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..

Answer 1

The compiler is already telling you what is wrong - but to quote section 7.3.2 of the C# 5 specification:

User-defined operator declarations always require at least one of the parameters to be of the class or struct type that contains the operator declaration. Thus, it is not possible for a user-defined operator to have the same signature as a predefined operator.

In other words, it would be fine to have:

class Foo
{
    public static int[,] operator *(int[,] arr1, Foo arr2)
    {
        ...
    }
}

... because then the second operand is the declaring type. But in your case both of the operands are int[,].

What you could do is add an extension method instead:

public static class ArrayExtensions
{
    public static int[,] Times(this int[,] arr1, int[,] arr2)
    {
        ...
    }
}

Then you could have:

int[,] x = ...;
int[,] y = ...;
int[,] z = x.Times(y);

Another solution - a better one, IMO, would be to declare your own Matrix class to encapsulate matrix operations. Then you can define your own multiplication operator:

public static Matrix operator *(Matrix left, Matrix right)

... and you're not interfering with int[,] arrays which aren't intended to be treated as matrices.