What is the generic version of a Hashtable?

Question

I have been learning the basics of generics in .NET. However, I don't see the generic equivalent of Hashtable. Please share some sample C# code for creating generic hashtable classes.

Answer 1

Dictionary<TKey, TValue>

Note that Dictionary is not a 100% drop in replacement for HashTable.

There is a slight difference in the way they handle NULLs. The dictionary will throw an exception if you try to reference a key that doesn't exist. The HashTable will just return null. The reason is that the value might be a value type, which cannot be null. In a Hashtable the value was always Object, so returning null was at least possible.

Answer 2

The generic version of Hashtable class is System.Collections.Generic.Dictionary class.

Sample code:

Dictionary<int, string> numbers = new Dictionary<int, string>( );
   numbers.Add(1, "one");
   numbers.Add(2, "two");
   // Display all key/value pairs in the Dictionary.
   foreach (KeyValuePair<int, string> kvp in numbers)
   {
      Console.WriteLine("Key: " + kvp.Key + "\tValue: " + kvp.Value);
   }

Answer 3

The generic version of a Hashtable is the Dictionary<TKey,TValue> class (link). Here is some sample code translated from using a Hashtable into the most direct equivalent of Dictionary (argument checking removed for sake of brevity)

public HashTable Create(int[] keys, string[] values) { 
  HashTable table = new HashTable();
  for ( int i = 0; i < keys.Length; i++ ) {
    table[keys[i]] = values[i];
  }
  return table;
}

public Dictionary<object,object> Create(int[] keys, string[] values) {
  Dictionary<object,object> map = Dictionary<object,object>();
  for ( int i = 0; i < keys.Length; i++) {
    map[keys[i]] = values[i];
  }
  return map;
}

That's a fairly direct translation. But the problem is that this does not actually take advantage of the type safe features of generics. The second function could be written as follows and be much more type safe and inccur no boxing overhead

public Dictionary<int,string> Create(int[] keys, string[] values) {
  Dictionary<int,string> map = Dictionary<int,string>();
  for ( int i = 0; i < keys.Length; i++) {
    map[keys[i]] = values[i];
  }
  return map;
}

Even better. Here's a completely generic version

public Dictionary<TKey,TValue> Create<TKey,TValue>(TKey[] keys, TValue[] values) {
  Dictionary<TKey,TValue> map = Dictionary<TKey,TValue>();
  for ( int i = 0; i < keys.Length; i++) {
    map[keys[i]] = values[i];
  }
  return map;
}

And one that is even further flexible (thanks Joel for pointing out I missed this)

public Dictionary<TKey,TValue> Create<TKey,TValue>(
    IEnumerable<TKey> keys, 
    IEnumerable<TValue> values) {

  Dictionary<TKey,TValue> map = Dictionary<TKey,TValue>();
  using ( IEnumerater<TKey> keyEnum = keys.GetEnumerator() ) 
  using ( IEnumerator<TValue> valueEnum = values.GetEnumerator()) {
    while (keyEnum.MoveNext() && valueEnum.MoveNext() ) { 
      map[keyEnum.Current] = valueEnum.Current;
    }
  }
  return map;
}

Answer 4

For those who are interested, I created a generic Hashtable wrapper class, which is useful for enforcing type safety and can be passed as a generic IDictionary, ICollection and IEnumerable type, whereas the non-generic Hashtable cannot. Below is the implementation.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Common.Collections.Generic
{
    public class Hashtable<TKey, TValue> : IDictionary<TKey, TValue>
        , ICollection<KeyValuePair<TKey, TValue>>
        , IEnumerable<KeyValuePair<TKey, TValue>>
        , IDictionary
        , ICollection
        , IEnumerable
    {
        protected Hashtable _items;
        /// <summary>
        /// Initializes a new, empty instance of the Hashtable class using the default initial capacity, load factor, hash code provider, and comparer.
        /// </summary>
        public Hashtable()
        {
            _items = new Hashtable();
        }
        /// <summary>
        /// Initializes a new, empty instance of the Hashtable class using the specified initial capacity, and the default load factor, hash code provider, and comparer.
        /// </summary>
        /// <param name="capacity">The approximate number of elements that the Hashtable object can initially contain. </param>
        public Hashtable(int capacity)
        {
            _items = new Hashtable(capacity);
        }
        /// <summary>
        /// Actual underlying hashtable object that contains the elements.
        /// </summary>
        public Hashtable Items { get { return _items; } }

