ICollection - Get single value

Question

What is the best way to get a value from a ICollection? We know the Collection is empty apart from that.

score 95 · Answer 1 · edited Oct 11 '21 at 14:21

95

You can use LINQ for this:.

var foo = myICollection.OfType<YourType>().FirstOrDefault();
// or use a query
var bar = (from x in myICollection.OfType<YourType>() where x.SomeProperty == someValue select x)
   .FirstOrDefault();

edited Oct 11 '21 at 14:21

Wai Ha Lee

8,598
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answered Dec 12 '08 at 17:18

2

FirstOrDefault is defined on ICollection or IEnumerable (non-generic)? Where? What would work though: myICollection.OfType().FirstOrDefault(); – Christoph Rüegg Dec 12 '08 at 17:42
3

This is the right answer. First and FirstOrDefault even use that IList optimisation internally where possible. – Daniel Earwicker Dec 12 '08 at 23:46
I was looking the FirstOrDefault() but intellisense was telling me that no way... until i added the using for System.Linq. So before doing any of this, check you added at the beginning of your code: `using System.Linq;` – Juan Ignacio Avendaño Huergo Oct 11 '21 at 14:15

Chris · Answer 2 · 2008-12-12T17:34:51.560

The simplest way to do this is:

foreach(object o in collection) {
  return o;
}

But this isn't particularly efficient if it's actually a generic collection because IEnumerator implements IDisposable, so the compiler has to put in a try/finally, with a Dispose() call in the finally block.

If it's a non-generic collection, or you know the generic collection implements nothing in its Dispose() method, then the following can be used:

IEnumerator en = collection.GetEnumerator();
en.MoveNext();
return en.Current;

If you know if may implement IList, you can do this:

IList iList = collection as IList;
if (iList != null) {
  // Implements IList, so can use indexer
  return iList[0];
}
// Use the slower way
foreach (object o in collection) {
  return o;
}

Likewise, if it's likely it'll be of a certain type of your own definition that has some kind of indexed access, you can use the same technique.

score 8 · Answer 3 · edited Oct 11 '21 at 14:20

8

collection.ToArray()[i]

This way is slow, but very simple to use.

edited Oct 11 '21 at 14:20

Wai Ha Lee

8,598
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answered Feb 10 '14 at 22:16

mehrdad seyrafi

3,084
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5

This copies the entire collection just to get one value. – Matt Feb 10 '14 at 22:38
2

"We know the Collection is empty apart from that.". For the sake of simplicity, the above would have its uses. Although not the best generic answer as it would be abused. – Skarsnik Mar 08 '16 at 00:30

bruno conde · Answer 4 · 2008-12-12T17:14:40.337

5

Without generics and because ICollection implements IEnumerable you can do like in example 1. With generics you simple need to do like example 2:

List<string> l = new List<string>();
l.Add("astring");

ICollection col1 = (ICollection)l;
ICollection<string> col2 = (ICollection<string>)l;

//example 1
IEnumerator e1 = col1.GetEnumerator();
if (e1.MoveNext())
    Console.WriteLine(e1.Current);

//example 2
if (col2.Count != 0)
    Console.WriteLine(col2.Single());

edited Dec 12 '08 at 17:14

answered Dec 12 '08 at 17:06

bruno conde

47,767
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example 2 isn't right because ICollection doesn't implement method Single(). – Dec 12 '08 at 17:18
This is a Linq extension method. Take a look at msdn doc : http://msdn.microsoft.com/en-us/library/y2fx0ty0.aspx – bruno conde Dec 12 '08 at 17:26

score 0 · Answer 5 · answered Apr 01 '18 at 09:42

If you know your collection has only one item, should only ever have one item, you can use the Linq extension method Single().

This converts a ICollection<T> into a T object containing the single item of that collection. If the length of the collection is 0, or more than one, this will throw an InvalidOperationException.

ICollection - Get single value

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