Develop Reference

Search for Value as if it were key in Dictionary [duplicate]

How do I get a Dictionary key by value in C#?
Dictionary<string, string> types = new Dictionary<string, string>()
{
{"1", "one"},
{"2", "two"},
{"3", "three"}
};
I want something like this:
getByValueKey(string value);
getByValueKey("one") must be return "1".
What is the best way do this? Maybe HashTable or SortedLists?

Values do not necessarily have to be unique, so you have to do a lookup. You can do something like this:
var myKey = types.FirstOrDefault(x => x.Value == "one").Key;
If values are unique and are inserted less frequently than read, then create an inverse dictionary where values are keys and keys are values.

You could do that:
By looping through all the KeyValuePair<TKey, TValue>'s in the dictionary (which will be a sizable performance hit if you have a number of entries in the dictionary)
Use two dictionaries, one for value-to-key mapping and one for key-to-value mapping (which would take up twice as much space in memory).
Use Method 1 if performance is not a consideration, and use Method 2 if memory is not a consideration.
Also, all keys must be unique, but the values are not required to be unique. You may have more than one key with the specified value.

I was in a situation where LINQ binding was not available and had to expand lambda explicitly. It resulted in a simple function:
public static T KeyByValue<T, W>(this Dictionary<T, W> dict, W val)
{
T key = default;
foreach (KeyValuePair<T, W> pair in dict)
{
if (EqualityComparer<W>.Default.Equals(pair.Value, val))
{
key = pair.Key;
break;
}
}
return key;
}
Call it like follows:
public static void Main()
{
Dictionary<string, string> dict = new Dictionary<string, string>()
{
{"1", "one"},
{"2", "two"},
{"3", "three"}
};
string key = dict.KeyByValue("two");
Console.WriteLine("Key: " + key);
}
It works on .NET 2.0 and in other limited environments.

public static string GetKeyFromValue(string valueVar)
{
foreach (string keyVar in dictionaryVar.Keys)
{
if (dictionaryVar[keyVar] == valueVar)
{
return keyVar;
}
}
return null;
}
Other people may have more efficient answers, but I find this personally more intuitive and it works in my case.

I have created a double-lookup class:
/// <summary>
/// dictionary with double key lookup
/// </summary>
/// <typeparam name="T1">primary key</typeparam>
/// <typeparam name="T2">secondary key</typeparam>
/// <typeparam name="TValue">value type</typeparam>
public class cDoubleKeyDictionary<T1, T2, TValue> {
private struct Key2ValuePair {
internal T2 key2;
internal TValue value;
}
private Dictionary<T1, Key2ValuePair> d1 = new Dictionary<T1, Key2ValuePair>();
private Dictionary<T2, T1> d2 = new Dictionary<T2, T1>();
/// <summary>
/// add item
/// not exacly like add, mote like Dictionary[] = overwriting existing values
/// </summary>
/// <param name="key1"></param>
/// <param name="key2"></param>
public void Add(T1 key1, T2 key2, TValue value) {
lock (d1) {
d1[key1] = new Key2ValuePair {
key2 = key2,
value = value,
};
d2[key2] = key1;
}
}
/// <summary>
/// get key2 by key1
/// </summary>
/// <param name="key1"></param>
/// <param name="key2"></param>
/// <returns></returns>
public bool TryGetValue(T1 key1, out TValue value) {
if (d1.TryGetValue(key1, out Key2ValuePair kvp)) {
value = kvp.value;
return true;
} else {
value = default;
return false;
}
}
/// <summary>
/// get key1 by key2
/// </summary>
/// <param name="key2"></param>
/// <param name="key1"></param>
/// <remarks>
/// 2x O(1) operation
/// </remarks>
/// <returns></returns>
public bool TryGetValue2(T2 key2, out TValue value) {
if (d2.TryGetValue(key2, out T1 key1)) {
return TryGetValue(key1, out value);
} else {
value = default;
return false;
}
}
/// <summary>
/// get key1 by key2
/// </summary>
/// <param name="key2"></param>
/// <param name="key1"></param>
/// <remarks>
/// 2x O(1) operation
/// </remarks>
/// <returns></returns>
public bool TryGetKey1(T2 key2, out T1 key1) {
return d2.TryGetValue(key2, out key1);
}
/// <summary>
/// get key1 by key2
/// </summary>
/// <param name="key2"></param>
/// <param name="key1"></param>
/// <remarks>
/// 2x O(1) operation
/// </remarks>
/// <returns></returns>
public bool TryGetKey2(T1 key1, out T2 key2) {
if (d1.TryGetValue(key1, out Key2ValuePair kvp1)) {
key2 = kvp1.key2;
return true;
} else {
key2 = default;
return false;
}
}
/// <summary>
/// remove item by key 1
/// </summary>
/// <param name="key1"></param>
public void Remove(T1 key1) {
lock (d1) {
if (d1.TryGetValue(key1, out Key2ValuePair kvp)) {
d1.Remove(key1);
d2.Remove(kvp.key2);
}
}
}
/// <summary>
/// remove item by key 2
/// </summary>
/// <param name="key2"></param>
public void Remove2(T2 key2) {
lock (d1) {
if (d2.TryGetValue(key2, out T1 key1)) {
d1.Remove(key1);
d2.Remove(key2);
}
}
}
/// <summary>
/// clear all items
/// </summary>
public void Clear() {
lock (d1) {
d1.Clear();
d2.Clear();
}
}
/// <summary>
/// enumerator on key1, so we can replace Dictionary by cDoubleKeyDictionary
/// </summary>
/// <param name="key1"></param>
/// <returns></returns>
public TValue this[T1 key1] {
get => d1[key1].value;
}
/// <summary>
/// enumerator on key1, so we can replace Dictionary by cDoubleKeyDictionary
/// </summary>
/// <param name="key1"></param>
/// <returns></returns>
public TValue this[T1 key1, T2 key2] {
set {
lock (d1) {
d1[key1] = new Key2ValuePair {
key2 = key2,
value = value,
};
d2[key2] = key1;
}
}
}

