public static class TestExtM
    {
        public static int M1(this string str)
        {
            return str.Length;
        }
        public static int M2(string str)
        {
            return str.Length;
        }

        public static IEnumerable<Type> M3()
        {
            var data = Assembly.GetCallingAssembly()
                .GetTypes()
                .Where(t => t.IsAbstract && t.IsSealed)
                .SelectMany(x => x.GetMethods())
                .Where(x => x.IsDefined(typeof(System.Runtime.CompilerServices.ExtensionAttribute)))
                .Select(x => x.GetParameters().First())
                .Select(x => x.ParameterType)
                .Distinct();
            return data;
        }
    }

 public interface IAdd<T> 
    {
        T Value { get; set; }
        T Add(T x);
    }
    public class IntAdd : IAdd<int>
    {
        public int Value { get; set; }
        public int Add(int x)
        {
            return Value + x;
        }

    }
    public class DoubleAdd : IAdd<double>
    {
        public double Value { get; set; }

        public double Add(double x)
        {
            return Value + x;
        }
    }
    public static class MulClass
    {
        public static T Mul<T>(this IAdd<T> add, int t)
        {
            T res = default(T);
            for (int i = 0; i < t; i++)
            {
                res = add.Add(res);
            }
            return res;
        }
    }

 IntAdd intAdd = new IntAdd { Value = 10 };
            var tmp1 = intAdd.Mul(10);
            Console.WriteLine(tmp1);
            DoubleAdd doubleAdd = new DoubleAdd { Value = 10 };
            var tmp2 = doubleAdd.Mul(10);
            Console.WriteLine(tmp2);