Question

Quick question, what’s the preferable way to programmaticaly ask “Is there exactly one element in this sequence that satisfies X condition?” using Linq?

i.e.

// Pretend that the .OneAndOnlyOne() method exists
int[] sequence = new int[] { 1, 1, 2, 3, 5, 8 };
Assert.IsTrue(sequence.OneAndOnlyOne(x => x == 2);
Assert.IsFalse(sequence.OneAndOnlyOne(x => x == 1);

something like this can be done with:

sequence.SingleOrDefault(x => x == 2) != null;

but that’s a little clunky.

I suppose I could roll my own extension method, but this seems to be a common pattern in my code and I want to make sure there’s a good clean way to do it. Is there a way using the built-in LINQ methods?

Answer 1

You could do:

bool onlyOne = source.Where(/*condition*/).Take(2).Count() == 1

Which will prevent count from enumerating a large sequence unnecessarily in the event of multiple matches.