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

Release Date:
January 26, 2021

Version:
.NET Framework 3.5, 4.7.2 and 4.8

Summary

This article describes the Cumulative Update Preview for 3.5, 4.7.2 and 4.8 for Windows 10, version 1809 and Windows Server, version 2019.

Additional information about this update

The following articles contain additional information about this update as it relates to individual product versions.

4598303 Description of the Cumulative Update for .NET Framework 3.5 and 4.7.2 for Windows 10, version 1809 and Windows Server, version 2019 (KB4598303)
4598300 Description of the Cumulative Update for .NET Framework 3.5 and 4.8 for Windows 10, version 1809 and Windows Server, version 2019 (KB4598300)

Known issues in this update

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 " />

How to get this update

Install this update

Release Channel	Available	Next Step
Windows Update and Microsoft Update	Yes	To download and install this update, go to Settings > Update & Security > Windows Update, and then select Check for updates.
Microsoft Update Catalog	Yes	To get the standalone package for this update, go to the Microsoft Update Catalog website.
Windows Server Update Services (WSUS)	No	You can import this update into WSUS manually. See the Microsoft Update Catalog for instructions. Note: This update is available on WSUS via 4598461.