ジェネリック型の加算は、難しい？

using System;
using System.Linq;

namespace Chapter01All
{
 
    class GenericDemo
    {
        static void Main()
        {     
            var testClass = new TestClass<int>();
            int a = 100; int b = 200;
            Console.WriteLine( testClass.AddWithDynamic(a, b));

            var testClass2 = new TestClass<double>();
            double c = 200.2; double d = 300.3;
            Console.WriteLine(testClass2.AddWithDynamic(c, d));
         }


    }
    public class GenericOperation<T>
    {
        public GenericOperation()
        {
            var availableT = new Type[]
            {
            typeof(int), typeof(uint), typeof(short), typeof(ushort), typeof(long), typeof(ulong), typeof(byte),
            typeof(decimal), typeof(double)
            };
            if (!availableT.Contains(typeof(T)))
            {
                throw new NotSupportedException();
            }
            /*
            var p1 = Expression.Parameter(typeof(T));
            var p2 = Expression.Parameter(typeof(T));
            Add = Expression.Lambda<Func<T, T, T>>(Expression.Add(p1, p2), p1, p2).Compile();
            Subtract = Expression.Lambda<Func<T, T, T>>(Expression.Subtract(p1, p2), p1, p2).Compile();
            Multiply = Expression.Lambda<Func<T, T, T>>(Expression.Multiply(p1, p2), p1, p2).Compile();
            Divide = Expression.Lambda<Func<T, T, T>>(Expression.Divide(p1, p2), p1, p2).Compile();
            Modulo = Expression.Lambda<Func<T, T, T>>(Expression.Modulo(p1, p2), p1, p2).Compile();
            Equal = Expression.Lambda<Func<T, T, bool>>(Expression.Equal(p1, p2), p1, p2).Compile();
            GreaterThan = Expression.Lambda<Func<T, T, bool>>(Expression.GreaterThan(p1, p2), p1, p2).Compile();
            GreaterThanOrEqual = Expression.Lambda<Func<T, T, bool>>(Expression.GreaterThanOrEqual(p1, p2), p1, p2).Compile();
            LessThan = Expression.Lambda<Func<T, T, bool>>(Expression.LessThan(p1, p2), p1, p2).Compile();
            LessThanOrEqual = Expression.Lambda<Func<T, T, bool>>(Expression.LessThanOrEqual(p1, p2), p1, p2).Compile();
            */
        }

        public Func<T, T, T> Add { get; private set; }
        public Func<T, T, T> Subtract { get; private set; }
        public Func<T, T, T> Multiply { get; private set; }
        public Func<T, T, T> Divide { get; private set; }
        public Func<T, T, T> Modulo { get; private set; }
        public Func<T, T, bool> Equal { get; private set; }
        public Func<T, T, bool> GreaterThan { get; private set; }
        public Func<T, T, bool> GreaterThanOrEqual { get; private set; }
        public Func<T, T, bool> LessThan { get; private set; }
        public Func<T, T, bool> LessThanOrEqual { get; private set; }
    }
    class TestClass<T>
    {
        private GenericOperation<T> go = new GenericOperation<T>();
        public T AddWithDynamic(T a, T b)
        {
            return (dynamic)a + (dynamic)b;
        }


        public T AddWithExpressionTree(T a, T b)
        {
            return go.Add(a, b);
        }

    }
}

　何故か？ネットから拾ったコードではExpressionがエラーになったので、コメントアウトしたんですが、一応動きました。何故か？難しので説明はできません。