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When to use .First and when to use .FirstOrDefault with LINQ?

Asked 2009-06-21T19:15:29.933
970

I've searched around and haven't really found a clear answer as to when you'd want to use .First and when you'd want to use .FirstOrDefault with LINQ.

  • When would you want to use .First? Only when you'd want to catch the exception if no results where returned?

    var result = List.Where(x => x == "foo").First();

  • And when would you want to use .FirstOrDefault? When you'd always want the default type if no result?

    var result = List.Where(x => x == "foo").FirstOrDefault();

  • And for that matter, what about Take?

    var result = List.Where(x => x == "foo").Take(1);
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3 Answers

22

First of all, Take is a completely different method. It returns an IEnumerable<T> and not a single T, so that's out.

Between First and FirstOrDefault, you should use First when you're sure that an element exists and if it doesn't, then there's an error.

By the way, if your sequence contains default(T) elements (e.g. null) and you need to distinguish between being empty and the first element being null, you can't use FirstOrDefault.

answered 2009-06-21T19:23:55.470
2
someList.First(); // exception if collection is empty.
someList.FirstOrDefault(); // first item or default(Type)

Which one to use? It should be decided by the business logic, and not the fear of exception/programm failure.

For instance, If business logic says that we can not have zero transactions on any working day (Just assume). Then you should not try to handle this scenario with some smart programming. I will always use First() over such collection, and let the program fail if something else screwed up the business logic.

Code:

var transactionsOnWorkingDay = GetTransactionOnLatestWorkingDay();
var justNeedOneToProcess = transactionsOnWorkingDay.First(): //Not FirstOrDefault()

I would like to see others comments over this.

answered 2009-06-21T22:25:08.537
1

Ok let me give my two cents. First / Firstordefault are for when you use the second constructor. I won't explain what it is, but it's when you would potentially always use one because you don't want to cause an exception.

person = tmp.FirstOrDefault(new Func<Person, bool>((p) =>
{
    return string.IsNullOrEmpty(p.Relationship);
}));
answered 2011-07-16T22:43:51.963

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