Thread-Safe collection with no order and no duplicates

Question

I need a thread-safe collection to hold items without duplicates. ConcurrentBag<T> allows non-unique items and HashSet<T> is not thread-safe. Is there a collection like this in .NET Framework 4.5?

Answer 1

Here's some code I just wrote that does this ConcurrentSet construct:

public class ConcurrentSet<T> : IEnumerable<T>, ISet<T>, ICollection<T>
{
    private readonly ConcurrentDictionary<T, byte> _dictionary = new ConcurrentDictionary<T, byte>();

    /// <summary>
    /// Returns an enumerator that iterates through the collection.
    /// </summary>
    /// <returns>
    /// A <see cref="T:System.Collections.Generic.IEnumerator`1"/> that can be used to iterate through the collection.
    /// </returns>
    public IEnumerator<T> GetEnumerator()
    {
        return _dictionary.Keys.GetEnumerator();
    }

    /// <summary>
    /// Returns an enumerator that iterates through a collection.
    /// </summary>
    /// <returns>
    /// An <see cref="T:System.Collections.IEnumerator"/> object that can be used to iterate through the collection.
    /// </returns>
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    /// <summary>
    /// Removes the first occurrence of a specific object from the <see cref="T:System.Collections.Generic.ICollection`1"/>.
    /// </summary>
    /// <returns>
    /// true if <paramref name="item"/> was successfully removed from the <see cref="T:System.Collections.Generic.ICollection`1"/>; otherwise, false. This method also returns false if <paramref name="item"/> is not found in the original <see cref="T:System.Collections.Generic.ICollection`1"/>.
    /// </returns>
    /// <param name="item">The object to remove from the <see cref="T:System.Collections.Generic.ICollection`1"/>.</param><exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only.</exception>
    public bool Remove(T item)
    {
        return TryRemove(item);
    }

    /// <summary>
    /// Gets the number of elements in the set.
    /// </summary>
    public int Count
    {
        get { return _dictionary.Count; }
    }

    /// <summary>
    /// Gets a value indicating whether the <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only.
    /// </summary>
    /// <returns>
    /// true if the <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only; otherwise, false.
    /// </returns>
    public bool IsReadOnly { get { return false; } }

    /// <summary>
    /// Gets a value that indicates if the set is empty.
    /// </summary>
    public bool IsEmpty
    {
        get { return _dictionary.IsEmpty; }
    }

    public ICollection<T> Values
    {
        get { return _dictionary.Keys; }
    }

    /// <summary>
    /// Adds an item to the <see cref="T:System.Collections.Generic.ICollection`1"/>.
    /// </summary>
    /// <param name="item">The object to add to the <see cref="T:System.Collections.Generic.ICollection`1"/>.</param><exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only.</exception>
    void ICollection<T>.Add(T item)
    {
        if(!Add(item))
            throw new ArgumentException("Item already exists in set.");
    }

    /// <summary>
    /// Modifies the current set so that it contains all elements that are present in both the current set and in the specified collection.
    /// </summary>
    /// <param name="other">The collection to compare to the current set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public void UnionWith(IEnumerable<T> other)
    {
        foreach (var item in other)
            TryAdd(item);
    }

    /// <summary>
    /// Modifies the current set so that it contains only elements that are also in a specified collection.
    /// </summary>
    /// <param name="other">The collection to compare to the current set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public void IntersectWith(IEnumerable<T> other)
    {
        var enumerable = other as IList<T> ?? other.ToArray();
        foreach (var item in this)
        {
            if (!enumerable.Contains(item))
                TryRemove(item);
        }
    }

    /// <summary>
    /// Removes all elements in the specified collection from the current set.
    /// </summary>
    /// <param name="other">The collection of items to remove from the set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public void ExceptWith(IEnumerable<T> other)
    {
        foreach (var item in other)
            TryRemove(item);
    }

    /// <summary>
    /// Modifies the current set so that it contains only elements that are present either in the current set or in the specified collection, but not both. 
    /// </summary>
    /// <param name="other">The collection to compare to the current set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public void SymmetricExceptWith(IEnumerable<T> other)
    {
        throw new NotImplementedException();
    }

    /// <summary>
    /// Determines whether a set is a subset of a specified collection.
    /// </summary>
    /// <returns>
    /// true if the current set is a subset of <paramref name="other"/>; otherwise, false.
    /// </returns>
    /// <param name="other">The collection to compare to the current set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public bool IsSubsetOf(IEnumerable<T> other)
    {
        var enumerable = other as IList<T> ?? other.ToArray();
        return this.AsParallel().All(enumerable.Contains);
    }

