In rare scenarios, you might need to detect unexpected input. For example, consider the following scenarios.

• During a mouse operation, the user opens the Start menu by pressing the Windows key or CTRL+ESC.

• During a mouse operation, the user switches to another program by pressing ALT+TAB.

• During a mouse operation, another program displays a window or a message box that takes focus away from the current application.

Mouse operations can include clicking and holding the mouse on a form or a control, or performing a mouse drag operation. If you have to detect when a form or a control loses mouse capture for these and related unexpected scenarios, you can use the MouseCaptureChanged event.

The following code example demonstrates the MouseCaptureChanged event for a Button control.

Private Sub Button1_MouseDown(ByVal sender As System.Object, _
ByVal e As MouseEventArgs) Handles Button1.MouseDown
    Debug.WriteLine("Button1_MouseDown")
End Sub

Private Sub Button1_MouseUp(ByVal sender As System.Object, _
ByVal e As MouseEventArgs) Handles Button1.MouseUp
    Debug.WriteLine("Button1_MouseUp")
End Sub

Private Sub Button1_MouseCaptureChanged(ByVal sender As System.Object, _
ByVal e As EventArgs) Handles Button1.MouseCaptureChanged
    Debug.WriteLine("Button1_MouseCaptureChanged")
End Sub

private void button1_MouseDown(object sender, MouseEventArgs e)
{
    System.Diagnostics.Debug.WriteLine("button1_MouseDown");
}

private void button1_MouseUp(object sender, MouseEventArgs e)
{
    System.Diagnostics.Debug.WriteLine("button1_MouseUp");
}

private void button1_MouseCaptureChanged(object sender, EventArgs e)
{
    System.Diagnostics.Debug.WriteLine("button1_MouseCaptureChanged");
}

To test this example, run it in the debugger by pressing F5. Open the Output window in Visual Studio so that you can see when events are raised. Click the Button and notice the following output.

button1_MouseDown

button1_MouseUp

button1_MouseCaptureChanged

Now, click and hold the left mouse button on the Button control. While still clicking the mouse, press ALT+TAB to switch to another program. Notice that the MouseCaptureChanged event is raised enabling you to potentially handle this scenario. Depending on your actions, the MouseUp event might not be raised. You can also try this test with the Windows key or CTRL+ESC.

button1_MouseDown

button1_MouseCaptureChanged

.NET Framework

.NET Framework Client Profile

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

Reference