A Dictionary<K,V> extension that works. I have been using it for a long time::
public static bool TryGetKey<K, V>(this IDictionary<K, V> instance, V value, out K key)
{
foreach (var entry in instance)
{
if (!entry.Value.Equals(value))
{
continue;
}
key = entry.Key;
return true;
}
key = default(K);
return false;
}
And use as :
public static void Main()
{
Dictionary<string, string> dict = new Dictionary<string, string>()
{
{"1", "one"},
{"2", "two"},
{"3", "three"}
};
string value="two";
if (dict.TryGetKey(value, out var returnedKey))
Console.WriteLine($"Found Key {returnedKey}");
else
Console.WriteLine($"No key found for value {value}");
}

Maybe something like this:
foreach (var keyvaluepair in dict)
{
if(Object.ReferenceEquals(keyvaluepair.Value, searchedObject))
{
//dict.Remove(keyvaluepair.Key);
break;
}
}

The order of the keys in Keys is unspecified, but it is the same as the associated values in Values (from the C# doc).
So an efficient way (in some situations) to do that for a collection of values looks like:
/// <summary>
/// Gets the 1st key matching each value
/// </summary>
public static IEnumerable<TKey> GetKeys<TKey,TValue>(this Dictionary<TKey, TValue> dic, IEnumerable<TValue> values) where TKey : notnull
{
//The order of the keys in Keys is unspecified, but it is the same as the associated values in Values
var dicKeys = dic.Keys.ToList();
var dicValues = dic.Values.ToList();
foreach (var value in values)
{
var i = dicValues.IndexOf(value); //Will return the index of the 1st found value, even when multiple values are present
//we could test if i==-1 there.
yield return dicKeys[i];
}
}

types.Values.ToList().IndexOf("one");
Values.ToList() converts your dictionary values into a List of objects.
IndexOf("one") searches your new List looking for "one" and returns the Index which would match the index of the Key/Value pair in the dictionary.
This method does not care about the dictionary keys, it simply returns the index of the value that you are looking for.
Keep in mind there may be more than one "one" value in your dictionary. And that is the reason there is no "get key" method.

