ModuloPower

static int ModuloPower(int value, int exponent, int modulus)
{
    int res = 1;

    while (exponent > 0)
    {
        if ((exponent & 1) != 0)
        {
            res = (res * value) % modulus;
        }
        value = (value * value) % modulus;
        exponent >>= 1;

    }
    return res;
}

static void PrintCalcModulo()
{
    int a, x, p;
    Console.Write(" Enter number a: ");
    a = Convert.ToInt32(Console.ReadLine());
    Console.Write(" Enter number x: ");
    x = Convert.ToInt32(Console.ReadLine());
    Console.Write(" Enter number p: ");
    p = Convert.ToInt32(Console.ReadLine());

    Console.WriteLine("\n {0}^{1} mod {2} = {3}", a, x, p, ModuloPower(a, x, p));
}