Feature Request: adding "when" clause to "yield return"

[jcouv]

@mspost commented on Wed Oct 18 2017

Instead of writing this:

     IEnumerable<string> foo()
     {
           var stringValue = myVariable.GetOtherValue();
           if (stringValue != null)
                yield return stringValue;
     }

I would like to be able to write this:

 IEnumerable<string> foo()
 {
       yield return myVariable.GetOtherValue() when v => v != null;
 }

This seems like a natural extension to the way 'when' is used by exceptions. Alternatively, given the way a property setter has a fixed value 'value', it could be done that way as well, like this:

     IEnumerable<string> foo()
     {
           yield return myVariable.GetOtherValue() when value != null;
     }

Since a property setter would never need a yield return, this may be a cleaner way to proceed.

---

@jcouv commented on Wed Oct 18 2017

This is a language design discussion, not a compiler bug. I'll move to csharplang repo.

[mspost]

Sorry. This was my first post and I got the wrong location.

[MkazemAkhgary]

use of value can bring ambiguity between local variables.

 var value = 0;
 yield return myVariable.GetOtherValue() when value != null;

this issue could be supported by #973 and #867

(var value = myVariable.GetOtherValue()) != null ? yield return value : yield continue;

yield continue is another proposal in this comment :))

[MkazemAkhgary]

with when clause it would be like

yield return (var result = myVariable.GetOtherValue()) when result != null;

when clause can access local and field variables, just like how its works after catch statements.

[orthoxerox]

I think declaration expressions will solve this problem for many language constructs instead of just yield return:

     IEnumerable<string> Foo()
     {
           if ((var stringValue = myVariable.GetOtherValue()) != null)
                yield return stringValue;
     }

[yaakov-h]

if (myVariable.GetOtherValue() is {} val) yield return val;

[jnm2]

We've already had discussions on why if is preferable to appending a condition to a return statement.
See #325, #453 and #724

[Thaina]

I would prefer condition expression

IEnumerable<string> Foo()
{
    (myVariable?.GetOtherValue() is var v && v != null) ? yield return v : yield break;
}
// or maybe
IEnumerable<string> Foo()
{
    yield (myVariable?.GetOtherValue() is var v && v != null) ? return v : break;
}

[MgSam]

@yaakov-h

if (myVariable.GetOtherValue() is {} val) yield return val;

Yuck, that is awful. Shocked that this got 5 upvotes. Completely non-obvious that the intention here is to do a null check. You save one line of code for the potential of hours of bugs later on when someone comes later, doesn't understand your intent, and refactors to something that misses the null check.

[MkazemAkhgary]

@yaakov-h how does that even compile?

[Joe4evr]

@MkazemAkhgary It's a proposed part of pattern matching.

[yaakov-h]

@MgSam I think it was upvoted not because it's a brilliant way to do this, but because it simply achieves the goals of the original post using C# language features that already exist and/or are actively being championed.

If you're concerned about refactoring breaking the code... write a unit test. 😉

[Thaina]

If it to be the same as pattern matching it should be return when _expression_ : obj;