ASP.NET

public class checkbox:CheckBox
{
    CheckBox chkbx;

	public checkbox()
	{
        chkbx = new CheckBox();
        chkbx.ID = "chk1";
        chkbx.Attributes["runat"] = "server";
        chkbx.AutoPostBack = true;
        
        chkbx.CheckedChanged += new EventHandler(myclick);
        Controls.Add(chkbx);
	}

    public void myclick(object sender, EventArgs e)
    {
        //if (sender is CheckBox)
        {
            //CheckBox cbox = sender as CheckBox;
            if (chkbx.Checked)
            {
                chkbx.Text = "checked";
            }
        }
    }
}

protected void Page_Load(object sender, EventArgs e)
    {
        checkbox cbx = new checkbox();
        form1.Controls.Add(cbx);
    }