How to check if all list items have the same value and return it, or return an “otherValue” if they don’t?

by 
var val = yyy.First().Value;
return yyy.All(x=>x.Value == val) ? val : otherValue;

Cleanest way I can think of. You can make it a one-liner by inlining val, but First() would be evaluated n times, doubling execution time.

To incorporate the “empty set” behavior specified in the comments, you simply add one more line before the two above: 

if(yyy == null || !yyy.Any()) return otherValue;

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