What is and how to use a multidelegate in C#

A multidelegate is an instance of a delegate that can point to more than one method. When a multidelegate is invoked, all the methods pointed by the delegate are executed in order.

This ability allows you to execute multiple methods in response to a single event or action.

In fact, in C# all delegates are multidelegates. We just have to use them slightly differently.

Multidelegate syntax

Multidelegates are created by combining delegate instances using the + operator or the Delegate.Combine method.

DelegateType multidelegate = MyDelegate1 + MyDelegate2;

Although, in general, we usually use += to combine delegates “in-place”.

Basic example

We can see it better with an example. Let’s suppose we have two methods, Method1 and Method2, both receiving a string. On the other hand, we have a MyDelegate definition that matches the signature of these methods.

We can create an instance of MyDelegate called multidelegate that executes both methods, creating two delegates and combining them with +.

static void Method1(string message)
{
    Console.WriteLine("Method1: " + message);
}

static void Method2(string message)
{
    Console.WriteLine("Method1: " + message);
}

delegate void MyDelegate(string message);

MyDelegate delegate1 = new MyDelegate(Method1);
MyDelegate delegate2 = new MyDelegate(Method2);

MyDelegate multidelegate = delegate1 + delegate2;
multidelegate("Hello World");

// Output:
// Method1: Hello World
// Method2: Hello World

However, as I said before, the usual thing is to use += to combine them directly. This would be done as follows:

MyDelegate multidelegate = Method1;
multidelegate += Method2;

This is equivalent to the creation of the previous multidelegate, without the need to create delegate1 and delegate2. It is the syntax that you will usually use, but it is good to see how it works “under the hood”.

Unlink a method

It is also possible to unlink a method that we have included in a multidelegate. For this, we can use the - operator or the Delegate.Remove method.

MyDelegate multidelegate2 = multidelegate - delegate1;

For this, similarly to the delegate combination case, the usual thing is to use the -= operator directly.

We see it easier with an example. In the previous case,

// this is what we had
MyDelegate multidelegate = Method1;
multidelegate += Method2;

// now I remove Method1 from the delegate
multidelegate -= Method1;

multidelegate("Hello world");

// Output:
// Method2: Hello World

In this case, it would call only Method2, because we have removed Method1 from the delegate.

This is a very common practice when we see events, because sometimes we will want to stop listening to them temporarily or definitively.