Delegates vs. Function Pointers, Addendum: Multicast Delegates

Until now, I've been focusing on only one of the differences between delegates and function pointers; namely, associated state.
Delegates have one other capability that function pointers do not.  A single function pointer can only point to one function.  .Net, on the other hand, supports multicast delegates – delegates that point to multiple functions.  You can combine two existing delegates using the + operator (or by calling Delegate.Combine) to create a single new delegate instance that points two all of the methods in the original two delegates.  This new delegate stores all of the methods from the original two delegates in a private array of delegates called InvocationList (the delegates in this array are ordinary non-multicast delegates that each only point to a single method). 

Note that delegates, like strings, are immutable.  Adding two delegates together creates a third delegate containing the methods from the first two; the original delegate instances are not affected.  For example, writing delegateField += SomeMethod creates a new delegate instance containing the methods originally in delegateField as well as SomeMethod, then stores this new instance in delegateField.

Similarly, the - operator (or Delegate.Remove) will remove the second operand from the first one (again, returning a new delegate instance).  If the second operand has multiple methods, all of them will be removed from the final delegate.  If some of the methods in the second operand appear multiple times in the original delegate, only the last occurrence of each one will be removed (the one most recently added).  The RemoveAll method will remove all occurrences.  If all of the methods were removed, it will return null; there is no such thing as an empty delegate instance.

Multicast delegates are not intended to be used with delegates that return values.  If you call a non-void delegate that contains multiple methods, it will return the return value of the last method in the delegate.  If you want to see the return values of all of the methods, you’ll need to loop over GetInvocationList() and call each delegate individually.

Multicast delegates also don’t play well with the new covariant and contravariant generic delegates in .Net 4.0.  You cannot combine two delegates unless their types match exactly, including variant generic parameters.

Function pointers cannot easily be combined the way multicast delegates can.  The only way to combine function pointers without cooperation from the code that calls the pointer is to make a function that uses a closure to call all of the function pointers you want to call.

In Javascript, that would look like this:

function combine() {
    var methods = arguments;

    return function() { 
        var retVal;
        for(var i = 0; i < methods.length; i++) 
            retVal = methods[i].apply(this, arguments);
        return retVal;
    };
}

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Great post! I really enjoyed reading the discussion on delegates versus function pointers. It's fascinating how delegates provide a higher level of abstraction and flexibility compared to function pointers.
However, I wanted to add something regarding multicast delegates. Multicast delegates are a powerful extension of delegates that allow multiple methods to be invoked with a single delegate instance. This comes in handy when you need to broadcast an event to multiple subscribers or execute a set of functions in a specific order.
One significant advantage of using multicast delegates is their ability to simplify event handling. By adding multiple event handlers to a multicast delegate, you can trigger all subscribed methods at once, making event management more efficient and ensuring consistent behavior across different subscribers.
Overall, multicast delegates offer a convenient way to handle scenarios where multiple methods need to be invoked simultaneously. They are certainly a valuable addition to the delegate functionality and can enhance the flexibility and power of your code. Keep up the great work!

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