NativeWindow.ReleaseHandle Method ()
Releases the handle associated with this window.
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
This method does not destroy the window handle. Instead, it sets the handle's window procedure to the default window procedure. It sets the Handle property to 0 and calls OnHandleChange to reflect the change.
A window automatically calls this method if it receives a native Win32 WM_NCDESTROY message, indicating that Windows has destroyed the handle.
The following code example demonstrates intercepting operating system window messages in a window procedure. The example creates a class that inherits from NativeWindow to accomplish this.
The MyNativeWindowListener class hooks into the window procedure of the form passed into the constructor, and overrides the WndProc method to intercepts the WM_ACTIVATEAPP window message. The class demonstrates the use of the AssignHandle and methods to identify the window handle the NativeWindow will use. The handle is assign based upon the Control.HandleCreated and Control.HandleDestroyed events. When the WM_ACTIVATEAPP window message is received, the class calls the form1ApplicationActivated method.
' NativeWindow class to listen to operating system messages. Friend Class MyNativeWindowListener Inherits NativeWindow ' Constant value was found in the "windows.h" header file. Private Const WM_ACTIVATEAPP As Integer = &H1C Private parent As Form1 Public Sub New(ByVal parent As Form1) AddHandler parent.HandleCreated, AddressOf Me.OnHandleCreated AddHandler parent.HandleDestroyed, AddressOf Me.OnHandleDestroyed Me.parent = parent End Sub ' Listen for the control's window creation and hook into it. Private Sub OnHandleCreated(ByVal sender As Object, ByVal e As EventArgs) ' Window is now created, assign handle to NativeWindow. AssignHandle(CType(sender, Form).Handle) End Sub Private Sub OnHandleDestroyed(ByVal sender As Object, ByVal e As EventArgs) ' Window was destroyed, release hook. ReleaseHandle() End Sub <System.Security.Permissions.PermissionSetAttribute(System.Security.Permissions.SecurityAction.Demand, Name:="FullTrust")> _ Protected Overrides Sub WndProc(ByRef m As Message) ' Listen for operating system messages Select Case (m.Msg) Case WM_ACTIVATEAPP ' Notify the form that this message was received. ' Application is activated or deactivated, ' based upon the WParam parameter. parent.ApplicationActivated(m.WParam.ToInt32() <> 0) End Select MyBase.WndProc(m) End Sub End Class
Available since 1.1