January 26, 2021-KB4598461 Cumulative Update Preview for .NET Framework 3.5, 4.7.2 and 4.8 for Windows 10, version 1809 and Windows Server, version 2019

Symptom

After installing this update, WPF apps may crash with a callstack similar to

Exception Info: System.NullReferenceException at System.Windows.Interop.HwndMouseInputProvider.HasCustomChrome(System.Windows.Interop.HwndSource, RECT ByRef) at System.Windows.Interop.HwndMouseInputProvider.GetEffectiveClientRect(IntPtr)
at System.Windows.Interop.HwndMouseInputProvider.PossiblyDeactivate(IntPtr, Boolean)
at System.Windows.Interop.HwndMouseInputProvider.Dispose()

This occurs when disposing an HwndSource whose RootVisual is null, a situation that arises in Visual Studio when docking or splitting windows, and could arise in other apps.

Workaround

To work around this problem, set two AppContext switches using one of the methods described in AppContext Class (System) under the heading “AppContext for library consumers”.  The switches are named Switch.System.Windows.Interop.MouseInput.OptOutOfMoveToChromedWindowFix and Switch.System.Windows.Interop.MouseInput.DoNotOptOutOfMoveToChromedWindowFix and both should be set to “true”.   The first switch avoids the crash, but re-introduces the bug fixed in the KBs.  The second switch is currently ignored, but will be recognized in a future .NET update that contains a fix for the null-reference crash;  it restores the original bug fix.

For example, using the app.config file method to apply the workaround at application scope:

<AppContextSwitchOverrides value="Switch.System.Windows.Interop.MouseInput.OptOutOfMoveToChromedWindowFix=true; Switch.System.Windows.Interop.MouseInput.DoNotOptOutOfMoveToChromedWindowFix=true " />