C# Dynamic Keyword — Run-time penalty?

Question

Does defining an instance as dynamic in C# mean:

1. The compiler does not perform compile-time type checking, but run-time checking takes place like it always

Answer 1

As far as I know, the answer is 3.

You can do this:

dynamic x = GetMysteriousObject();
x.DoLaundry();

Since the compiler does no type checking on x, it will compile this code, the assumption being that you know what you're doing.

But this means extra run-time checking has to occur: namely, examining x's type, seeing if it has a DoLaundry method accepting no arguments, and executing it.

In other words the above code is sort of like doing this (I'm not saying it's the same, just drawing a comparison):

object x = GetMysteriousObject();

MethodInfo doLaundry = x.GetType().GetMethod(
    "DoLaundry",
    BindingFlags.Instance | BindingFlags.Public
);

doLaundry.Invoke(x, null);

This is definitely not trivial, though that isn't to say you're going to be able to see a performance issue with your naked eye.

I believe the implementation of dynamic involves some pretty sweet behind-the-scenes caching that gets done for you, so that if you run this code again and x is the same type, it'll run a lot faster.

Don't hold me to that, though. I don't have all that much experience with dynamic; this is merely how I understand it to work.