Using an unassigned local variable to create an anonymous function that closes to itself

Question

Why is this declaration + assignment causing an error:

// Use of unassigned local variable 'handler'. SessionEndingEventHandler handler = (sender, e) => { isShuttingDown = true; SystemEvents.SessionEnding -= handler; }; 

while this is not so:

 SessionEndingEventHandler handler = null; handler = (sender, e) => { isShuttingDown = true; SystemEvents.SessionEnding -= handler; }; 

In it is intuitive that the first statement should cause an error, but not immediately understand why the second does not.

Also, how can I determine if the SystemEvents.SessionEnding event was SystemEvents.SessionEnding canceled after calling handler(null, null) ? GetInvocationList only works with delegates.

 SystemEvents.SessionEnding += handler; handler(null, null); 

Answer 1

This is for the same reason that you expect this to happen:

 int i = 1 - i; 

The right part of the operator is calculated before the assignment and at the time of its calculation the variable has not yet been assigned.

If you think lambdas / delegates are changing things, consider the following statement:

 int i = ((Action)() => 1 - i)(); 

Since you create a lambda before assigning i , it is possible that i can be used before any value is assigned to it. The fact that you do not expect this to happen in your case will not change the situation from the point of view of the compiler - you need to explicitly assign a value to the variable before using it. If this is a null value, then at least the compiler knows that you are considering the possibility that it will be null when you receive it.

As for your last question, then SessionEndingEventHandler is a delegate. So this will work fine:

 var unsubscribed = SystemEvents.SessionEnding == null || !SystemEvents.SessionEnding.GetInvocationList().Contains(handler);