Form.KeyPreview Property
Definition
Important
Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.
Gets or sets a value indicating whether the form will receive key events before the event is passed to the control that has focus.
public:
property bool KeyPreview { bool get(); void set(bool value); };public bool KeyPreview { get; set; }member this.KeyPreview : bool with get, setPublic Property KeyPreview As BooleanProperty Value
true if the form will receive all key events; false if the currently selected control on the form receives key events. The default is false.
Examples
The following example demonstrates setting a form's KeyPreview property to true and handling the key events at the form level. To run the example, paste the following code in a blank form.
using namespace System::Windows::Forms;
// 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.
// This method makes sure the control is selectable and the
// mnemonic is correct before displaying the message box
// and triggering the click event.
public ref class MyMnemonicButton: public Button
{
protected:
bool ProcessMnemonic( char inputChar )
{
if ( CanSelect && IsMnemonic( inputChar, this->Text ) )
{
MessageBox::Show( "You've raised the click event "
"using the mnemonic." );
this->PerformClick();
return true;
}
return false;
}
};
// Declare the controls contained on the form.
public ref class Form1: public System::Windows::Forms::Form
{
private:
MyMnemonicButton^ button1;
public private:
System::Windows::Forms::ListBox^ ListBox1;
public:
Form1()
: Form()
{
// Set KeyPreview object to true to allow the form to process
// the key before the control with focus processes it.
this->KeyPreview = true;
// Add a MyMnemonicButton.
button1 = gcnew MyMnemonicButton;
button1->Text = "&Click";
button1->Location = System::Drawing::Point( 100, 120 );
this->Controls->Add( button1 );
// Initialize a ListBox control and the form itself.
this->ListBox1 = gcnew System::Windows::Forms::ListBox;
this->SuspendLayout();
this->ListBox1->Location = System::Drawing::Point( 8, 8 );
this->ListBox1->Name = "ListBox1";
this->ListBox1->Size = System::Drawing::Size( 120, 95 );
this->ListBox1->TabIndex = 0;
this->ListBox1->Text = "Press a key";
this->ClientSize = System::Drawing::Size( 292, 266 );
this->Controls->Add( this->ListBox1 );
this->Name = "Form1";
this->Text = "Form1";
this->ResumeLayout( false );
// Associate the event-handling method with the
// KeyDown event.
this->KeyDown += gcnew KeyEventHandler( this, &Form1::Form1_KeyDown );
}
private:
// 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.
void Form1_KeyDown( Object^ /*sender*/, KeyEventArgs^ e )
{
ListBox1->Items->Add( e->KeyCode );
e->Handled = false;
}
};
[System::STAThreadAttribute]
int main()
{
Application::Run( gcnew Form1 );
}
using System.Windows.Forms;
public class Form1 :
System.Windows.Forms.Form
// Declare the controls contained on the form.
{
private MyMnemonicButton button1;
internal System.Windows.Forms.ListBox ListBox1;
public Form1() : base()
{
// Set KeyPreview object to true to allow the form to process
// the key before the control with focus processes it.
this.KeyPreview = true;
// Add a MyMnemonicButton.
button1 = new MyMnemonicButton();
button1.Text = "&Click";
button1.Location = new System.Drawing.Point(100, 120);
this.Controls.Add(button1);
// Initialize a ListBox control and the form itself.
this.ListBox1 = new System.Windows.Forms.ListBox();
this.SuspendLayout();
this.ListBox1.Location = new System.Drawing.Point(8, 8);
this.ListBox1.Name = "ListBox1";
this.ListBox1.Size = new System.Drawing.Size(120, 95);
this.ListBox1.TabIndex = 0;
this.ListBox1.Text = "Press a key";
this.ClientSize = new System.Drawing.Size(292, 266);
this.Controls.Add(this.ListBox1);
this.Name = "Form1";
this.Text = "Form1";
this.ResumeLayout(false);
// Associate the event-handling method with the
// KeyDown event.
this.KeyDown += new KeyEventHandler(Form1_KeyDown);
}
// 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 void Form1_KeyDown(object sender, KeyEventArgs e)
{
ListBox1.Items.Add(e.KeyCode);
e.Handled = false;
}
[System.STAThreadAttribute]
public static void Main()
{
Application.Run(new Form1());
}
}
// 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 : Button
{
// This method makes sure the control is selectable and the
// mneumonic is correct before displaying the message box
// and triggering the click event.
protected override bool ProcessMnemonic(char inputChar)
{
if (CanSelect && IsMnemonic(inputChar, this.Text))
{
MessageBox.Show("You've raised the click event " +
"using the mnemonic.");
this.PerformClick();
return true;
}
return false;
}
}
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
Remarks
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 KeyPreview 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.
Note
If a form has no visible or enabled controls, it automatically receives all keyboard events.
Note
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 KeyPreview.
