How do I get the first element from an IEnumerable<T> in .net?

Question

I often want to grab the first element of an IEnumerable<T> in .net, and I haven't found a nice way to do it. The best I've come up with is:

foreach(Elem e in enumerable) {
  // do something with e
  break;
}

Yuck! So, is there a nice way to do this?

Answer 1

If you can use LINQ you can use:

var e = enumerable.First();

This will throw an exception though if enumerable is empty: in which case you can do use:

var e = enumerable.FirstOrDefault(myDefault);

If you can't use LINQ, then your approach is technically correct and no different than creating an enumerator using the 'GetEnumerator' and 'MoveNext' methods to retrieve the first result (this example assumes enumerable is an IEnumerable):

Elem e = myDefault;
using (IEnumerator<Elem> enumer = enumerable.GetEnumerator()) {
    if (enumer.MoveNext()) e = enumer.Current;
}

Joel Coehoorn mentions .Single() in the comments; this will also work, if you are expecting your enumerable to contain exactly one element - however it will throw an exception if it is either empty or larger than one element. There is a corresponding SingleOrDefault() method that covers this scenario in a similar fashion to FirstOrDefault().

Edit: Thanks Marc Gravell for pointing out that I need to dispose of my IEnumerator object after using it - I've edited the non-LINQ example to display the 'using' keyword to implement this pattern.

Answer 2

FirstOrDefault ?

Elem e = enumerable.FirstOrDefault();
//do something with e

Answer 3

Well, you didn't specify which version of .Net you're using.

Assuming you have 3.5, another way is the ElementAt method:

var e = enumerable.ElementAt(0);

Answer 4

Just in case you're using .NET 2.0 and don't have access to LINQ:

 static T First<T>(IEnumerable<T> items)
 {
     using(IEnumerator<T> iter = items.GetEnumerator())
     {
         iter.MoveNext();
         return iter.Current;
     }
 }

This should do what you're looking for...it uses generics so you to get the first item on any type IEnumerable.

Call it like so:

List<string> items = new List<string>() { "A", "B", "C", "D", "E" };
string firstItem = First<string>(items);

Or

int[] items = new int[] { 1, 2, 3, 4, 5 };
int firstItem = First<int>(items);

You could modify it readily enough to mimic .NET 3.5's IEnumerable.ElementAt() extension method:

static T ElementAt<T>(IEnumerable<T> items, int index)
{
    using(IEnumerator<T> iter = items.GetEnumerator())
    {
        for (int i = 0; i <= index; i++, iter.MoveNext()) ;
        return iter.Current;
    }
}

Calling it like so:

int[] items = { 1, 2, 3, 4, 5 };
int elemIdx = 3;
int item = ElementAt<int>(items, elemIdx);

Of course if you do have access to LINQ, then there are plenty of good answers posted already...

Answer 5

Use FirstOrDefault or a foreach loop as already mentioned. Manually fetching an enumerator and calling Current should be avoided. foreach will dispose your enumerator for you if it implements IDisposable. When calling MoveNext and Current you have to dispose it manually (if aplicable).