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?

Solution 1

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

---

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)

---

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

Solution 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?

Solution 3

Practically, they are the same. But I prefer second one since one exception is thrown while in the first two. Exceptions are expensive.

Solution 4

I think it's OK to write

var item = list.SingleOrDefault(x => x.HasFoo);
if (item == null) ...

but you can also write

if (list.Any(x => x.HasFoo)) ...

if you don't actually need access to the value.

Solution 5

If you ALWAYS EXPECT  one element in the list, just use 

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

and catch exception at the top level method, where you will log details of exception and show friendly message to the user.

If you sometimes expect 0 or more than 1 elements, the safest method will be

var item = list.FirstOrDefault(x => x.HasFoo);
if (item == null) 
{ 
// empty list processing, not necessary throwing exception
}

I assumed, that it is not important to verify, are more than 1 record exist or not.

Similar question was discussed in Code Project article LINQ: Single vs. SingleOrDefault

View more solutions