C#: What are virtual events and how can they be used?

A virtual event is simply one which can be overridden in a derived class.

Are you happy with the concept of a virtual property, with a getter and setter which can be overridden? If so, you can think of a virtual event in exactly the same way: instead of a getter and setter, there's an "add" operation and a "remove" operation. These can be virtual, so handled polymorphically. You implement them the same way you implement any other virtual/overridden member.

Example:

using System;

class Base
{
    public virtual event EventHandler Foo
    {
        add
        {
            Console.WriteLine("Base Foo.add called");
        }
        remove
        {
            Console.WriteLine("Base Foo.remove called");
        }
    }
}

class Derived : Base
{
    public override event EventHandler Foo
    {
        add
        {
            Console.WriteLine("Derived Foo.add called");
        }
        remove
        {
            Console.WriteLine("Derived Foo.remove called");
        }
    }
}

class Test
{
    static void Main()
    {
        Base x = new Derived();

        x.Foo += (sender, args) => {};
    }
}

Note that the event itself is not responsible for what happens when it is raised - just the add/remove side. (In C#, anyway; the CLR itself has the notion of raising, but we'll ignore that for the moment.)

You may also want to read my article on events if you're slightly hazy on the difference between an event and a delegate.

Personally I find it very rare that I want a virtual event.