There are 3 static methods on TypeBuilder to get an instantiated generic MemberInfo given a generic definition:

https://docs.microsoft.com/en-us/dotnet/api/system.reflection.emit.typebuilder.getconstructor
https://docs.microsoft.com/en-us/dotnet/api/system.reflection.emit.typebuilder.getfield
https://docs.microsoft.com/en-us/dotnet/api/system.reflection.emit.typebuilder.getmethod

The issue is, the first 2 method (GetConstructor and GetField) have their checks in the wrong order, resulting in dead code. For example:

The first check says "if the type isn't a TypeBuilderInstantiation, throw". Then the next check is checking if the type is a TypeBuilder. Both TypeBuilder and TypeBuilderInstantiation are sealed types that inherit from TypeInfo. So if the type isn't a TypeBuilderInstantiation, an exception is thrown, and the check for type is TypeBuilder is dead code.

I thought I could just remove the dead code from these two methods, but looking at the last one: TypeBuilder::GetMethod:

This has the checks flipped, which makes more sense. If the type passed in IsGenericTypeDefinition (which can only be true if it is a TypeBuilder), then it calls MakeGenericType, which will return a TypeBuilderInstantiation.

My assumption is the GetConstructor and GetField methods were supposed to be written this way, but it was a mistake. I looked back in internal source history, and it appears all 3 of these methods were written this way when the feature was first developed.

If the code is truly not necessary, we should just delete the dead code. But I think the checks should be flipped to match the GetMethod method.

Note: we should also add unit tests for this scenario when fixing this bug.