C# Delegates

Delegates

People often find it difficult to see the difference between events and delegates. C# doesn't help matters by allowing you to declare field-like events which are automatically backed by a delegate variable of the same name. This article aims to clarify the matter for you. Another source of confusion is the overloading of the term "delegate". Sometimes it is used to mean a delegate type, and at other times it can be used to mean an instance of a delegate type. I'll use "delegate type" and "delegate instance" to distinguish between them, and "delegate" when talking about the whole topic in a general sense.

Delegate types

In some ways, you can think of a delegate type as being a bit like an interface with a single method. It specifies the signature of a method, and when you have a delegate instance, you can make a call to it as if it were a method with the same signature. Delegates provide other features, but the ability to make calls with a particular signature is the reason for the existence of the delegate concept. Delegates hold a reference to a method, and (for instance methods) a reference to the target object the method should be called on.

Delegates types are declared with the delegate keyword. They can appear either on their own or nested within a class, as shown below.

namespace DelegateArticle
{
public delegate string FirstDelegate (int x);

public class Sample
{
public delegate void SecondDelegate (char a, char b);
}
}

This code declares two delegate types. The first is DelegateArticle.FirstDelegate which has a single parameter of type int and returns a string. The second is DelegateArticle.Sample.SecondDelegate which has two char parameters, and doesn't return anything (because the return type is specified as void).

Note that the delegate keyword doesn't always mean that a delegate type is being declared. The same keyword is used when creating instances of the delegate type, using anonymous methods.

The types declared here derive from System.MulticastDelegate, which in turn derives from System.Delegate. In practice, you'll only see delegate types deriving from MulticastDelegate. The difference between Delegate and MulticastDelegate is largely historical; in betas of .NET 1.0 the difference was significant (and annoying) - Microsoft considered merging the two types together, but decided it was too late in the release cycle to make such a major change. You can pretty much pretend that they're only one type.

Any delegate type you create has the members inherited from its parent types, one constructor with parameters of object and IntPtr and three extra methods: Invoke, BeginInvoke and EndInvoke. We'll come back to the constructor in a minute. The methods can't be inherited from anything, because the signatures vary according to the signature the delegate is declared with. Using the sample code above, the first delegate has the following methods:


public string Invoke (int x);
public System.IAsyncResult BeginInvoke(int x, System.AsyncCallback callback, object state);
public string Endinvoke(IAsyncResult result);

As you can see, the return type of Invoke and EndInvoke matches that of the declaration signature, as are the parameters of Invoke and the first parameters of BeginInvoke. We'll see the purpose of Invoke in the next section, and cover BeginInvoke and EndInvoke in the section on advanced usage. It's a bit premature to talk about calling methods when we don't know how to create an instance, however. We'll cover that (and more) in the next section.
Delegate instances: the basics

Now we know how a delegate type is declared and what it contains, let's look at how to create an instance of such a type, and what we can do with it.
Creating delegate instances

Note: this article doesn't cover the features of C# 2.0 and 3.0 for creating delegate instances. I cover the C# 2.0 features in my article about C# 2.0. When I eventually get round to writing about C# 3.0, I'm sure I'll cover the new features for lambda functions etc at that point. By concentrating on the explicit manner of creating instances in C# 1.0/1.1, I believe it will be easier to understand what's going on under the hood. When you understand the basics, it's clearly worth knowing the features these later versions provide - but if you try to use them without having a firm grasp on the basics, you may well get confused.

As mentioned earlier, the key points of data in any particular delegate instance are the method the delegate refers to, and a reference to call the method on (the target). For static methods, no target is required. The CLR itself supports other slightly different forms of delegate, where either the first argument passed to a static method is held within the delegate, or the target of an instance method is provided as an argument when the method is called. See the documentation for System.Delegate for more information on this if you're interested, but don't worry too much about it - it's largely irrelevant from a C# point of view. C# only supports creating what MSDN calls a closed instance delegate type.

So, now that we know the two pieces of data required to create an instance (along with the type itself, of course), how do we tell the compiler what they are? We use what the C# specification calls a delegate-creation-expression which is of the form new delegate-type (expression). The expression must either be another delegate of the same type (or a compatible delegate type in C# 2.0) or a method group - the name of a method and optionally a target, specified as if you were calling the method, but without the arguments or brackets. Creating copies of a delegate is fairly rare, so we will concentrate on the more common form. A few examples are listed below:

// The following two creation expressions are equivalent,
// where InstanceMethod is an instance method in the class
// containing the creation expression (or a base class).
// The target is "this".
FirstDelegate d1 = new FirstDelegate(InstanceMethod);
FirstDelegate d2 = new FirstDelegate(this.InstanceMethod);

// Here we create a delegate instance referring to the same method
// as the first two examples, but with a different target.
FirstDelegate d3 = new FirstDelegate(anotherInstance.InstanceMethod);

// This delegate instance uses an instance method in a different class,
// specifying the target to call the method on
FirstDelegate d4 = new FirstDelegate(instanceOfOtherClass.OtherInstanceMethod);

// This delegate instance uses a static method in ths class containing
// the creation expression (or a base class).
FirstDelegate d5 = new FirstDelegate(StaticMethod);

// This delegate instance uses a static method in a different class
FirstDelegate d6 = new FirstDelegate(OtherClass.OtherStaticMethod);

The constructor we mentioned earlier has two parameters - an object and an IntPtr. The object is a reference to the target (or null for static methods) and the IntPtr is a pointer to the method itself.

One point to note is that delegate instances can refer to methods and targets which wouldn't normally be visible at the point the call is actually made. For instance, a private method can be used to create a delegate instance, and then the delegate instance can be returned from a public member. Alternatively, the target of an instance may be an object which the eventual caller knows nothing about. However, both the target and the method must be accessible to the creating code. In other words, if (and only if) you can call a particular method on a particular object, you can use that method and target for delegate creation. Access rights are effectively ignored at call time. Speaking of which...
Calling delegate instances

Delegate instances are called just as if they were the methods themselves. For instance, to call the delegate referred to by variable d1 above, we could write:

string result = d1(10);

The method referred to by the delegate instance is called on the target object (if there is one), and the result is returned. Producing a complete program to demonstrate this without including a lot of seemingly irrelevant code is tricky. However, here's a program which gives one example of a static method and one of an instance method. DelegateTest.StaticMethod could be written as just StaticMethod in the same way that (within an instance method) you could write InstanceMethod instead of this.InstanceMethod - I've included the class name just to make it clear how you would reference methods from other classes.

using System;

public delegate string FirstDelegate (int x);

class DelegateTest
{
string name;

static void Main()
{
FirstDelegate d1 = new FirstDelegate(DelegateTest.StaticMethod);

DelegateTest instance = new DelegateTest();
instance.name = "My instance";
FirstDelegate d2 = new FirstDelegate(instance.InstanceMethod);

Console.WriteLine (d1(10)); // Writes out "Static method: 10"
Console.WriteLine (d2(5)); // Writes out "My instance: 5"
}

static string StaticMethod (int i)
{
return string.Format ("Static method: {0}", i);
}

string InstanceMethod (int i)
{
return string.Format ("{0}: {1}", name, i);
}
}

The C# syntax is just a short-hand for calling the Invoke method provided by each delegate type. Delegates can also be run asynchronously if they provide BeginInvoke/EndInvoke methods