Feature request: Allow null-propagation operator for event accessors

[SLaks]

Why can't I write code like this?

Process.GetProcessById(2)?.Exited += delegate { };

Error:

CS0079 The event 'Process.Exited' can only appear on the left hand side of += or -=

This would be especially useful when writing property setters that add and remove event handlers to their values if non-null.

[DavidKarlas]

Bump, Is this bug or?

[gafter]

the ?. Operator never produces an lvalue, so this is by design.

[SLaks]

Can this be reopened as a feature request?
This would be very useful for code like this.

[DavidKarlas]

Maybe add also properties setters into request like:

Object?.Prop = false;

I think it's very good feature request compared to some futuristic requests for C#7 ;)

[SLaks]

Note that the delegate expression (eg, x?.SomeEvent += ExpensiveFunction().SomeMethod) would still be evaluated if x is null.

[MrMatthewLayton]

@SLaks you're not the only one that has raised this:

http://stackoverflow.com/questions/32519200/c-sharp-6-0-null-propagation-operator-property-assignment/32715351#32715351

Congrats for also adding delegates/events to the mix; I never considered this!

[jnm2]

What would it take to not evaluate the RHS if the object is null? That would seem far more helpful at first blush.

It makes me happy that this is being considered. I've wished for this on a regular basis writing UI code.

[jnm2]

Just for context, I ran into wanting this again today.

[miloush]

Just for the record, I expected this to work for attaching event handlers too, though I now understand what the issue is.

object o = null;
INotifyCollectionChanged c = null;

c?.CollectionChanged += OnChanged;
(o as INotifyCollectionChanged)?.CollectionChanged += OnChanged;
((INotifyCollectionChanged)o)?.CollectionChanged += OnChanged;

@SLaks

Note that the delegate expression (eg, x?.SomeEvent += ExpensiveFunction().SomeMethod) would still be evaluated if x is null.

Not if it was rewritten to if (x != null) x.SomeEvent += ... or am I missing something? Do you want it to be evaluated?

[salmelo]

Would just like to add a vote to this (and null conditional lvalues in general) for whatever that's worth.

I run into wanting this very frequently when writing code to bridge UI and internal objects.

Edit: Ideally I would want the right hand side not to be evaluated at all in the case of null, much as if you had written the if statement necessary to produce the desired effect now.

[SLaks]

@miloush: You're right; it should check the LHS first.

[jnm2]

@SLaks Could you migrate this to https://github.com/dotnet/csharplang please?

[Misiu]

Whats the status of this?

I'd like to do this:

TabListControl?.SelectedIndexChanged += this.OnSelectedIndexChanged;

but I must add extra check

if (TabListControl != null)
{
  TabListControl.SelectedIndexChanged += this.OnSelectedIndexChanged;
}

I've looked at Null-coalescing assignments, but this isn't something I could use in this specific case.

[jnm2]

Issue moved to dotnet/csharplang #737 via ZenHub

[CyrusNajmabadi]

Closing out as this is a language request. I believe @RikkiGibson is championing allowing exactly this for the next version of C#.

[RikkiGibson]

dotnet/csharplang#6045