Why are delegates reference types?

Question

Quick note on the accepted answer: I disagree with a small part of Jeffrey's answer, namely the point that since Delegate had to be a reference type, it follows that all delegates are reference types. (It simply isn't true that a multi-level inheritance chain rules out value types; all enum types, for example, inherit from System.Enum, which in turn inherits from System.ValueType, which inherits from System.Object, all reference types.) However I think the fact that, fundamentally, all delegates in fact inherit not just from Delegate but from MulticastDelegate is the critical realization here. As Raymond points out in a comment to his answer, once you've committed to supporting multiple subscribers, there's really no point in not using a reference type for the delegate itself, given the need for an array somewhere.

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See update at bottom.

It has always seemed strange to me that if I do this:

Action foo = obj.Foo;

I am creating a new Action object, every time. I'm sure the cost is minimal, but it involves allocation of memory to later be garbage collected.

Given that delegates are inherently themselves immutable, I wonder why they couldn't be value types? Then a line of code like the one above would incur nothing more than a simple assignment to a memory address on the stack*.

Even considering anonymous functions, it seems (to me) this would work. Consider the following simple example.

Action foo = () => { obj.Foo(); };

In this case foo does constitute a closure, yes. And in many cases, I imagine this does require an actual reference type (such as when local variables are closed over and are modified within the closure). But in some cases, it shouldn't. For instance in the above case, it seems that a type to support the closure could look like this: I take back my original point about this. The below really does need to be a reference type (or: it doesn't need to be, but if it's a struct it's just going to get boxed anyway). So, disregard the below code example. I leave it only to provide context for answers the specfically mention it.

struct CompilerGenerated
{
    Obj obj;

    public CompilerGenerated(Obj obj)
    {
        this.obj = obj;
    }

    public void CallFoo()
    {
        obj.Foo();
    }
}

// ...elsewhere...

// This would not require any long-term memory allocation
// if Action were a value type, since CompilerGenerated
// is also a value type.
Action foo = new CompilerGenerated(obj).CallFoo;

Does this question make sense? As I see it, there are two possible explanations:

• Implementing delegates properly as value types would have required additional work/complexity, since support for things like closures that do modify values of local variables would have required compiler-generated reference types anyway.
• There are some other reasons why, under the hood, delegates simply can't be implemented as value types.

In the end, I'm not losing any sleep over this; it's just something I've been curious about for a little while.

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Update: In response to Ani's comment, I see why the CompilerGenerated type in my above example might as well be a reference type, since if a delegate is going to comprise a function pointer and an object pointer it'll need a reference type anyway (at least for anonymous functions using closures, since even if you introduced an additional generic type parameter—e.g., Action<TCaller>—this wouldn't cover types that can't be named!). However, all this does is kind of make me regret bringing the question of compiler-generated types for closures into the discussion at all! My main question is about delegates, i.e., the thing with the function pointer and the object pointer. It still seems to me that could be a value type.

In other words, even if this...

Action foo = () => { obj.Foo(); };

...requires the creation of one reference type object (to support the closure, and give the delegate something to reference), why does it require the creation of two (the closure-supporting object plus the Action delegate)?

_{*Yes, yes, implementation detail, I know! All I really mean is short-term memory storage.}

Answer 1

The question boils down to this: the CLI (Common Language Infrastructure) specification says that delegates are reference types. Why is this so?

One reason is clearly visible in the .NET Framework today. In the original design, there were two kinds of delegates: normal delegates and "multicast" delegates, which could have more than one target in their invocation list. The MulticastDelegate class inherits from Delegate. Since you can't inherit from a value type, Delegate had to be a reference type.

In the end, all actual delegates ended up being multicast delegates, but at that stage in the process, it was too late to merge the two classes. See this blog post about this exact topic:

We abandoned the distinction between Delegate and MulticastDelegate towards the end of V1. At that time, it would have been a massive change to merge the two classes so we didn’t do so. You should pretend that they are merged and that only MulticastDelegate exists.

In addition, delegates currently have 4-6 fields, all pointers. 16 bytes is usually considered the upper bound where saving memory still wins out over extra copying. A 64-bit MulticastDelegate takes up 48 bytes. Given this, and the fact that they were using inheritance suggests that a class was the natural choice.

