A new update for .NET Framework is now available for the AppContext class. The behavior of the AppContext class was recently regressed. The update returns the class to the correct behavior. This update affects the .NET Framework 4.6 and later. The update is not required on Windows 10.
The AppContext class was introduced in the .NET Framework 4.6. It’s primary use is to enable developers to opt into new behavior in the .NET Framework that is not enabled by default. The regression prevents developers from opting into new behavior.
You are only encouraged to install this update if you have used the AppContext class directly and are experiencing problems. Otherwise, you will get this update in one of the regular broader releases over the next one to two months.
Getting the Update
This update is available via the Microsoft Update Catalog.
Downloading KBs from Microsoft Update Catalog
You can download patches from the table below. See .NET Framework Monthly Rollups Explained for an explanation on how to use this table to download patches from Microsoft Update Catalog.
Product Version Preview of Quality Rollup KB Windows 8.1
Windows RT 8.1
Windows Server 2012 R2Catalog
4038922.NET Framework 4.6, 4.6.1, 4.6.2, 4.7 4038922 Windows Server 2012 Catalog
4038921.NET Framework 4.6, 4.6.1, 4.6.2, 4.7 4038921 Windows 7
Windows Server 2008 R2 SP1Catalog
4038923.NET Framework 4.6, 4.6.1, 4.6.2, 4.7 4038923 Windows Server 2008 SP2 Catalog
4038923.NET Framework 4.6 4038923
Source: https://blogs.msdn.microsoft.com/dotnet/2017/08/18/net-framework-update-for-appcontext/