Validating WinForms TextBox (in C#)

Question

In TextBox_Leave event i need to check whether numbers entered in textbox is in serial number or not.If it is not in order then i need to display a message as \"number\" is

Answer 1

Alternatively you can also use Validating event of the text box.

  private void textBox1_Validating( object sender, CancelEventArgs e )
  {
      if ( textBox1.Text == "3" )
          e.Cancel = true;
  }

The text-box wont loose focus until it receives a valid input.