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

To answer your question, no. Sadly there is nothing built in, although it would often be useful when working with IEnumerable. You’ll have to roll your own.

Update

Instead of using workarounds, here’s an example of an efficient and immutable SingletonList that implements IList<T>:

Usage

SingletonList<int> bling = new SingletonList<int>(10);    

Code

public class SingletonList<T> : IList<T>
{
    private readonly T _item;

    public SingletonList(T item)
    {
        _item = item;
    }

    public IEnumerator<T> GetEnumerator()
    {
        yield return _item;
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public void Add(T item)
    {
        throw new NotSupportedException("Add not supported.");
    }

    public void Clear()
    {
        throw new NotSupportedException("Clear not supported.");
    }

    public bool Contains(T item)
    {
        if (item == null) return _item == null;

        return item.Equals(_item);
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        if (array == null) throw new ArgumentNullException("array");

        array[arrayIndex] = _item;
    }

    public bool Remove(T item)
    {
        throw new NotSupportedException("Remove not supported.");
    }

    public int Count
    {
        get { return 1; }
    }

    public bool IsReadOnly
    {
        get { return true; }
    }

    public int IndexOf(T item)
    {
        return Contains(item) ? 0 : -1;
    }

    public void Insert(int index, T item)
    {
        throw new NotSupportedException("Insert not supported.");
    }

    public void RemoveAt(int index)
    {
        throw new NotSupportedException("RemoveAt not supported.");
    }

    public T this[int index]
    {
        get
        {
            if (index == 0) return _item;

            throw new IndexOutOfRangeException();
        }
        set { throw new NotSupportedException("Set not supported."); }
    }
}