Thread.CurrentPrincipal throws PlatformNotSupportedException on .NET Core 3.x

[bording]

We're currently setting a WindowsPrincipal principal policy during endpoint startup, even when we are running on non-Windows OSes.

This is relatively harmless when using .NET Core 2.x because the current principal, for example via Thread.CurrentPrincipal, will be null when running on non-Windows OSes.

However, when using .NET Core 3.x, this presents a bigger problem. Accessing the principal when it is a WindowsPrincipal on a non-Windows OS instead throws a PlatformNotSupportedException.

The way to fix this with the least impact would to call AppDomain.CurrentDomain.SetPrincipalPolicy(PrincipalPolicy.WindowsPrincipal) only when we are running on Windows since this will maintain the .NET Core 2.x behavior of getting a null value from things like Thread.CurrentPrincipal.

Who's affected

Anyone using .NET Core 3.x on macOS or Linux and accesses Thread.CurrentPrincipal in their code.

Symptoms

After starting the endpoint, getting the value of Thread.CurrentPrincipal will throw a PlatformNotSupportedException.

Backported to

• 7.2.3
• 7.1.2