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();
}

Par o Impar mediante procedimiento

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 numero18
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
//Variables
int numero;
string resultado = "";

private void button2_Click(object sender, EventArgs e)
{
Application.Exit();
}
//Entrada
private void textBox1_TextChanged(object sender, EventArgs e)
{
numero = int.Parse(textBox1.Text);
}
//Proceso

Función para eliminar espacios en blanco

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 numero15
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

//Variables
string textoentrada, textosalida;

private void textBox1_TextChanged(object sender, EventArgs e)
{
textoentrada = textBox1.Text;
}


//Proceso
private void button1_Click(object sender, EventArgs e)
{
textosalida = textosinespacios(textoentrada);
textBox2.Text = textosalida.ToString();
}

Sumar 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 numero13
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
//Variables
int n1, n2, s;
private void textBox1_TextChanged(object sender, EventArgs e)
{
n1 = int.Parse(textBox1.Text);
}

private void textBox2_TextChanged(object sender, EventArgs e)
{
n2 = int.Parse(textBox2.Text);
}

private void button1_Click(object sender, EventArgs e)
{
//Proceso
s = Sumar(n1, n2);
textBox3.Text = s.ToString();
}