Select single item from a list

Question

Using LINQ what is the best way to select a single item from a list if the item may not exists in the list?

I have come up with two solutions, neither of which I like. I use a where clause to select the list of items (which I know will only be one), I can then check the count and make a Single call on this list if count is one, the other choice is to use a foreach and just break after getting the item.

Neither of these seem like a good approach, is there a better way?

Answer 1

You can use IEnumerable.First() or IEnumerable.FirstOrDefault().

The difference is that First() will throw if no element is found (or if no element matches the conditions, if you use the conditions). FirstOrDefault() will return default(T) (null if it's a reference type).

Answer 2

Maybe I'm missing something here, but usually calling .SingleOrDefault() is the way to go to return either the single element in the list, or a default value (null for reference or nullable types) is the list is empty. It generates an exception if the list contains more than one element.

Use FirstOrDefault() to take care of the case where you could have more than one)

Answer 3

List<string> items = new List<string>();

items.Find(p => p == "blah");

or

items.Find(p => p.Contains("b"));

but this allows you to define what you are looking for via a match predicate...

I guess if you are talking linqToSql then:

example looking for Account...

DataContext dc = new DataContext();

Account item = dc.Accounts.FirstOrDefault(p => p.id == 5);

If you need to make sure that there is only 1 item (throws exception when more than 1)

DataContext dc = new DataContext();

Account item = dc.Accounts.SingleOrDefault(p => p.id == 5);

Answer 4

There are two easy ways, depending on if you want to deal with exceptions or get a default value.

You can use the First<T>() or the FirstOrDefault<T>() extension method to get the first result or default(T).

var list = new List<int> { 1, 2, 4 };
var result = list.Where(i => i == 3).First(); // throws InvalidOperationException
var result = list.Where(i => i == 3).FirstOrDefault(); // = 0

Answer 5

Use the FirstOrDefault selector.

var list = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

var firstEven = list.FirstOrDefault(n => n % 2 == 0);

if (firstEven == 0)
    Console.WriteLine("no even number");
else 
    Console.WriteLine("first even number is {0}", firstEven);

Just pass in a predicate to the First or FirstOrDefault method and it'll happily go round' the list and picks the first match for you.

If there isn't a match, FirstOrDefault will returns the default value of whatever datatype the list items is.

Hope this helps :-)

Answer 6

SingleOrDefault() is what you need

cheers

Answer 7

Just to complete the answer, If you are using the LINQ syntax, you can just wrap it since it returns an IEnumerable:

(from int x in intList
 where x > 5
 select x * 2).FirstOrDefault()