Should I use Single() or SingleOrDefault() if there is a chance that the element won't be found?

Question

What would you prefer to see?

try {   var item = list.Single(x => x.HasFoo); } catch(InvalidOperationException e) {   throw new InvalidOperationException("Exactly one item with foo expected, none found", e); } 

Or:

var item = list.SingleOrDefault(x => x.HasFoo); if (item == null)       throw new InvalidOperationException("Exactly one item with foo expected, none found"); 

What's the best practice here? Which one makes the exception more comprehensible?

Answer 1

• Use SingleOrDefault() if 0 or 1 items are expected
• Use Single() if 1, not 0 or 2 and more, item is expected

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Also keep in mind that there are a number of possible scenarios:

• You got 0 when 0 or 1 was expected (ok)
• You got 1 when 0 or 1 was expected (ok)
• You got 2 or more when 0 or 1 was expected (error)

And:

• You got 0 when 1 was expected (error)
• You got 1 when 1 was expected (ok)
• You got 2 or more when 1 was expected (error)

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And don't forget about First(), FirstOrDefault() and Any()

Answer 2

I would write:

var item = list.Single(x => x.HasFoo); 

If the case where this does not return a single item is so common you need a friendlier error message, then is it really an exception at all?