    /// <summary>
    /// Determines whether the current set is a superset of a specified collection.
    /// </summary>
    /// <returns>
    /// true if the current set is a superset of <paramref name="other"/>; otherwise, false.
    /// </returns>
    /// <param name="other">The collection to compare to the current set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public bool IsSupersetOf(IEnumerable<T> other)
    {
        return other.AsParallel().All(Contains);
    }

    /// <summary>
    /// Determines whether the current set is a correct superset of a specified collection.
    /// </summary>
    /// <returns>
    /// true if the <see cref="T:System.Collections.Generic.ISet`1"/> object is a correct superset of <paramref name="other"/>; otherwise, false.
    /// </returns>
    /// <param name="other">The collection to compare to the current set. </param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public bool IsProperSupersetOf(IEnumerable<T> other)
    {
        var enumerable = other as IList<T> ?? other.ToArray();
        return this.Count != enumerable.Count && IsSupersetOf(enumerable);
    }

    /// <summary>
    /// Determines whether the current set is a property (strict) subset of a specified collection.
    /// </summary>
    /// <returns>
    /// true if the current set is a correct subset of <paramref name="other"/>; otherwise, false.
    /// </returns>
    /// <param name="other">The collection to compare to the current set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public bool IsProperSubsetOf(IEnumerable<T> other)
    {
        var enumerable = other as IList<T> ?? other.ToArray();
        return Count != enumerable.Count && IsSubsetOf(enumerable);
    }

    /// <summary>
    /// Determines whether the current set overlaps with the specified collection.
    /// </summary>
    /// <returns>
    /// true if the current set and <paramref name="other"/> share at least one common element; otherwise, false.
    /// </returns>
    /// <param name="other">The collection to compare to the current set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public bool Overlaps(IEnumerable<T> other)
    {
        return other.AsParallel().Any(Contains);
    }

    /// <summary>
    /// Determines whether the current set and the specified collection contain the same elements.
    /// </summary>
    /// <returns>
    /// true if the current set is equal to <paramref name="other"/>; otherwise, false.
    /// </returns>
    /// <param name="other">The collection to compare to the current set.</param><exception cref="T:System.ArgumentNullException"><paramref name="other"/> is null.</exception>
    public bool SetEquals(IEnumerable<T> other)
    {
        var enumerable = other as IList<T> ?? other.ToArray();
        return Count == enumerable.Count && enumerable.AsParallel().All(Contains);
    }

    /// <summary>
    /// Adds an element to the current set and returns a value to indicate if the element was successfully added. 
    /// </summary>
    /// <returns>
    /// true if the element is added to the set; false if the element is already in the set.
    /// </returns>
    /// <param name="item">The element to add to the set.</param>
    public bool Add(T item)
    {
        return TryAdd(item);
    }

    public void Clear()
    {
        _dictionary.Clear();
    }

    public bool Contains(T item)
    {
        return _dictionary.ContainsKey(item);
    }

    /// <summary>
    /// Copies the elements of the <see cref="T:System.Collections.Generic.ICollection`1"/> to an <see cref="T:System.Array"/>, starting at a particular <see cref="T:System.Array"/> index.
    /// </summary>
    /// <param name="array">The one-dimensional <see cref="T:System.Array"/> that is the destination of the elements copied from <see cref="T:System.Collections.Generic.ICollection`1"/>. The <see cref="T:System.Array"/> must have zero-based indexing.</param><param name="arrayIndex">The zero-based index in <paramref name="array"/> at which copying begins.</param><exception cref="T:System.ArgumentNullException"><paramref name="array"/> is null.</exception><exception cref="T:System.ArgumentOutOfRangeException"><paramref name="arrayIndex"/> is less than 0.</exception><exception cref="T:System.ArgumentException"><paramref name="array"/> is multidimensional.-or-The number of elements in the source <see cref="T:System.Collections.Generic.ICollection`1"/> is greater than the available space from <paramref name="arrayIndex"/> to the end of the destination <paramref name="array"/>.-or-Type <paramref name="T"/> cannot be cast automatically to the type of the destination <paramref name="array"/>.</exception>
    public void CopyTo(T[] array, int arrayIndex)
    {
        Values.CopyTo(array, arrayIndex);
    }

    public T[] ToArray()
    {
        return _dictionary.Keys.ToArray();
    }

    public bool TryAdd(T item)
    {
        return _dictionary.TryAdd(item, default(byte));
    }

    public bool TryRemove(T item)
    {
        byte donotcare;
        return _dictionary.TryRemove(item, out donotcare);
    }
}