How to initialize an event

Question

I have a class with an event like this

public class A
{
    public event Func<string, string> Message;

    public void Calling()
    {
        Message("Hello world!");
    }
}

If I call the Calling() method and no one has subscribed to the Message event yet, it is null and throws an exception.

How can I initialize my event?

More in-depth discussion about events can be found [here](http://stackoverflow.com/q/3102918/945456) and [here](http://stackoverflow.com/q/786383/945456). — Jeff B, Nov 05 '14 at 19:24

Peter Duniho · Answer 1 · 2019-11-22T16:33:49.957

The event is initialized. To the value null. The fix is to check for null first. The canonical approach looks like this:

public void Calling()
{
    Func<string, string> handler = Message;

    if (handler != null)
    {
        handler("Hello world!");
    }
}

Note the event value is copied to a local before the check. This ensures against concurrent changes to the event between the time it's checked for null and the time it's invoked. In many programs, this is not needed (no concurrency) and you can safely just check the event itself directly.

If you are using a version of C# (6.0 or later) that supports the null-conditional operator, the above can be simplified nicely as this:

public void Calling()
{
    Message?.Invoke("Hello world!");
}

The compiler will generate the intermediate copy of the reference for you, check it for null, and only call the Invoke() method if it's non-null.

score 6 · Accepted Answer · answered Nov 05 '14 at 19:01

6

You don't init your event rather you need to check for null in your Calling method:

public void Calling()
{
    if (Message != null)
        Message("Hello World!");
}

answered Nov 05 '14 at 19:01

Brian Driscoll

19,373
3
46
65

score 3 · Answer 3 · answered Nov 05 '14 at 19:03

Check for null before invoking your event in class to see if anyone has subscribed to it or not.

public void Calling()
{
    if (Message != null)
        Message("Hello world!");
}

See: Events Tutorial

Invoking an event: Once a class has declared an event, it can treat that event just like a field of the indicated delegate type. The field will either be null, if no client has hooked up a delegate to the event, or else it refers to a delegate that should be called when the event is invoked. Thus, invoking an event is generally done by first checking for null and then calling the event.

score 1 · Answer 4 · answered Nov 05 '14 at 19:02

You can't.

Events are, as you have found, null by default, which is why good practice demands that you check them before using them:

public void Calling()
{
    if (Message != null)
       Message("Hello World");
}

You aren't using the return value there; kind of odd, but perhaps its just because this is an illustration.

You could also initialize it to a blank function in your constructor, but thats just wasted resources:

public A()
{
    Message += (s) => { return String.Empty; };
}

Now it calls a do-nothing function every time the event is raised, when a simple null check solved the problem. Definitely not recommended, but it is the only way to "initialize" an event.

score 1 · Answer 5 · answered Nov 05 '14 at 19:05

Although, this isn't your question, I'm going to throw this in there. You can check to see if anyone is listening to the event by "null-checking" it. This ensures that it's not going to execute the method (and subsequently throw a NullReferenceException).

public void Calling()
{
  if (Message != null)
    Message("Hello World"); // Why are you ignoring the return value?
}

As for your actual question, something has to have a reference to an object of type A (as defined in your example.

public class B
{
  A MyA;

  public B()
  { 
    MyA = new A();
  }
}

Then it has to subscribe to the event of Message.

public class B()
{
  A MyA;

  public B()
  { 
    MyA = new A();
    MyA.Message += MessageHandler;
  }

  public string MessageHandler(string s)
  {
    // Do other code here and ensure you're returning a string as defined in your Message event.
    return s;
  }
}

Note that the MessageHandler can be named anything. I just used that for clarity's sake. It can also be anonymous methods or lambdas rather than a named method.

How to initialize an event

5 Answers5