        /// <summary>
        /// Adds an element with the specified key and value into the Hashtable.
        /// </summary>
        /// <param name="key">Key of the new element to add.</param>
        /// <param name="value">Value of the new elment to add.</param>
        public void Add(TKey key, TValue value)
        {
            _items.Add(key, value);
        }
        /// <summary>
        /// Adds an element with the specified key and value into the Hashtable.
        /// </summary>
        /// <param name="item">Item containing the key and value to add.</param>
        public void Add(KeyValuePair<TKey, TValue> item)
        {
            _items.Add(item.Key, item.Value);
        }

        void IDictionary.Add(object key, object value)
        {
            this.Add((TKey)key, (TValue)value);
        }
        /// <summary>
        /// Add a list of key/value pairs to the hashtable.
        /// </summary>
        /// <param name="collection">List of key/value pairs to add to hashtable.</param>
        public void AddRange(IEnumerable<KeyValuePair<TKey, TValue>> collection)
        {
            foreach (var item in collection)
                _items.Add(item.Key, item.Value);
        }
        /// <summary>
        /// Determines whether the Hashtable contains a specific key.
        /// </summary>
        /// <param name="key">Key to locate.</param>
        /// <returns>True if key is found, otherwise false.</returns>
        public bool ContainsKey(TKey key)
        {
            return _items.ContainsKey(key);
        }
        /// <summary>
        /// Determines whether the Hashtable contains a specific key.
        /// </summary>
        /// <param name="item">Item containing the key to locate.</param>
        /// <returns>True if item.Key is found, otherwise false.</returns>
        public bool Contains(KeyValuePair<TKey, TValue> item)
        {
            return _items.ContainsKey(item.Key);
        }

        bool IDictionary.Contains(object key)
        {
            return this.ContainsKey((TKey)key);
        }
        /// <summary>
        /// Gets an ICollection containing the keys in the Hashtable.
        /// </summary>
        public ICollection<TKey> Keys
        {
            get { return _items.ToList<TKey>(); }
        }

        ICollection IDictionary.Keys
        {
            get { return this.Keys.ToList(); }
        }
        /// <summary>
        /// Gets the value associated with the specified key.
        /// </summary>
        /// <param name="key">The key of the value to get.</param>
        /// <param name="value">When this method returns, contains the value associated with the specified key,
        /// if the key is found; otherwise, the default value for the type of the value parameter. This parameter 
        /// is passed uninitialized.</param>
        /// <returns>true if the hashtable contains an element with the specified key, otherwise false.</returns>
        public bool TryGetValue(TKey key, out TValue value)
        {
            value = (TValue)_items[key];
            return (value != null);
        }
        /// <summary>
        /// Gets an ICollection containing the values in the Hashtable.
        /// </summary>
        public ICollection<TValue> Values
        {
            get { return _items.Values.ToList<TValue>(); }
        }

