ICollection - Get single value

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

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();

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.

collection.ToArray()[i]

This way is slow, but very simple to use.

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());

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.