44 lines
1.6 KiB
Markdown
44 lines
1.6 KiB
Markdown
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## PreviewLostKeyboardFocus is called repeatedly if its handler shows a Windows Forms message box
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### Scope
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Edge
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### Version Introduced
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4.5
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### Source Analyzer Status
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Available
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### Change Description
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Beginning in the .NET Framework 4.5, calling `System.Windows.Forms.MessageBox.Show` from a `UIElement.PreviewLostKeyboardFocus`
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handler will cause the handler to re-fire when the message box is closed, potentially resulting in an infinite loop of
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message boxes.
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- [ ] Quirked
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- [ ] Build-time break
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### Recommended Action
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There are two options to work around this issue -
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1. It may be avoided by calling `System.Windows.MessageBox.Show` instead of `System.Windows.Forms.MessageBox.Show`.
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2. It may be avoided by showing the message box from a `LostKeyboardFocus` event handler (as opposed to a `PreviewLostKeyboardFocus` event handler).
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### Affected APIs
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* `M:System.Windows.ContentElement.add_PreviewLostKeyboardFocus(System.Windows.Input.KeyboardFocusChangedEventHandler)`
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* `M:System.Windows.IInputElement.add_PreviewLostKeyboardFocus(System.Windows.Input.KeyboardFocusChangedEventHandler)`
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* `M:System.Windows.UIElement.add_PreviewLostKeyboardFocus(System.Windows.Input.KeyboardFocusChangedEventHandler)`
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* `M:System.Windows.UIElement3D.add_PreviewLostKeyboardFocus(System.Windows.Input.KeyboardFocusChangedEventHandler)`
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### Categories
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Windows Forms
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Windows Presentation Foundation (WPF)
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[More information](http://stackoverflow.com/questions/17988219/system-windows-forms-messagebox-in-netversion-4-0-4-5-behaves-differently-in-wi)
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<!--
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### Notes
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Scan XAML for PreviewLostKeyboardFocus use and then examine the handlers
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-->
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<!-- breaking change id: 131 -->
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