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

Question

How does a virtual event work? How would you override it? How would that work? And in what cases would you do that?

Would it for example be an ok replacement for pro

Answer 1

Also note that in C# a derived class is not able to fire the event purely defined in base class (no matter what modifier it has). So we either need have a new or overriding event for the derived class and in most cases an overriding one is preferred if the same event is meant to be fired.

Answer 2

For Foo to be raised there are 2 alternatives which I find missing from the accepted answer.

public class Base 
{
    public virtual event EventHandler Foo;

    public void Bar()
    {
        RaiseFoo();
    }

    protected virtual void RaiseFoo()
    {
        Foo?.Invoke(this, EventArgs.Empty);
    }
}

public class Derived : Base
{
    // In this case we use the base Foo as a backing store.
    public override event EventHandler Foo
    {
        add { base.Foo += value; }
        remove { base.Foo -= value; }
    }
}

public class Derived2 : Base
{
    public override event EventHandler Foo;

    // In this case we raise the overriden Foo.
    protected override void RaiseFoo()
    {
        Foo?.Invoke(this, EventArgs.Empty);
    }
}

class Test
{
    static void Main()
    {
        Base x = new Derived();
        x.Foo += (sender, e) => { };
        x.Bar();
        Base x2 = new Derived2();
        x2.Foo += (sender, e) => { };
        x2.Bar();
    }
}

Note that this doesn't work:

public class Derived3 : Base
{
    public override event EventHandler Foo
    {
        add { base.Foo += value; }
        remove { base.Foo -= value; }
    }

    protected override void RaiseFoo()
    {
        Foo?.Invoke(this, EventArgs.Empty);
    }
}

Answer 3

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.