Add some helper method to help instantiate generic types and methods

[lovettchris]

Turns out instantiating a generic type or method is non-trivial if you want the newly written assembly to actually work properly. I found these helper methods useful, perhaps others would also. I wasn't sure where the best place to test these helpers would be, do you have any "read/modify/write" assembly tests ? That's the scenario where I found these new helpers to be useful. Actually, we'd want tests that "read/modify/write/execute" to verify the updated assembly loads and runs properly. For example, suppose my assembly contains this generic class:

    class Foo<T>
    {
        public static void Bar<U>(T x, U y)
        {
            Console.WriteLine("Hey this is even more fun: {0}, {1}", x, y);
        }
    }

and i want to read the assembly and add a call to Bar, then write it back out. I can now do this:

var fooType = module.GetType("HelloWorld.Foo`1");
var barMethod = (from m in fooType.Methods where m.Name == "Bar" select m).FirstOrDefault();
var intType = module.ImportReference(typeof(int));

// instantiate Foo<int>.Bar<int>(int x, int y)
var instance = module.ImportGenericMethodInstance(barMethod, intType, intType);
    
// now add call to the method with two constant integer values 10 and 20
// in a test method somewhere:
MethodDefinition testMethod = ... 
var processor = testMethod.Body.GetILProcessor();
var loadint = Instruction.Create(OpCodes.Ldc_I4, 10);
processor.InsertBefore(testMethod.Body.Instructions[0], loadint);
var loadsecondInt = Instruction.Create(OpCodes.Ldc_I4, 20);
processor.InsertAfter(loadint, loadsecondInt);
var callBar = Instruction.Create(OpCodes.Call, instance);
processor.InsertAfter(loadsecondInt, callBar);

// now I can write out the modified assembly and it works.