protected virtual void OnClick(object sender, EventArgs e)

Protected Overridable Sub OnClick(sender As Object, e As EventArgs)

Raising an event invokes the event handler through a delegate.

The OnClick method also allows derived classes to handle the event without attaching a delegate. This is the preferred technique for handling the event in a derived class.

Notes to Inheritors:

When overriding OnClick in a derived class, be sure to call the base class's OnClick method so that registered delegates receive the event.