public class SearchTextBox
  : TextBox
{

  public SearchTextBox() : base() { }

  /// <summary>
  /// Run search if enter is pressed while in the textbox.
  /// </summary>
  protected override bool ProcessCmdKey( ref Message msg, Keys keyData )
  {
    //-- You could also define other keys, such as the tab key here.
    if (keyData == Keys.Enter)
    {
      //-- Run the search
      if (this.SearchFunction != null)
      {
        int count = this.SearchFunction( this.Text );
        MessageBox.Show( string.Format( "{0} items found" );
      }
      else
      {
        //-- Notify develper that they haven't setup a search function.
        MessageBox.Show( string.Format( "No search function defined. Search term is '{0}'", this.Text ) );
      }
    }

    //-- Whatever key is used to run the search 
    //   should also just do whatever it normally does.
    return base.ProcessCmdKey( ref msg, keyData );
  }

  /// <summary>
  /// Define the function that runs the search. This
  /// function accepts a string search term and return
  /// number of items found. You could also use a 
  /// normal delegate, so you don't have to have a return
  /// value.  
  /// </summary>
  public Func<string, int> SearchFunction { get; set; }

  /// <summary>
  /// Override to always return a string (and never null).
  /// </summary>
  public override string Text
  {
    get
    {
      string text = string.Empty;
      if (!string.IsNullOrEmpty( base.Text ))
      {
        text = base.Text;
      }
      return text;
    }
    set
    {
      base.Text = value;
    }
  }
}