Why is event null not handled by delegate_问答_开发者

When calling a delegate you always have to check if it is not null. This is an often cause for errors. Since delegates are more or less just a list of functions, I would assume, that this could have been easily checked by the delegate itself.

Does anybody 开发者_JAVA百科know, why it has been implemented as it is?

This may be stating the obvious, but you can declare your event to point to a no-op handler and then you don't need to check for null when invoking.

public event EventHandler MyEvent = delegate { };

Then you can call the handlers pointed to by MyEvent without checking for null.

The delegate itself can't check it. Since it is null you can't call methods on it.

The C# compiler on the other hand could automatically insert the null-check. But I don't know what behavior to use if the delegate is a function with a return-value.

One might argue that the C# compiler should insert that check for void-functions to avoid boilerplate code in that common case. But that decision is now in the past, you can only fix it without breaking existing programs if you get a time-machine.

You can use extension methods(since they can be called on a null object) to automate the null-check for events:
http://blogs.microsoft.co.il/blogs/shayf/archive/2009/05/25/a-handy-extension-method-raise-events-safely.aspx
And since a parameter is a temp variable you don't need to manually assign the event to a temp variable for thread-safety.

Internally, the compiler will generate 2 methods add_MyEvent and remove_MyEvent for each event declared in a class. When you write, MyEvent += ..., the compiler will actually generate a call to add_MyEvent which in turn will call System.Delegate.Combine.

Combine takes 2 delegates in parameter and creates a new (multicast) delegate from the 2 original delegates, handling the case when one of them is null (which is the case the first time you call +=).

I guess the compiler could have been a bit smarter and also handled the event call so that when you call MyEvent(), it would actually generate a null check and actually invoke the delegate only if not null. (It would be nice to have Eric Lippert's view on that one).

The reason is performance (actually, that's my best guess). Events and delegates are made with a lot of compiler magic, stuff that cannot be replicated by mere C# code. But underneath it goes something like this:

A delegate is a compiler-generated class that inherits from MulticastDelegate class, which itself derives from the Delegate class. Note these two classes are magic and you can't inherit from them yourself (well, maybe you can, but you won't be able to use them very well).

An event however is implemented something like this:

private MyEventDelegateClass __cgbe_MyEvent; // Compiler generated backing field

public event MyEventDelegateClass MyEvent
{
    add
    {
        this.__cgbe_MyEvent = (MyEventDelegateClass)Delegate.Combine(this.__cgbe_MyEvent, value);
    }
    remove
    {
        this.__cgbe_MyEvent = (MyEventDelegateClass)Delegate.Remove(this.__cgbe_MyEvent, value);
    }
    get
    {
        return this.__cgbe_MyEvent;
    }
}

OK, so this isn't real code (nor is it exactly the way it is in real life), but it should give you an idea of what's going on.

The point is that the backing field initially really is null. That way there is no overhead for creating an instance of MyEventDelegateClass when creating your object. And that's why you have to check for null before invoking. Later on, when handlers get added/removed, an instance of MyEventDelegateClass is created and assigned to the field. And when the last handler is removed, this instance is also lost and the backing field is reset to null again.

This is the principle of "you don't pay for what you don't use". As long as you don't use the event, there will be no overhead for it. No extra memory, no extra CPU cycles.

I'd guess the reasons are history and consistency.

It is consistent with the way other types are handled; e.g. there's no language or platform assistence in dealing with nulls - it's the programmers responsibility to ensure the null placeholder is never actually used - there's no means of only optionally calling a method if the object reference you wish to call it on is non-null either, after all.

It's history, since null references happen to be included by default in most types, even though this isn't necessary. That is, rather than treating reference types like value types and require an additional "nullability" annotation to permit nullability, reference types are always nullable. I'm sure there were reasons for this back in the day when Java and .NET were designed, but it introduces many unnecessary bugs and complexities which are easy to avoid in a strongly typed language (e.g., .NET value types). But given the historical inclusion of null as a type-system wide "invalid" value, so to speak, it's only natural to do so for delegates as well.

If a delegate instance is null, how can it "check itself"? It's null... that means it does not exist, basically. There is nothing there to check itself in the first place. That's why your code which attempts to call the delegate must do that checking, first.