Guidelines for events since non-nullable references

Question

When working with C# 8 and the new non-nullable references, I realized that events are treated like fields. This means that they will cause a warning 90% of the time since they won't be initialized until someone subscribes to it.

Consider the following event:

public event EventHandler IdleTimeoutReached; 

You get the following warning on the line of the constructor declaration.

CS8618 C# Non-nullable event is uninitialized. Consider declaring the event as nullable.

If you make it nullable, the warning disappears of course. However, this looks very weird to me.

public event EventHandler? IdleTimeoutReached; 

An alternative would be assigning it a no-op, but this seems even worse.

public event EventHandler IdleTimeoutReached = new EventHandler((o, e) => { }); 

What's the correct way to handle this situation and get rid of the warning without just disabling it? Are there any official guidelines for this?

Answer 1

You should make the event nullable, because it is indeed nullable. It may look a bit strange but that's the correct way of expressing your intent.

It has always been necessary to check whether an event is null before raising it within the class. In this case nullable reference checking will warn you if you try to invoke it without checking for null.

    public class Publisher     {         public event EventHandler? IdleTimeoutReached;          protected virtual void RaiseIdleTimeoutEvent()         {             // `IdleTimeoutReached` will be null if no subscribers; compiler gives warning about possible null reference             IdleTimeoutReached.Invoke(this, EventArgs.Empty);              // No compiler warning             IdleTimeoutReached?.Invoke(this, EventArgs.Empty);         }     } 

The nullable annotation really only adds value for the class invoking the event. It won't affect callers at all because the += / -= syntax takes care of correctly assigning/removing those delegates.

So you should use:

public event EventHandler? IdleTimeoutReached; 

For events with EventArgs, use:

public event EventHandler<IdleTimeoutEventArgs>? IdleTimeoutReached;