I want to create multidimensional Array with different random numbers

I want to create multidimensional Array with Random r = new Random(); Each element should be generated randomly but there must be all different elements. Non of them should be same. But also i can's use any function. Here is array but there are same elements. I want to all the elements were different.

   for (int i=0; i<3; i++)
        {
            for(int j=0; j<4; j++)
            {
                arr[i, j] = rand.Next(1,25);
            }
        }