Overriding method is invoked by event

// Program.cs
using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var instance = new DerivedClass();
            instance.Init();
            instance.DoSomething(); // outputs "DerivedClass"
        }
    }

    class EventSource
    {
        public event EventHandler SomeEvent;

        public void DoSomething()
        {
            if (this.SomeEvent != null) this.SomeEvent(this, EventArgs.Empty);
        }
    }

    class BaseClass
    {
        private readonly EventSource _source = new EventSource();

        public void Init()
        {
            _source.SomeEvent += _source_SomeEvent;
        }

        protected virtual void _source_SomeEvent(object sender, EventArgs e)
        {
            Console.WriteLine("BaseClass");
        }

        public void DoSomething()
        {
            _source.DoSomething();
        }
    }

    class DerivedClass : BaseClass
    {
        protected override void _source_SomeEvent(object sender, EventArgs e)
        {
            Console.WriteLine("DerivedClass");
        }
    }
}