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

I'm playing around with the C# reflection API. I can easily load Type information of classes, methods etc. in an assembly, however, now I wonder how can I load and read the code inside a method?

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.

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

Code Type	Can get with Reflection
The source code, i.e. the original C# or VB.NET	No
The symbolic IL code	No
The JITed assembly code	No
The IL bytes, i.e. the actual bytes that IL is compiled to	Yes

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?

I'd like to provide an example of how one could explore the code inside a method. As others have explained, this can't be done easily using the native .NET Reflection API. However, using the Mono.Reflection API, you can disassemble the code programmatically using the GetInstructions() method and inspect it at runtime.

For example, the following code inspects a method and computes the number of calls inside it. As a use case for such a code, say I am a teacher (which I am) and instruct my fellow students to program a given method without using any other method, then using this code in unit tests I can verify that the given constrain is respected.

public static class MethodInfoUtil
{
    public static int NbOfInnerCalls(this MethodInfo mi)
    {
        return mi.GetInstructions().Count(
            instruction => instruction.OpCode.FlowControl == FlowControl.Call);
    }
}

Example Console program:

class Program
{
    static int Add(int a, int b) => a + b;
    static int Doubling(int a) => Add(a, a);
    static int Quadrupling(int a) => Add(Add(a, a), Add(a, a));

    static void Main(string[] args)
    {
        Console.WriteLine("Inner method calls");
        Console.WriteLine("        Add: {0}", ((Func<int, int, int>)Add).Method.NbOfInnerCalls());
        Console.WriteLine("   Doubling: {0}", ((Func<int, int>)Doubling).Method.NbOfInnerCalls());
        Console.WriteLine("Quadrupling: {0}", ((Func<int, int>)Quadrupling).Method.NbOfInnerCalls());
    }
}

// Output:
// Inner method calls
//         Add: 0
//    Doubling: 1
// Quadrupling: 3

No
This is a feature slated for the next version of C#. You can use the CodeDom to get more info than reflection, but you cannot interrogate the parse tree yet.

Well there is always mono, in mono the compiler is a service, and you could get the parse trees at runtime.

The better question is why you want to?

Yes, there must be a way to achieve this: The .NET Reflector tool does this, too. Can't tell you how it's done there, though.