using System;
using System.Linq;

namespace ConsoleApplication1
{
    class Vector<T> { public T[] Data { get; set; } }

    class Program
    {
        static object dotproduct<T, U>(Vector<T> t, Vector<U> u)
        {
            if (t.Data.Length != u.Data.Length)
                throw new Exception("error");

            return Enumerable.Range(0, u.Data.Length)
                .Select(x => Some_Common_Library.Mul(t.Data[x], u.Data[x]))
                .Aggregate(Some_Common_Library.Add);
        }

        static void Main(string[] args)
        {
            var v1 = new Vector<short> { Data = new short[] { 1, 2, 3 } };
            var v2 = new Vector<char> { Data = new[] { '1', '2', '3' } };

            var res = dotproduct(v1, v2);

            if (res.GetType() != typeof(int))
                throw new Exception("error");

            Console.WriteLine(res);
            Console.ReadLine();
        }
    }

    static class Some_Common_Library
    {
        public static object Add<T, U>(T t, U u)
        {
            if ((t.GetType() == typeof(int)) && (u.GetType() == typeof(int)))
                return (int)((int)(object)t + (int)(object)u);

            // ... more cases ...

            throw new Exception("error");
        }

        public static object Mul<T, U>(T t, U u)
        {
            if ((t.GetType() == typeof(short)) && (u.GetType() == typeof(char)))
                return (int)((short)(object)t * (char)(object)u);

            // ... more cases ...

            throw new Exception("error");
        }
    }
}

ARK>            if ((t.GetType() == typeof(short)) && (u.GetType() == typeof(char)))
ARK>                return (int)((short)(object)t * (char)(object)u);
ARK>

    if ((t.GetType() == typeof(short)) && (u.GetType() == typeof(char)))
        return (short)(object)t * (char)(object)u;