// form designerthis.Load += Form1_Load;// Form1.csprivate void Form1_Load(object sender, EventArgs e){    MessageBox.Show("a");}--------------------------------------------------------if override OnLoad abstract method which is defined in Form class, call base.OnLoad(e) is required, otherwise, this.Load event cannot get called.// form designerthis.Load += Form1_Load;// Form1.csprivate void Form1_Load(object sender, EventArgs e){    MessageBox.Show("a");}protected override void OnLoad(EventArgs e){    base.OnLoad(e); // if this is commented, event cannot invoke.}