delegate double mathAction(double a, double b);

double Sum(double x, double y)
{
    return x + y;
}

double Mul(double x, double y)
{
    return x * y;
}

// in code
mathAction m1 = new mathAction(Sum);
double result1 = m1(3, 5); // result = Sum(3, 5)

mathAction m2 = new mathAction(Sum);
double result2 = m2(3, 5); // result = Mul(3, 5)