## 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](http://stackoverflow.com/questions/17988219/system-windows-forms-messagebox-in-netversion-4-0-4-5-behaves-differently-in-wi)

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    ### Notes
    Scan XAML for PreviewLostKeyboardFocus use and then examine the handlers
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