All the methods that are called by events require two arguments:
object sender
EventArgs e

The event calls a method using those 2 parameters, so we can’t directly add a custom argument.

If we have this code and we want to display the string ‘s’ in a MessageBox…

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private void Form1_Load (object sender, EventArgs e)
{
      string s = "Hello!";
      button.Click += new EventHandler(show_msg);  // our string is not included in the call
}

private void show_msg(object sender, EventArgs e, string s) 
// this gives an error, because the function is called with only 2 arguments, but it expects 3
{
      MessageBox.Show(s);
}

Clearly this will not work because we are limited to those two parameters.

The Solution

The easiest solution is to use a delegate to call our method.

We get the shortest code using the lambda operator, whose symbol is =>.

  • It is recommended for beginners because of its simplicity.
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private void Form1_Load (object sender, EventArgs e)
{
      string s = "Hello!";
      button.Click += (sender2, e2) => show_msg(sender2, e2, s);
}

private void show_msg (object sender, EventArgs e, string s)
{
      MessageBox.Show(s);
}

Without using the lambda operator, it can be rewritten using a delegate

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private void Form1_Load (object sender, EventArgs e)
{
       string s = "Hello!";

       button.Click += delegate(object sender2, EventArgs e2)
       {
             show_msg(sender2, e2, s);
       };
}

private void show_msg(object sender, EventArgs e, string s)
{
      MessageBox.Show(s);
}

That’s all :).