What does plus equals(+=) operator means here? [duplicate]

Question

I was working out sample code of Windows phone and often I see statements with += operator.

I know about add assignment operator which does the below operation

+= means a = a + b;  // used for both adding number and string concatenation

But this is new to me

phNumChoseTask = new PhoneNumberChooserTask();
phNumChoseTask.Completed += new EventHandler<PhoneNumberResult>(phoneNumberChooserTask_Completed);

Here how does += works?

Answer 1

In the current context += means subscribe. In other words it's like you are telling subscribe my method (the right operand) to this event (the left operand), this way, when the event is raised, your method will be called. Also, it is a good practice to unsubscribe (-= from this event, when you have finished your work ( but before you dispose you object ) in order to prevent your method being called and to prevent resource leaks. FMI look here.

Answer 2

The += operator is used to specify a method that will be called in response to an event; such methods are called event handlers. The use of the += operator in this context is referred to as subscribing to an event.

Other usage, It can also be used as assignment operator:

a=a+b;

can be written as

 a+=b;