Are nullable annotations on events meaningful/allowed?

[AlekseyTs]

This issues is created for a

            // PROTOTYPE(NullableReferenceTypes): are annotations on events meaningful/allowed?

that was in AnnotationWithoutNonNullTypes unit-test.

[Joe4evr]

What if for an automatic/field-like event, the backing field is implicitly considered nullable? Essentially:

public event EventHandler OnFoo; //Compiler generates: private EventHandler? m_OnFoo;

private void Foo()
{
    //....
    OnFoo(this, EventArgs.Empty); //warning: event could NRE if there are no subscribers
}

A special fix would be suggested to use null-safe invocation: OnFoo?.Invoke(this, EventArgs.Empty).

[jcouv]

Closing. Events can be set to null, so annotating them is meaningful:

using System;
public class C {
    event Action E;
    public void M() 
    {
        E = null;
        E();
    }
}

[AlekseyTs]

If we allow annotations for events:

• Do we require non-nullable field-like events to be set to non-null value in a constructor?
• Do we warn on event1 += null?

It feels like it is very common for an event to not have any subscribers. It would also be good to get clarity on what do we consider a safety issue around an event? An invocation of a field-like event that might have no subscribers? Something else.

[jcouv]

I added some references to this issue in the code. In particular, if nullability is meaningful for events, we'll also want to honor annotations ([Disallow/Allow/Maybe/NotNull]).

[jasonmalinowski]

@jcouv I see this is marked as "archived" in the project board -- it is still being tracked?

[jcouv]

@jasonmalinowski I don't see this issue marked as "archived":

[jasonmalinowski]

@jcouv That's the little icon before the word "event". I had no idea that was a feature until seeing it just now.

[jcouv]

@jasonmalinowski Indeed. Not sure what happened. Restored it now. Thanks!

[jcouv]

Tagging @agocke I'm pushing this whole bucket (nullability analysis of events) post C# 8 RTM

[jcouv]

This was discussed in LDM today (7/17/2019) with the conclusion that ? annotations on events are meaningful and affect the adder, setter and backing field (therefore the .Invoke() as well).

[RikkiGibson]

@AlekseyTs is this issue still relevant?