Question

When I write an event handler for csharp, it looks like this:

public void FooHandler(object sender, EventArgs e)
{
    //do stuff..
    this.doSomething();  //Does the "this" keyword mean something in this context?
}

Does the “this” keyword mean something in this context?

EDIT:

Let’s say I also have this code:

public class GizmoManager {
    public void Manage() {
        g = new Gizmo();
        g.Foo += new EventHandler(FooHandler);
    }
}

What would the this (within FooHandler) refer to?

Answer 1

Yes, it’s a reference to object for which FooHandler() is called. Delegates are capable of referencing both static and non-static methods. When talking about non-static ones, this is a reference to object instance.

class A
{
   public delegate void MyDelegate(object sender, int x);
   public event MyDelegate TheEvent;

   public void func()
   {
     if(TheEvent != null) TheEvent(this, 123);
   }
}

class B
{
   public B()
   {
     A a = new A();
     a.TheEvent += handler;
     a.func();
   }

   public void handler(object sender, int x)
   {
      // "sender" is a reference to object of type A that we've created in ctor
      // "x" is 123
      // "this" is a reference to B (b below)
   } 
}

B b = new B(); // here it starts

Some more details. Your code:

g = new Gizmo();
g.Foo += new EventHandler(FooHandler);

could be re-written like this

g = new Gizmo();
g.Foo += new EventHandler(this.FooHandler); // look here

In this case this is the same this that you have in your handler 😉

And even more, if you have some problems with understanding this:

class X
{
  int a;

  public X(int b)
  {
    this.a = b; // this stands for "this object"
    // a = b is absolutely the same
  }

  public X getItsThis()
  {
    return this;
  }
}

X x = new X();
X x2 = x.getItsThis();
// x and x2 refer to THE SAME object
// there's still only one object of class X, but 2 references: x and x2