one of the parameters of a binary operator must be the containing type c#

Question

public static int[,] operator *(int[,] arr1, int[,] arr2)
    {
        int sum;
        int[,] res = new int[arr1.GetLength(0), arr2.GetLength(1)];
        for (int i = 0; i < arr1.GetLength(0); i++)
        {
            for (int j = 0; j < arr2.GetLength(1); j++)
            {
                sum = 0;
                for (int k = 0; k < arr1.GetLength(1); k++)
                {
                    sum = sum + (arr1[i, k] * arr2[k, j]);
                }
                res[i, j] = sum;
                //Console.Write("{0} ", res[i, j]);
            }
            //Console.WriteLine();
        }

        return res;
    }

Here i am trying to overload * operator to multiply two matrices.. but the compiler keeps on showing me the error that..

" one of the parameters of a binary operator must be the containing type c#"

please tell me what is the problem in my code and how to solve it..