Answer 2

There is only one reason that Delegate needs to be a class, but it's a big one: while a delegate could be small enough to allow efficient storage as a value type (8 bytes on 32-bit systems, or 16 bytes on 64-bit systems), there's no way it could be small enough to efficiently guarantee if one thread attempts to write a delegate while another thread attempts to execute it, the latter thread wouldn't end up either invoking the old method on the new target, or the new method on the old target. Allowing such a thing to occur would be a major security hole. Having delegates be reference types avoids this risk.

Actually, even better than having delegates be structure types would be having them be interfaces. Creating a closure requires creating two heap objects: a compiler-generated object to hold any closed-over variables, and a delegate to invoke the proper method on that object. If delegates were interfaces, the object which held the closed-over variables could itself be used as the delegate, with no other object required.

Answer 3

Imagine if delegates were value types.

public delegate void Notify();

void SignalTwice(Notify notify) { notify(); notify(); }

int counter = 0;
Notify handler = () => { counter++; }
SignalTwice(handler);
System.Console.WriteLine(counter); // what should this print?

Per your proposal, this would internally be converted to

struct CompilerGenerated
{
    int counter = 0;
    public Execute() { ++counter; }
};

Notify handler = new CompilerGenerated();
SignalTwice(handler);
System.Console.WriteLine(counter); // what should this print?

If delegate were a value type, then SignalEvent would get a copy of handler, which means that a brand new CompilerGenerated would be created (a copy of handler) and passed to SignalEvent. SignalTwice would execute the delegate twice, which increments the counter twice in the copy. And then SignalTwice returns, and the function prints 0, because the original was not modified.

Answer 4

Here's an uninformed guess:

If delegates were implemented as value-types, instances would be very expensive to copy around since a delegate-instance is relatively heavy. Perhaps MS felt it would be safer to design them as immutable reference types - copying machine-word sized references to instances are relatively cheap.

A delegate instance needs, at the very least:

• An object reference (the "this" reference for the wrapped method if it is an instance method).
• A pointer to the wrapped function.
• A reference to the object containing the multicast invocation list. Note that a delegate-type should support, by design, multicast using the same delegate type.

Let's assume that value-type delegates were implemented in a similar manner to the current reference-type implementation (this is perhaps somewhat unreasonable; a different design may well have been chosen to keep the size down) to illustrate. Using Reflector, here are the fields required in a delegate instance:

System.Delegate: _methodBase, _methodPtr, _methodPtrAux, _target
System.MulticastDelegate: _invocationCount, _invocationList

If implemented as a struct (no object header), these would add up to 24 bytes on x86 and 48 bytes on x64, which is massive for a struct.

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On another note, I want to ask how, in your proposed design, making the CompilerGenerated closure-type a struct helps in any way. Where would the created delegate's object pointer point to? Leaving the closure type instance on the stack without proper escape analysis would be extremely risky business.

Answer 5

I can tell that making delegates as reference types is definitely a bad design choice. They could be value types and still support multi-cast delegates.

Imagine that Delegate is a struct composed of, let's say: object target; pointer to the method

It can be a struct, right? The boxing will only occur if the target is a struct (but the delegate itself will not be boxed).

You may think it will not support MultiCastDelegate, but then we can: Create a new object that will hold the array of normal delegates. Return a Delegate (as struct) to that new object, which will implement Invoke iterating over all its values and calling Invoke on them.

So, for normal delegates, that are never going to call two or more handlers, it could work as a struct. Unfortunately, that is not going to change in .Net.

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As a side note, variance does not requires the Delegate to be reference types. The parameters of the delegate should be reference types. After all, if you pass a string were an object is required (for input, not ref or out), then no cast is needed, as string is already an object.

Answer 6

I saw this interesting conversation on the Internet:

Immutable doesn't mean it has to be a value type. And something that is a value type is not required to be immutable. The two often go hand-in-hand, but they are not actually the same thing, and there are in fact counter-examples of each in the .NET Framework (the String class, for example).

And the answer:

The difference being that while immutable reference types are reasonably common and perfectly reasonable, making value types mutable is almost always a bad idea, and can result in some very confusing behaviour!

Taken from here

So, in my opinion the decision was made by language usability aspects, and not by compiler technological difficulties. I love nullable delegates.

Answer 7

I guess one reason is support for multi cast delegates Multi cast delegates are more complex than simply a few fields indicating target and method.

Another thing that's only possible in this form is delegate variance. This kind of variance requires a reference conversion between the two types.

Interestingly F# defines it's own function pointer type that's similar to delegates, but more lightweight. But I'm not sure if it's a value or reference type.