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 xref:System.Windows.Forms.MessageBox.Show%2A?displayProperty=nameWithType from a xref:System.Windows.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 xref:System.Windows.MessageBox.Show%2A?displayProperty=nameWithType instead of xref:System.Windows.Forms.MessageBox.Show%2A?displayProperty=nameWithType.
2. It may be avoided by showing the message box from a xref:System.Windows.UIElement.LostKeyboardFocus event handler (as opposed to a xref:System.Windows.UIElement.PreviewLostKeyboardFocus?displayProperty=name 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