Question

why doesn’t the element get swapped

public static void SwapArray(int[,] arr)
    {
        for (int i = 0; i < arr.GetLength(0); i++)
        {
            for (int j = 0; j < arr.GetLength(0); j++)
            {
                int temp = arr[i, j];
                arr[i, j] = arr[j, i];
                arr[j, i] = temp;
            }
        }
    }

even if the parameter is without a ref modifier the array doesn’t change.
a copy of the reference is passed as a parameter right?

Answer 1

There is an error in your algorithm. For every i and j, your loop swaps arr[i,j] and arr[j,i] twice.

For example arr[3,1] gets swapped with arr[1,3] once for i=3, j=1 and once for i=1, j=3. So the result is the original matrix. You should change the j-loop to

for (int j = 0; j < i; j++)