ILReader | interprete CIL-bytes of methods and delegates bodies

I had to massage your code quite a bit to get it to compile and run. Once I did this, it ran perfectly, so I'm not sure which part of my massaging fixed the problem you had. The changes I made:

1. When you define your tb (by calling DefineType), you need to make sure that it inherits from baseCrudController. Since you declare baseCrudController later, I'm not convinced you were doing this. But it's hard to say, because this code isn't in your question
2. In your call to tb.DefineMethod, strings in C# need double quotes, not single quotes
3. In that same call, you need typeof(Foo), new[] { typeof(int) } not typeof(Foo), typeof(int), as you need to pass an array of parameter types
4. You need to use typeof(HttpGetAttribute), not typeof(HttpGet)
5. HttpGetAttribute has multiple constructors, and you can't rely on the order that they're returned in. Explicitly specify that you want the parameterless constructor
6. You don't need the nops, and you don't need the stloc/ldloc after the call. Those are only added in Debug builds to help the debugger
7. In fact, since you haven't declared a local slot 0, the stloc.0 and ldloc.0 are invalid. When you're using System.Reflection.Emit, if you need to store a value into a local slot, use a LocalBuilder and don't explicitly address the slots by number.
8. When you call the base method, its name is "DoGet" and not "GetInBase"

Make all of those changes and you get this, which works: