private void btnCalculae_Click(object sender, System.EventArgs e)
        {
            long number = Convert.ToInt64 (txtNumber.Text);
            long factorial = 1;
               lblFactorial.Text = factorial.ToString("n20");
 
            // calculate factorial
 
            while ( number > 0 && number <= 20)
            {
                factorial *= number;
                number++;
            } // end while loop
 
            txtNumber.Text = "";
            txtNumber.Focus();
        }
 
        private void btnExit_Click(object sender, System.EventArgs e)
        {
           this.Close();
        }
    }
}

public int fact(int n)
{
    if (n==0) return 1;
    else return n*fact(n-1)
}
 
Console.WriteLine("Type a number up to 20:");
int n = Convert.ToInt32(Console.ReadLine());
Console.WriteLine(this.fact(n));