Question

Here is the sum for a diagonal top-left to bottom-right:

public int sumarDiagonal()
{
    int x = 0;
    for (int F = 0; F < Filas; F++)
    {
        for (int c = 0; c < Columnas; c++)
        {
            if (F == c)
            {
                x += m[F,c];
            }
        }
    }
    return x;
}

How can I go from top-right to bottom-left?

Answer 1

Your original code with two nested loops is not very efficient.
Better do it like this:

public int sumarDiagonal() 
{ 
 int x = 0; 
 for (int i = 0; i < Math.Min(Filas,Columnas); ++i) 
  x += m[i,i]; 
 return x; 
} 

public int sumarAntiDiagonal() 
{ 
 int x = 0; 
 for (int i = 0; i < Math.Min(Filas,Columnas); ++i) 
  x += m[Filas - 1 - i,i]; 
 return x; 
}