现在你可以在2.0应用程序中使用HashSet<T> - 只需引用System.Core.dll，你就可以开始使用了。

注意：这需要您安装免费且与Visual Studio分开的.NET 3.5框架。安装完成后，您将拥有包含HashSet类型的新System.Core程序集。由于.NET框架版本2.0-3.5都共享相同的CLR，因此您可以在2.0应用程序中使用此程序集而不会出现任何问题。

这是我为2.0写的一个使用Dictionary<T，object>的内部实现。它不是3.5 HashSet<T>的完全匹配，但它对我来说做得到。

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
}

你可以使用 Iesi.Collections（NHibernate 使用）或Mono 的 HashSet。

C5库还有一个HashSet实现。

我认为PowerCollections 库应该满足您的需求。这是一个开源库，其中包含几个在.NET中缺失的集合类，包括Set<T>、Bag<T>、MultiDictionary等。它运行在.NET 2.0上。我现在已经使用它好几年了，非常满意。

您可以使用using指令将字典别名为哈希集。虽然并不完全相同，但它可能会在以后简化您的工作。