What is the purpose of the delegates are immutable in c#?

Question

I was reading a book illustrated C # 2012 in the section Combining Delegates point not notice that? Purpose of the delegates are immutable.

Combining

Answer 1

Thread-safety and speed are the primary concerns here. A delegate update is not atomic, it requires updating 6 fields and a list. Making it atomic so it cannot get corrupted requires a lock, far too expensive for such a basic operation that rarely needs to be thread-safe.

By making it immutable, the delegate object cannot get corrupted since it always sets fields on an object that nobody has a reference to yet. Reassigning the delegate object reference is atomic, a basic .NET memory model guarantee. So no need for a lock anymore. The trade-off is less efficient memory use, it is a small penalty.

Do keep in mind that the thread-safe delegate update does not automatically make your code thread-safe as well. The test-and-fire code needs to copy the reference to avoid NRE and you can make a callback to a method that was already unsubscribed.

Answer 2

Delegates are pointers to a method, either concrete or anonymous (well, even anonymous methods are compiled with some compiler-generated identifier).

Would be reasonable that something that points to some concrete thing be mutable? I don't think so. Each instance represents a pointer to some method. Do you want to point to some other method? Create a new pointer. That is, you instantiate a new delegate.

In the other hand, the addition of two or more delegates has this result because + operator can be overloaded. That is, delegates can be part of an addition but internally +'s overload is creating a new delegate with an invocation list.