using System;
using System.Reflection.Emit;

namespace MSILCalc
{
    class Program
    {
        private static readonly Type ReturnType = typeof(int);
        private static readonly Type[] ParameterTypes = { typeof(int), typeof(int), typeof(int), typeof(int) };

        private static Func<int, int, int, int, int> Parse(string equation)
        {
            DynamicMethod dynamicMethod = new DynamicMethod("", ReturnType, ParameterTypes);
            ILGenerator ilGenerator = dynamicMethod.GetILGenerator();

            foreach (char c in equation)
            {
                switch (c)
                {
                    case 'a':
                        ilGenerator.Emit(OpCodes.Ldarg_0);
                        break;
                    case 'b':
                        ilGenerator.Emit(OpCodes.Ldarg_1);
                        break;
                    case 'c':
                        ilGenerator.Emit(OpCodes.Ldarg_2);
                        break;
                    case 'd':
                        ilGenerator.Emit(OpCodes.Ldarg_3);
                        break;
                    case '+':
                        ilGenerator.Emit(OpCodes.Add);
                        break;
                    case '*':
                        ilGenerator.Emit(OpCodes.Mul);
                        break;
                }
            }

            ilGenerator.Emit(OpCodes.Ret);

            Delegate compiled = dynamicMethod.CreateDelegate(typeof(Func<int, int, int, int, int>));
            return (Func<int, int, int, int, int>)compiled;
        }

        static void Main(string[] args)
        {
            Func<int, int, int, int, int> intMethod = Parse("abcd+*+");
            int result1 = intMethod(1, 2, 3, 4);
            int result2 = intMethod(4, 3, 2, 1);
        }
    }
}