Factorial mediante una Función

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace numero19
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

private void button2_Click(object sender, EventArgs e)
{
Application.Exit();
}
//Variables
int numero, f, i;

private void textBox1_TextChanged(object sender, EventArgs e)
{
numero = int.Parse(textBox1.Text);
}
//Proceso

private void button1_Click(object sender, EventArgs e)
{
f = Factorial(numero);
textBox2.Text = f.ToString();
}




private static int Factorial (int num)
{
//Variables
int i, f;
//Proceso
f = 1;
for (i = 1; i <=num; i++)
f *= i;
//Salida
return f;
}
}
}