Is there something similar to singletonList in C#

Question

Java has Collections.singletonList(T) which returns a List<T> of exactly one element. Is there something similar in C# that returns an IList?

Answer 1

IEnumerable<T> enumerable = Enumerable.Repeat(t, 1);

Creates an IEnumerable with one element.

Answer 2

Array implements IList; and the length cannot be modified via Add (as ReadOnly is true).

Thus, SingletonList<int> could be implemented as easily as:

var slist = new int[] { 5 };

You may want to wrap it in a System.Collections.ObjectModel.ReadOnlyCollection<T> so that the single value cannot be changed (if the Java Singleton list works like this). E.g.

var slist = new System.Collections.ObjectModel.ReadOnlyCollection<int>(new int[] { 5 });

You can also create an extension method.

public static IList<T> AsSingletonList<T>(this IEnumerable<T> source)
{
    foreach (var item in source)
    {
        return new System.Collections.ObjectModel.ReadOnlyCollection<T>(new T[] { item });
    }
    return new System.Collections.ObjectModel.ReadOnlyCollection<T>(new T[] { default(T) });
}

Or one that asserts there is exactly one value in the source:

public static IList<T> AsSingletonList<T>(this IEnumerable<T> source)
{
    IList<T> result = null;
    foreach (var item in source)
    {
        if (result != null)
            throw new ArgumentOutOfRangeException("source", "Source had more than one value.");
        result = new System.Collections.ObjectModel.ReadOnlyCollection<T>(new T[] { item });
    }

    if (result == null)
        throw new ArgumentOutOfRangeException("source", "Source had no values.");
    return result;
}

Edit: Used ReadOnlyCollection<T> to prevent mutation of the single value.

Note: While I think the other answers are correct, the List<T> by default has a capacity of 10 - which is a tiny bit wasteful.