Proposal: "while" as a LINQ query expression clause

[HaloFour]

Copied from dotnet/roslyn#15638

Low effort/reward proposal:

I propose that the existing C# keyword while be added as a LINQ query expression clause. Unlike where which filters out elements that do not meet the condition while would include all elements as long as the condition is met. Since while is already a keyword this would not break any existing syntax.

var q = from i in numbers
where i % 2 == 0
while i < 10
select i * 2;

// translated into
var q = numbers
    .Where(i => i % 2 == 0)
    .TakeWhile(i => i < 10)
    .Select(i => i * 2);

[orthoxerox]

Should we have an issue for each new LINQ clause, or will you collect them all here?

[rubberduck203]

What does while i < 10 accomplish that where i < 10 doesn't? Do they accomplish the same thing, or is there a semantic difference?

[jnm2]

@ckuhn203 While skips the rest of the items as soon as any item doesn't match. Where only skips the items that don't match.
0,1,2 while i % 2 == 0 gives 0.
0,1,2 where i % 2 == 0 gives 0,2.

[HaloFour]

@orthoxerox I'd think that each would be managed separately (although if you'd be suggesting a family of clauses I could see them proposed as one). My thoughts for this particular proposal was that it's relatively low hanging fruit in that it's syntactically unambiguous given that while is a reserved keyword and it maps easily to the existing TakeWhile extension method.

[orthoxerox]

@HaloFour have you thought about extending this to SkipWhile()?

[vladd]

Is this new operator easily translatable to SQL?

(And I'm afraid it's not easily parallelizable, as it depends on the element ordering.)

[jnm2]

@vladd it would be possible but complicated to translate to SQL. However, there's no requirement for any ORM to implement support for this. For example, Entity Framework has never supported First, Single, SingleOrDefault, etc, in a subquery.

[orthoxerox]

@vladd It's an existing LINQ method, TakeWhile. Adding it to query syntax doesn't affect LINQ providers in any way.