Make optional SafeHandle parameters accept null

[AArnott]

While the .NET interop layer may not permit null for SafeHandle parameters, we can certainly support it by making the extern method private and exposing a wrapper to it that checks for null SafeHandle args and does the right thing.

Example:

[DllImport(Entrypoint = "CreateFileW")]
private static extern SafeFileHandle CreateFile_Private(char* path, [Optional, In] SafeHandle hTemplate);

internal static SafeFileHandle CreateFile(char* path, SafeHandle? hTemplate)
{
   return CreateFile_Private(path, hTemplate ?? NullSafeHandle.NullHandle);
}

[jnm2]

When the project langversion is >= 8, could you also put #nullable enable or at least #nullable annotations in the file or around the parameter declaration and use SafeHandle? as the parameter type so that IntelliSense shows me that nulls are anticipated?

[AArnott]

@jnm2 yes, I have that in mind to eventually do. I filed an issue to track.