Erroneous "Use of unassigned local variable" for out parameter used inside collection expression

[JakeYallop]

Erroneous "Use of unassigned local variable" for out parameter used inside collection expression

[CyrusNajmabadi]

This is unrelated to collection expressions. The problem is simply with:

!V1("s", out var s1) || !V2("s2", out var s2)

Here there is a codepath where you execute [s1, s2] without executing both of those clauses. Namely, if !V1("s", out var s1) is true, then the || is short circuited, and the ? ... clause runs. In that case !V2("s2", out var s2) never runs, so s2 is never assigned. The compiler tells you this.

var v1Result = V1("s", out var s3);
var v2Result = V2("s2", out var s4);

This is not the same case. Here, you are unilaterally running both calls. And so s3 and s4 are always assigned.