internal class Base {
    public event EventHandler<CustomEventArgs> TestEvent;
    protected void OnTestEvent() {
        if(TestEvent != null) {
            TestEvent(this, EventArgs.Empty);
        }
    }
    public void TestMethod() {
        OnTestEvent();
    }
}
internal class Derived : Base {
}
Main() {
    Derived d = new Derived();
    d.TestEvent += (s, e) => Console.WriteLine("Что-то делаем");
    d.OnTestEvent();
}