How to trap keystrokes in .NET controls by using Visual Basic .NET or Visual Basic 2005

For a Microsoft Visual C# .NET version of this article, see
320584 .

IN THIS TASK

Summary

This step-by-step article demonstrates how to trap keystrokes in Windows Forms controls. By using the sample code in this article, you can intercept almost any individual keystroke. You also can intercept key combinations, including CTRL and ALT. The Print Screen key is not affected by this technique. Additionally, some keystrokes from keyboards with additional keys, such as keys that control a Web browser or a CD-ROM player, might not be captured.

For most purposes, the standard KeyUp, KeyDown, and KeyPress events are enough to capture and handle keystrokes. However, not all controls raise these events for all keystrokes under all conditions.

For example, consider the DataGrid control: If no data has been assigned to the grid, the arrow keys (LEFT ARROW, RIGHT ARROW, UP ARROW, and DOWN ARROW) raise only the KeyUp event. Other keys, such as A or 4, raise all three events. If the
DataGrid is currently displaying data, none of the standard keyboard events are raised for the navigation keys. Keystrokes such as A or 4 raise no events, raise only KeyUp, or raise all three events, depending on what is currently selected in the control. In these situations, you can follow the steps in this article to capture keystrokes, regardless of the state of the control.


The code samples in this article are written to work with the DataGrid, because this is the control for which this feature is most frequently requested. You can use this same approach with other .NET controls.

Set Up the Key Trap

To trap keystrokes in a Windows Forms control, you derive a new class that is based on the class of the control that you want. You override the
ProcessCmdKey method. In this overridden method, you will place the code to process the keystrokes that you want to trap. The following sample code is an example of the basic structure for such a class:
Class MyDataGrid
Inherits DataGrid

Protected Overrides Function ProcessCmdKey( _
ByRef msg As Message, _
ByVal keyData As Keys _
) As Boolean

End Function
End Class

Implement the Overridden Method

The system passes two parameters to the ProcessCmdKey method: msg and keyData. The msg parameter contains the Windows Message, such as WM_KEYDOWN. The keyData parameter contains the key code of the key that was pressed. If CTRL or ALT was also pressed, the keyData parameter contains the ModifierKey information.

Using the msg parameter is not mandatory; you can ignore it. It is good practice, however, to test the message. In this example, you test WM_KEYDOWN to verify that this is a keystroke event. You also test WM_SYSKEYDOWN, so that it is possible to catch keystroke combinations that involve control keys (primarily ALT and CTRL).

To trap specific keys, you can evaluate the keyCode by comparing it to the Keys enumeration. The following code sample demonstrates how to catch the keystrokes UP ARROW, DOWN ARROW, TAB, CTRL+M, and ALT+Z:
Const WM_KEYDOWN As Integer = &H100
Const WM_SYSKEYDOWN As Integer = &H104

If ((msg.Msg = WM_KEYDOWN) Or (msg.Msg = WM_SYSKEYDOWN)) Then
Select Case (keyData)
Case Keys.Down
Console.WriteLine("Down Arrow Captured")
Case Keys.Up
Console.WriteLine("Up Arrow Captured")
Case Keys.Tab
Console.WriteLine("Tab Key Captured")
Case (Keys.Control Or Keys.M)
Console.WriteLine("<CTRL> + m Captured")
Case (Keys.Alt Or Keys.Z)
Console.WriteLine("<ALT> + z Captured")
End Select
End If

Build an Example

The following example shows how to trap keystrokes with the DataGrid control.
  1. Create a new Windows Control Library project in Visual Basic .NET or in Visual Basic 2005.
  2. View the properties for the class UserControl1, and then change the name to
    MyDataGrid.
  3. View the code for the Control Library, and then change the following line of code
    Inherits System.Windows.Forms.UserControl
    to the following:
    Inherits System.Windows.Forms.DataGrid
  4. Add the following method to the MyDataGrid class:
    Protected Overrides Function ProcessCmdKey(ByRef msg As Message, _
    ByVal keyData As Keys) As Boolean
    Const WM_KEYDOWN As Integer = &H100
    Const WM_SYSKEYDOWN As Integer = &H104

    If ((msg.Msg = WM_KEYDOWN) Or (msg.Msg = WM_SYSKEYDOWN)) Then
    Select Case (keyData)
    Case Keys.Down
    Me.Parent.Text = "Down Arrow Captured"
    Case Keys.Up
    Me.Parent.Text = "Up Arrow Captured"
    Case Keys.Tab
    Me.Parent.Text = "Tab Key Captured"
    Case (Keys.Control Or Keys.M)
    Me.Parent.Text = "<CTRL> + M Captured"
    Case (Keys.Alt Or Keys.Z)
    Me.Parent.Text = "<ALT> + Z Captured"
    End Select
    End If

    Return MyBase.ProcessCmdKey(msg, keyData)
    End Function
  5. Build the project.
  6. Create a new Windows Application project in Visual Basic .NET or in Visual Basic 2005. By default, Form1 is created.

    Note You must change the code in Visual Basic 2005. By default, Visual Basic creates two files for the project when you create a Windows Forms project. If the form is named Form1, the two files that represent the form are named Form1.vb and Form1.Designer.vb. You write the code in the Form1.vb file. The Windows Forms Designer writes the code in the Form1.Designer.vb file. The Windows Forms Designer uses the partial keyword to divide the implementation of Form1 into two separate files. This behavior prevents the designer-generated code from being interspersed with your code.

    For more information about the new Visual Basic 2005 language enhancements, visit the following Microsoft Developer Network (MSDN) Web site: For more information about partial classes and the Windows Forms Designer, visit the following MSDN Web site:
  7. On the Tools menu, click Customize Toolbox.
  8. Click the .NET Framework Components tab.
  9. Click Browse, find the control/DLL that was just created, and then click OK.
  10. The control MyDataGrid now appears in the toolbox. Place one on Form1.NOTE: You can use the code in the remaining steps to create sample data for the grid to display.

  11. Add the following code to the namespace of the Form. You can place the code either before or after the Form Class definition.
    ' Structure is to provide sample data for the example.
    Public Structure gridData
    Private mmake As String
    Private myear As Integer

    Public Sub New(ByVal n As String, ByVal y As Integer)
    mmake = n
    myear = y
    End Sub

    Public Property Make() As String
    Get
    Return mmake
    End Get

    Set(ByVal Value As String)
    Make = Value
    End Set
    End Property

    Public Property Year() As Integer
    Get
    Return myear
    End Get
    Set(ByVal Value As Integer)
    myear = Value
    End Set
    End Property
    End Structure
  12. Add the following code to the form class immediately following the "Windows Form Designer generated code" section:
    protected dataArray(5) As gridData
  13. Add the following code to the Load event of Form1:
    ' Create some sample data.
    dataArray(0) = New gridData("ford", 1999)
    dataArray(1) = New gridData("chevrolet", 1999)
    dataArray(2) = New gridData("plymouth", 1988)
    dataArray(3) = New gridData("honda", 1999)
    dataArray(4) = New gridData("fiat", 1987)

    ' Assign the data to the grid.
    MyDataGrid1.DataSource = dataArray
  14. Run the sample, and try the various keystrokes that are being trapped (UP ARROW, DOWN ARROW, TAB, CTRL+M, and ALT+Z). The caption of the form is updated to show which keystroke was pressed.
Properties

Article ID: 320583 - Last Review: Oct 30, 2008 - Revision: 1

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