dotnet/Documentation/compatibility/previewlostkeyboardfocus-is-called-repeatedly-if-its-handler-shows-a-message-box.md

PreviewLostKeyboardFocus is called repeatedly if its handler shows a Windows Forms message box

Scope

Edge

Version Introduced

4.5

Source Analyzer Status

Available

Change Description

Beginning in the .NET Framework 4.5, calling System.Windows.Forms.MessageBox.Show from a UIElement.PreviewLostKeyboardFocus handler will cause the handler to re-fire when the message box is closed, potentially resulting in an infinite loop of message boxes.

• Quirked
• Build-time break

Recommended Action

There are two options to work around this issue -

1. It may be avoided by calling System.Windows.MessageBox.Show instead of System.Windows.Forms.MessageBox.Show.
2. It may be avoided by showing the message box from a LostKeyboardFocus event handler (as opposed to a PreviewLostKeyboardFocus event handler).

Affected APIs

• M:System.Windows.ContentElement.add_PreviewLostKeyboardFocus(System.Windows.Input.KeyboardFocusChangedEventHandler)
• M:System.Windows.IInputElement.add_PreviewLostKeyboardFocus(System.Windows.Input.KeyboardFocusChangedEventHandler)
• M:System.Windows.UIElement.add_PreviewLostKeyboardFocus(System.Windows.Input.KeyboardFocusChangedEventHandler)
• M:System.Windows.UIElement3D.add_PreviewLostKeyboardFocus(System.Windows.Input.KeyboardFocusChangedEventHandler)

Categories

Windows Forms Windows Presentation Foundation (WPF)

More information