Here’s one I wrote for 2.0 that uses a Dictionary<T, object> internally. It’s not an exact match of the 3.5 HashSet<T>, but it does the job for me.
using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.Serialization;
public class HashSet<T> : ICollection<T>, ISerializable, IDeserializationCallback
{
private readonly Dictionary<T, object> dict;
public HashSet()
{
dict = new Dictionary<T, object>();
}
public HashSet(IEnumerable<T> items) : this()
{
if (items == null)
{
return;
}
foreach (T item in items)
{
Add(item);
}
}
public HashSet<T> NullSet { get { return new HashSet<T>(); } }
#region ICollection<T> Members
public void Add(T item)
{
if (null == item)
{
throw new ArgumentNullException("item");
}
dict[item] = null;
}
/// <summary>
/// Removes all items from the <see cref="T:System.Collections.Generic.ICollection`1"/>.
/// </summary>
/// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only. </exception>
public void Clear()
{
dict.Clear();
}
public bool Contains(T item)
{
return dict.ContainsKey(item);
}
/// <summary>
/// Copies the items 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 items 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-<paramref name="arrayIndex"/> is equal to or greater than the length of <paramref name="array"/>.-or-The number of items 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 T cannot be cast automatically to the type of the destination <paramref name="array"/>.</exception>
public void CopyTo(T[] array, int arrayIndex)
{
if (array == null) throw new ArgumentNullException("array");
if (arrayIndex < 0 || arrayIndex >= array.Length || arrayIndex >= Count)
{
throw new ArgumentOutOfRangeException("arrayIndex");
}
dict.Keys.CopyTo(array, arrayIndex);
}
/// <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 dict.Remove(item);
}
/// <summary>
/// Gets the number of items contained in the <see cref="T:System.Collections.Generic.ICollection`1"/>.
/// </summary>
/// <returns>
/// The number of items contained in the <see cref="T:System.Collections.Generic.ICollection`1"/>.
/// </returns>
public int Count
{
get { return dict.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;
}
}
#endregion
public HashSet<T> Union(HashSet<T> set)
{
HashSet<T> unionSet = new HashSet<T>(this);
if (null == set)
{
return unionSet;
}
foreach (T item in set)
{
if (unionSet.Contains(item))
{
continue;
}
unionSet.Add(item);
}
return unionSet;
}
public HashSet<T> Subtract(HashSet<T> set)
{
HashSet<T> subtractSet = new HashSet<T>(this);
if (null == set)
{
return subtractSet;
}
foreach (T item in set)
{
if (!subtractSet.Contains(item))
{
continue;
}
subtractSet.dict.Remove(item);
}
return subtractSet;
}
public bool IsSubsetOf(HashSet<T> set)
{
HashSet<T> setToCompare = set ?? NullSet;
foreach (T item in this)
{
if (!setToCompare.Contains(item))
{
return false;
}
}
return true;
}
public HashSet<T> Intersection(HashSet<T> set)
{
HashSet<T> intersectionSet = NullSet;
if (null == set)
{
return intersectionSet;
}
foreach (T item in this)
{
if (!set.Contains(item))
{
continue;
}
intersectionSet.Add(item);
}
foreach (T item in set)
{
if (!Contains(item) || intersectionSet.Contains(item))
{
continue;
}
intersectionSet.Add(item);
}
return intersectionSet;
}
public bool IsProperSubsetOf(HashSet<T> set)
{
HashSet<T> setToCompare = set ?? NullSet;
// A is a proper subset of a if the b is a subset of a and a != b
return (IsSubsetOf(setToCompare) && !setToCompare.IsSubsetOf(this));
}
public bool IsSupersetOf(HashSet<T> set)
{
HashSet<T> setToCompare = set ?? NullSet;
foreach (T item in setToCompare)
{
if (!Contains(item))
{
return false;
}
}
return true;
}
public bool IsProperSupersetOf(HashSet<T> set)
{
HashSet<T> setToCompare = set ?? NullSet;
// B is a proper superset of a if b is a superset of a and a != b
return (IsSupersetOf(setToCompare) && !setToCompare.IsSupersetOf(this));
}
public List<T> ToList()
{
return new List<T>(this);
}
#region Implementation of ISerializable
/// <summary>
/// Populates a <see cref="T:System.Runtime.Serialization.SerializationInfo"/> with the data needed to serialize the target object.
/// </summary>
/// <param name="info">The <see cref="T:System.Runtime.Serialization.SerializationInfo"/> to populate with data. </param><param name="context">The destination (see <see cref="T:System.Runtime.Serialization.StreamingContext"/>) for this serialization. </param><exception cref="T:System.Security.SecurityException">The caller does not have the required permission. </exception>
public void GetObjectData(SerializationInfo info, StreamingContext context)
{
if (info == null) throw new ArgumentNullException("info");
dict.GetObjectData(info, context);
}
#endregion
#region Implementation of IDeserializationCallback
/// <summary>
/// Runs when the entire object graph has been deserialized.
/// </summary>
/// <param name="sender">The object that initiated the callback. The functionality for this parameter is not currently implemented. </param>
public void OnDeserialization(object sender)
{
dict.OnDeserialization(sender);
}
#endregion
#region Implementation of IEnumerable
/// <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>
/// <filterpriority>1</filterpriority>
public IEnumerator<T> GetEnumerator()
{
return dict.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>
/// <filterpriority>2</filterpriority>
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
#endregion
}