

        CalculateOrder()
End Sub
  31. In the Class Name combo box, select txtQuantity3
  32. In the Method Name combo box, Leave Click and implement the event as follows:
      
    Private Sub txtQuantity3Leave(ByVal sender As Object, 
                                   ByVal e As System.EventArgs) 
                                   Handles txtQuantity3.Leave
        Dim Quantity As Integer
        Dim UnitPrice As Double, SubTotal As Double

        Try
            Quantity = CInt(txtQuantity3.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            UnitPrice = CDbl(txtUnitPrice3.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        SubTotal = Quantity * UnitPrice
        txtSubTotal3.Text = CStr(SubTotal)

        CalculateOrder()
End Sub
  33. In the Class Name combo box, select txtQuantity4
  34. In the Method Name combo box, Leave Click and implement the event as follows:
      
    Private Sub txtQuantity4Leave(ByVal sender As Object, 
                                   ByVal e As System.EventArgs) 
                                   Handles txtQuantity4.Leave
        Dim Quantity As Integer
        Dim UnitPrice As Double, SubTotal As Double

        Try
            Quantity = CInt(txtQuantity4.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            UnitPrice = CDbl(txtUnitPrice4.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        SubTotal = Quantity * UnitPrice
        txtSubTotal4.Text = CStr(SubTotal)

        CalculateOrder()
End Sub
  35. In the Class Name combo box, select txtQuantity5
  36. In the Method Name combo box, select Leave and implement the event as follows:
      
    Private Sub txtQuantity5Leave(ByVal sender As Object, 
                                   ByVal e As System.EventArgs) 
                                   Handles txtQuantity5.Leave
        Dim Quantity As Integer
        Dim UnitPrice As Double, SubTotal As Double

        Try
            Quantity = CInt(txtQuantity5.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            UnitPrice = CDbl(txtUnitPrice5.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        SubTotal = Quantity * UnitPrice
        txtSubTotal5.Text = CStr(SubTotal)

        CalculateOrder()
End Sub
  37. In the Class Name combo box, select txtQuantity6
  38. In the Method Name combo box, select Leave and implement the event as follows:
      
    Private Sub txtQuantity6Leave(ByVal sender As Object, 
                                   ByVal e As System.EventArgs) 
                                   Handles txtQuantity6.Leave
        Dim Quantity As Integer
        Dim UnitPrice As Double, SubTotal As Double

        Try
            Quantity = CInt(txtQuantity6.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        Try
            UnitPrice = CDbl(txtUnitPrice6.Text)
        Catch ex As FormatException
            MsgBox("Invalid Value")
        End Try

        SubTotal = Quantity * UnitPrice
        txtSubTotal6.Text = CStr(SubTotal)

        CalculateOrder()
End Sub
  39. In the Class Name combo box, select btnRemove1
  40. In the Method Name combo box, select Click and implement the event as follows:
      
    Private Sub btnRemove1Click(ByVal sender As Object, 
                                 ByVal e As System.EventArgs) 
                                 Handles btnRemove1.Click
        txtPartNumber1.Text = ""
        txtPartName1.Text = ""
        txtUnitPrice1.Text = "0.00"
        txtQuantity1.Text = "0"
        txtSubTotal1.Text = "0.00"
        CalculateOrder()
End Sub
  41. In the Class Name combo box, select btnRemove2
  42. In the Method Name combo box, select Click and implement the event as follows:
      
    Private Sub btnRemove2Click(ByVal sender As Object, 
                                 ByVal e As System.EventArgs) 
                                 Handles btnRemove2.Click
        txtPartNumber2.Text = ""
        txtPartName2.Text = ""
        txtUnitPrice2.Text = "0.00"
        txtQuantity2.Text = "0"
        txtSubTotal2.Text = "0.00"
        CalculateOrder()
End Sub
  43. In the Class Name combo box, select btnRemove3
  44. In the Method Name combo box, select Click and implement the event as follows:
      
    Private Sub btnRemove3Click(ByVal sender As Object, 
                                 ByVal e As System.EventArgs) 
                                 Handles btnRemove3.Click
        txtPartNumber3.Text = ""
        txtPartName3.Text = ""
        txtUnitPrice3.Text = "0.00"
        txtQuantity3.Text = "0"
        txtSubTotal3.Text = "0.00"
        CalculateOrder()
End Sub
  45. In the Class Name combo box, select btnRemove4
  46. In the Method Name combo box, select Click and implement the event as follows:
      
    Private Sub btnRemove4Click(ByVal sender As Object, 
                                 ByVal e As System.EventArgs) 
                                 Handles btnRemove4.Click
        txtPartNumber4.Text = ""
        txtPartName4.Text = ""
        txtUnitPrice4.Text = "0.00"
        txtQuantity4.Text = "0"
        txtSubTotal4.Text = "0.00"
        CalculateOrder()
End Sub
  47. In the Class Name combo box, select btnRemove5
  48. In the Method Name combo box, select Click and implement the event as follows:
      
    Private Sub btnRemove5Click(ByVal sender As Object, 
                                 ByVal e As System.EventArgs) 
                                 Handles btnRemove5.Click
        txtPartNumber5.Text = ""
        txtPartName5.Text = ""
        txtUnitPrice5.Text = "0.00"
        txtQuantity5.Text = "0"
        txtSubTotal5.Text = "0.00"
        CalculateOrder()
End Sub
  49. In the Class Name combo box, select btnRemove6
  50. In the Method Name combo box, select Click and implement the event as follows:
      
    Private Sub btnRemove6Click(ByVal sender As Object, 
                                 ByVal e As System.EventArgs) 
                                 Handles btnRemove6.Click
        txtPartNumber6.Text = ""
        txtPartName6.Text = ""
        txtUnitPrice6.Text = "0.00"
        txtQuantity6.Text = "0"
        txtSubTotal6.Text = "0.00"
        CalculateOrder()
End Sub
  51. In the Class Name combo box, select btnClose
  52. In the Method Name combo box, select Click and implement the event as follows:
      
    Private Sub btnCloseClick(ByVal sender As Object, 
                               ByVal e As System.EventArgs) 
                               Handles btnClose.Click
        End
End Sub
  53. Execute the application to test it
     
    College Park Auto Parts - Form Result
  54. 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 Function FindString(s As String) As Integer

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 Function FindString(s As String, startIndex As Integer) As Integer

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 Function FindStringExact(s As String) As Integer

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 Function FindStringExact(s As String, startIndex As Integer) As Integer

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:
      
    Public Sub 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.FlatStyle = FlatStyle.Standard

    Controls.Add(cbxAcademicDisciplines)

End Sub
    Combo Box
  • 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
    Combo Box
  • 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:

Public Sub 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)

End Sub

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:

Public Sub 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)

        End Sub

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:

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:

Combo Box

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 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:
     
    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 Append

    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
     
    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
     

    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

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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