Use closure for event handler.

This is a sample program.
A number displayed on a label increases when a button is clicked.

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

       this.button1.Click += new EventHandler(button1_Click);
   }

   int num = 0;

   void button1_Click(object sender, EventArgs e)
   {
       this.label1.Text = num.ToString();
       num++;
   }
}

This code uses a member field "num".


I rewrite this code using closure.

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

       this.button1.Click += make_button1_Click(0);
   }

   EventHandler make_button1_Click(int num)
   {
       return delegate(object sender, EventArgs e)
       {
           this.label1.Text = num.ToString();
           num++;
       };
   }
}

This code uses no member field.
Closure memorize the variable "num".