using System;
using System.Reflection;
using System.Reflection.Emit;
using System.Text;

namespace EmitSample
{
	class Program
	{
		static void Main(string[] args)
		{
			IRun run = CreateClass.Create();
			string result = run.Run();
			Console.WriteLine("Result: {0}", result);

			Console.ReadKey();
		}
	}
	
	class CreateClass
	{
		private static string _filename = "dynamicAssem.dll";

		public static IRun Create()
		{
			AssemblyBuilder assemblyBuilder =
				AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName("dynamicAssem"), 
				AssemblyBuilderAccess.RunAndSave);

			ModuleBuilder moduleBuilder = assemblyBuilder.DefineDynamicModule("dynamicModule", _filename, true);

			TypeBuilder dynClassBuilder = moduleBuilder.DefineType("DynamicType", TypeAttributes.Public | TypeAttributes.Class);

			dynClassBuilder.AddInterfaceImplementation(typeof (IRun));

			MethodBuilder runBuilder = dynClassBuilder.DefineMethod(
				"Run", MethodAttributes.Public | MethodAttributes.Virtual, typeof(string), new Type[0]);

			//HelloWorld(runBuilder);
			MoreComplexMethod(runBuilder);

			MethodInfo runMethodInfo = typeof (IRun).GetMethod("Run");
			dynClassBuilder.DefineMethodOverride(runBuilder, runMethodInfo);

			Type t = dynClassBuilder.CreateType();

			assemblyBuilder.Save(_filename);
			return (IRun)Activator.CreateInstance(t);
		}

		private static void HelloWorld(MethodBuilder runBuilder)
		{
			ILGenerator il = runBuilder.GetILGenerator();

			il.EmitWriteLine("Hello World.");
			il.Emit(OpCodes.Ldnull);
			il.Emit(OpCodes.Ret);
		}

		public static void MoreComplexMethod(MethodBuilder runBuilder)
		{
			ILGenerator il = runBuilder.GetILGenerator();

			Label endOfMethod = il.DefineLabel();
			Type stringBuilderType = typeof (StringBuilder);
			il.DeclareLocal(stringBuilderType);
			il.DeclareLocal(typeof(string));
			
			il.Emit(OpCodes.Newobj, stringBuilderType.GetConstructor(new Type[0]));
			il.Emit(OpCodes.Stloc_0);
			il.Emit(OpCodes.Ldloc_0);
			il.Emit(OpCodes.Ldstr, "Hello World (complex)");
			il.Emit(OpCodes.Call, stringBuilderType.GetMethod("Append", new Type[] {typeof(string)}));
			il.Emit(OpCodes.Call, stringBuilderType.GetMethod("ToString", new Type[0]));
			il.Emit(OpCodes.Stloc_1);
			il.Emit(OpCodes.Br_S, endOfMethod);
			il.MarkLabel(endOfMethod);
			il.Emit(OpCodes.Ldloc_1);
			il.Emit(OpCodes.Ret);
		}
	}

	public interface IRun
	{
		string Run();
	}
}