Can I use reflection to inspect the code in a method?

That depends on what you mean by "read the code." There are 4 forms of the code.

Take a look at MethodBase.GetMethodBody() for the last one. You can get the IL bytes, the local variables, exception frames etc.

You sort of can. The relevant function is MethodBase.GetMethodBody.

It's not exactly the most useful API. You can get some basic information about what's inside the method, and you can obtain the IL as a byte array. That's about it.

There's a slightly better API in the Mono.Cecil library, which exposes a MethodDefinition class with its own MethodBody implementation which contains actual Instructions, so you don't have to interpret the raw byte code. Still, if you're looking to get C# code out of it à la Reflector, you're going to be sorely disappointed. Also, Cecil isn't very well documented.

If you still want to try, then good luck.

If you don't need to do this real-time, have a look at Reflector. You can disassemble any .NET assembly (including the MS core DLLs) and see the code in your language of choice. This can be very educational.

Update Has anyone tried using Reflector on Reflector to figure out how this is done?

Basic Answer:

You can't with the reflection API (System.Reflection).

The reason is that the reflection api is designed to work on Metadata (Type of Classes, Name and Signature of Methods, ...) but not on the data level (which would be the IL-stream itself).

Extended Answer:

You can emit (but not read) IL with System.Reflection.Emit (e.g. ILGenerator Class).

Through MethodInfo.GetMethodBody() you can get the binary IL-stream for the implementation of a method. But thats usually completely useless by itself.

There are external libraries (like Cecil) that you can use to read/modify/add/delete code inside a method.