        ICollection IDictionary.Values
        {
            get { return this.Values.ToList(); }
        }
        /// <summary>
        /// Gets or sets the value associated with the specified key.
        /// </summary>
        /// <param name="key">The key whose value to get or set. </param>
        /// <returns>The value associated with the specified key. If the specified key is not found, 
        /// attempting to get it returns null, and attempting to set it creates a new element using the specified key.</returns>
        public TValue this[TKey key]
        {
            get
            {
                return (TValue)_items[key];
            }
            set
            {
                _items[key] = value;
            }
        }
        /// <summary>
        /// Removes all elements from the Hashtable.
        /// </summary>
        public void Clear()
        {
            _items.Clear();
        }
        /// <summary>
        /// Copies all key/value pairs in the hashtable to the specified array.
        /// </summary>
        /// <param name="array">Object array to store objects of type "KeyValuePair&lt;TKey, TValue&gt;"</param>
        /// <param name="arrayIndex">Starting index to store objects into array.</param>
        public void CopyTo(Array array, int arrayIndex)
        {
            _items.CopyTo(array, arrayIndex);
        }
        /// <summary>
        /// Copies all key/value pairs in the hashtable to the specified array.
        /// </summary>
        /// <param name="array">Object array to store objects of type "KeyValuePair&lt;TKey, TValue&gt;"</param>
        /// <param name="arrayIndex">Starting index to store objects into array.</param>
        public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
        {
            _items.CopyTo(array, arrayIndex);
        }
        /// <summary>
        /// Gets the number of key/value pairs contained in the Hashtable.
        /// </summary>
        public int Count
        {
            get { return _items.Count; }
        }
        /// <summary>
        /// Gets a value indicating whether the Hashtable has a fixed size.
        /// </summary>
        public bool IsFixedSize
        {
            get { return _items.IsFixedSize; }
        }
        /// <summary>
        /// Gets a value indicating whether the Hashtable is read-only.
        /// </summary>
        public bool IsReadOnly
        {
            get { return _items.IsReadOnly; }
        }
        /// <summary>
        /// Gets a value indicating whether access to the Hashtable is synchronized (thread safe).
        /// </summary>
        public bool IsSynchronized
        {
            get { return _items.IsSynchronized; }
        }
        /// <summary>
        /// Gets an object that can be used to synchronize access to the Hashtable.
        /// </summary>
        public object SyncRoot
        {
            get { return _items.SyncRoot; }
        }
        /// <summary>
        /// Removes the element with the specified key from the Hashtable.
        /// </summary>
        /// <param name="key">Key of the element to remove.</param>
        public void Remove(TKey key)
        {
            _items.Remove(key);
        }
        /// <summary>
        /// Removes the element with the specified key from the Hashtable.
        /// </summary>
        /// <param name="item">Item containing the key of the element to remove.</param>
        public void Remove(KeyValuePair<TKey, TValue> item)
        {
            this.Remove(item.Key);
        }

        bool IDictionary<TKey, TValue>.Remove(TKey key)
        {
            var numValues = _items.Count;
            _items.Remove(key);
            return numValues > _items.Count;
        }

        bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> item)
        {
            var numValues = _items.Count;
            _items.Remove(item.Key);
            return numValues > _items.Count;
        }

        void IDictionary.Remove(object key)
        {
            _items.Remove(key);
        }
        /// <summary>
        /// Returns an enumerator that iterates through the hashtable.
        /// </summary>
        /// <returns>An enumerator for a list of key/value pairs.</returns>
        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
        {
            foreach (DictionaryEntry? item in _items)
                yield return new KeyValuePair<TKey, TValue>((TKey)item.Value.Key, (TValue)item.Value.Value);
        }
        /// <summary>
        /// Returns an enumerator that iterates through the hashtable.
        /// </summary>
        /// <returns>An enumerator for a list of key/value pairs as generic objects.</returns>
        IEnumerator IEnumerable.GetEnumerator()
        {
            return this.GetEnumerator();
        }

        IDictionaryEnumerator IDictionary.GetEnumerator()
        {
            // Very old enumerator that no one uses anymore, not supported.
            throw new NotImplementedException(); 
        }

        object IDictionary.this[object key]
        {
            get
            {
                return _items[(TKey)key];
            }
            set
            {
                _items[(TKey)key] = value;
            }
        }
    }
}

I have done some testing of this Hashtable vs Dictionary and found the two perform about the same when used with a string key and string value pair, except the Hashtable seems to use less memory. The results of my test are as follows:

TestInitialize Dictionary_50K_Hashtable
Number objects 50000, memory usage 905164
Insert, 22 milliseconds.
A search not found, 0 milliseconds.
Search found, 0 milliseconds.
Remove, 0 milliseconds.
Search found or not found, 0 milliseconds.
TestCleanup Dictionary_50K_Hashtable

TestInitialize Dictionary_50K_Dictionary
Number objects 50000, memory usage 1508316
Insert, 16 milliseconds.
A search not found, 0 milliseconds.
Search found, 0 milliseconds.
Remove, 0 milliseconds.
Search found or not found, 0 milliseconds.
TestCleanup Dictionary_50K_Dictionary

Answer 5

The generic version of System.Collection.Hashtable is System.Collections.Generic.Dictionary<TKey, TValue>.