Gets or sets a value indicating whether the form will receive key events before the event is passed to the control that has focus.
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
When this property is set to true, the form will receive all KeyPress, KeyDown, and KeyUp events. After the form's event handlers have completed processing the keystroke, the keystroke is then assigned to the control with focus. For example, if the property is set to true and the currently selected control is a TextBox, after the keystroke is handled by the event handlers of the form the TextBox control will receive the key that was pressed. To handle keyboard events only at the form level and not allow controls to receive keyboard events, set the KeyPressEventArgs.Handled property in your form's KeyPress event handler to true.
You can use this property to process most keystrokes in your application and either handle the keystroke or call the appropriate control to handle the keystroke. For example, when an application uses function keys, you might want to process the keystrokes at the form level rather than writing code for each control that might receive keystroke events.
If a form has no visible or enabled controls, it automatically receives all keyboard events.
A control on a form may be programmed to cancel any keystrokes it receives. Since the control never sends these keystrokes to the form, the form will never see them regardless of the setting of .
The following code example demonstrates setting a form's property to true and handling the key events at the form level. To run the example, paste the following code in a blank form.
Imports System.Windows.Forms Imports System.Security.Permissions Public Class Form1 Inherits System.Windows.Forms.Form ' Declare the controls contained on the form. Private WithEvents button1 As MyMnemonicButton Friend WithEvents ListBox1 As System.Windows.Forms.ListBox Public Sub New() MyBase.New() ' Set KeyPreview object to true to allow the form to process ' the key before the control with focus processes it. Me.KeyPreview = True ' Add a MyMnemonicButton. button1 = New MyMnemonicButton button1.Text = "&Click" button1.Location = New System.Drawing.Point(100, 120) Me.Controls.Add(button1) ' Initialize a ListBox control and the form itself. Me.ListBox1 = New System.Windows.Forms.ListBox Me.SuspendLayout() Me.ListBox1.Location = New System.Drawing.Point(8, 8) Me.ListBox1.Name = "ListBox1" Me.ListBox1.Size = New System.Drawing.Size(120, 95) Me.ListBox1.TabIndex = 0 Me.ListBox1.Text = "Press a key" Me.ClientSize = New System.Drawing.Size(292, 266) Me.Controls.Add(Me.ListBox1) Me.Name = "Form1" Me.Text = "Form1" Me.ResumeLayout(False) End Sub ' The form will handle all key events before the control with ' focus handles them. Show the keys pressed by adding the ' KeyCode object to ListBox1. Ensure the processing is passed ' to the control with focus by setting the KeyEventArg.Handled ' property to false. Private Sub Form1_KeyDown(ByVal sender As Object, _ ByVal e As KeyEventArgs) Handles MyBase.KeyDown ListBox1.Items.Add(e.KeyCode) e.Handled = False End Sub <System.STAThreadAttribute()> Public Shared Sub Main() Application.Run(New Form1) End Sub End Class ' This button is a simple extension of the button class that overrides ' the ProcessMnemonic method. If the mnemonic is correctly entered, ' the message box will appear and the click event will be raised. Public Class MyMnemonicButton Inherits Button ' This method makes sure the control is selectable and the ' mneumonic is correct before displaying the message box ' and triggering the click event. <System.Security.Permissions.UIPermission( _ System.Security.Permissions.SecurityAction.Demand, Window:=UIPermissionWindow.AllWindows)> _ Protected Overrides Function ProcessMnemonic( _ ByVal inputChar As Char) As Boolean If (CanSelect And IsMnemonic(inputChar, Me.Text)) Then MessageBox.Show("You've raised the click event " _ & "using the mnemonic.") Me.PerformClick() Return True End If Return False End Function End Class
Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2