When does the loadoperation completed event fire?

Question

Class DomainContext has method Invoke which return instance of InvokeOperation and often we can see next code

InvokeOperation op = domainConextInstance.Invoke(...);
op.Completed +={...};

My first thought - it should not work: after all event can arise earlier than we will subscribe for it.

I made an experiment

InvokeOperation op = domainConextInstance.Invoke(...);
Thread.Sleep(5000); //or 25000
op.Completed +={...};

But I found that this code works correctly, But how? Can you explain this to me?

And what pattern does this construct use?

Answer 1

It's hard to know without seeing any of the code for DomainContext - but it sounds like the code which adds a handler for the Completed event calls the handler immediately if the operation has already completed.

Assuming you have the code for InvokeOperation , I'd definitely look at the declaration of the Completed event to discover the "magic".

Answer 2

Assuming, you are talking about WCF RIA Services SDK, Jon is right. InvokeOperation has a property IsComplete . The add part of the Completed event checks this property. In case of a completed operation it does not add the passed event handler but calls it immediately.

You can validate this by inspecting OperationBase (base class of InvokeOperation) in System.ServiceModel.DomainServices.Client.dll with a disassemler tool like dotPeek.