.net - What is [DllImport("QCall")]?

Question

Many methods in the .Net library are implemented in native code. Those that come from the framework itself are marked with [MethodImpl(MethodImplOptions.InternalCall)]. Those that come from some unmanaged DLL are marked with [DllImport] (e.g. [DllImport("kernel32.dll")]). So far nothing unusual.

But while writing answer for another question, I discovered there are many methods marked with [DllImport("QCall")]. They seem to be internal implementation of .Net (e.g. GC._Collect()).

My question is: What exactly does [DllImport("QCall")] mean? What is the difference between [DllImport("QCall")] and [MethodImpl(MethodImplOptions.InternalCall)]?

Answer 1

I asked some people in the .Net team about this.

QCalls are calls to native methods within the CLR runtime. They behave like other [DllImport]s, but they're faster because they make specific (undocumented) assumptions about what the native methods do, so they can skip various marshalling and GC and exception checks.

InternalCall is different; it's for calls for special reflection-style things which are generated at runtime (this wasn't very clear).