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()