What's the point of MethodImplOptions.InternalCall?

Question

Many methods in the BCL are marked with the [MethodImpl(MethodImplOptions.InternalCall)] attribute. This indicates that the \"method is implemented within the

Answer 1

The method is native, TRULY native, implemented into the Runtime itself. Don't forget, that CLR comes from C++ after all. It's like in compiler. In real world, CIL is not truly executed. It's JIT-ted and then executed, like a compiler, by a C/C++ runtime. Math.Pow is most probably [speculation] a call into the native C/C++ math.h pow method, and it's implementation-defined - now NET and Mono implement the CLR.

In UnityEngine.dll, [MethodImpl(MethodImplOptions.InternalCall)] is used on most native external methods. These C++ methods however use the Mono C++ CLR Library directly and they can co-operate with C# in way more intimate way than P/INVOKE itself. And way more performant (PInvoke hides some implementation details and makes some hard to understand)

However, the only way to use [MethodImpl(MethodImplOptions.InternalCall)] is if you are the CLR runtime itself - I only tested Mono like this, however. I don't know if it is even possible to alter the Microsoft's CLR Implementation, but with Mono you are free to abuse this feature.

Also, don't mess this with [MethodImpl(MethodImplOptions.Unmanaged)] - it's whole different story.

More about internal calls and how Mono works, here: http://www.mono-project.com/docs/advanced/embedding/

Disclaimer: I am NOT related to Unity or Mono!