The below code only works if it contains unique value data:
public string getKey(string Value)
{
if (dictionary.ContainsValue(Value))
{
var ListValueData = new List<string>();
var ListKeyData = new List<string>();
var Values = dictionary.Values;
var Keys = dictionary.Keys;
foreach (var item in Values)
{
ListValueData.Add(item);
}
var ValueIndex = ListValueData.IndexOf(Value);
foreach (var item in Keys)
{
ListKeyData.Add(item);
}
return ListKeyData[ValueIndex];
}
return string.Empty;
}

I have a very simple way to do this. It worked out perfect for me.
Dictionary<string, string> types = new Dictionary<string, string>();
types.Add("1", "one");
types.Add("2", "two");
types.Add("3", "three");
Console.WriteLine("Please type a key to show its value: ");
string rLine = Console.ReadLine();
if(types.ContainsKey(rLine))
{
string value_For_Key = types[rLine];
Console.WriteLine("Value for " + rLine + " is" + value_For_Key);
}

Key-key pair dictionary with two way access by keys? [duplicate]

I am looking for a generic, bidirectional 1 to 1 Dictionary class in C# (2), ie. a BiDictionaryOneToOne<T, S> which is guaranteed to only contain one of each value and key (up to RefEquals anyway), and which can be searched using either key or value. Anyone know of one, or should I just implement it myself? I can't believe that I'm the first person to need this...
There is a BiDictionary in the answers to this question, but it is not for unique elements (and also does not implement RemoveByFirst(T t) or RemoveBySecond(S s)).
Thanks!

OK, here is my attempt (building on Jon's - thanks), archived here and open for improvement :
/// <summary>
/// This is a dictionary guaranteed to have only one of each value and key.
/// It may be searched either by TFirst or by TSecond, giving a unique answer because it is 1 to 1.
/// </summary>
/// <typeparam name="TFirst">The type of the "key"</typeparam>
/// <typeparam name="TSecond">The type of the "value"</typeparam>
public class BiDictionaryOneToOne<TFirst, TSecond>
{
IDictionary<TFirst, TSecond> firstToSecond = new Dictionary<TFirst, TSecond>();
IDictionary<TSecond, TFirst> secondToFirst = new Dictionary<TSecond, TFirst>();
#region Exception throwing methods
/// <summary>
/// Tries to add the pair to the dictionary.
/// Throws an exception if either element is already in the dictionary
/// </summary>
/// <param name="first"></param>
/// <param name="second"></param>
public void Add(TFirst first, TSecond second)
{
if (firstToSecond.ContainsKey(first) || secondToFirst.ContainsKey(second))
throw new ArgumentException("Duplicate first or second");
firstToSecond.Add(first, second);
secondToFirst.Add(second, first);
}
/// <summary>
/// Find the TSecond corresponding to the TFirst first
/// Throws an exception if first is not in the dictionary.
/// </summary>
/// <param name="first">the key to search for</param>
/// <returns>the value corresponding to first</returns>
public TSecond GetByFirst(TFirst first)
{
TSecond second;
if (!firstToSecond.TryGetValue(first, out second))
throw new ArgumentException("first");
return second;
}
/// <summary>
/// Find the TFirst corresponing to the Second second.
/// Throws an exception if second is not in the dictionary.
/// </summary>
/// <param name="second">the key to search for</param>
/// <returns>the value corresponding to second</returns>
public TFirst GetBySecond(TSecond second)
{
TFirst first;
if (!secondToFirst.TryGetValue(second, out first))
throw new ArgumentException("second");
return first;
}
/// <summary>
/// Remove the record containing first.
/// If first is not in the dictionary, throws an Exception.
/// </summary>
/// <param name="first">the key of the record to delete</param>
public void RemoveByFirst(TFirst first)
{
TSecond second;
if (!firstToSecond.TryGetValue(first, out second))
throw new ArgumentException("first");
firstToSecond.Remove(first);
secondToFirst.Remove(second);
}
/// <summary>
/// Remove the record containing second.
/// If second is not in the dictionary, throws an Exception.
/// </summary>
/// <param name="second">the key of the record to delete</param>
public void RemoveBySecond(TSecond second)
{
TFirst first;
if (!secondToFirst.TryGetValue(second, out first))
throw new ArgumentException("second");
secondToFirst.Remove(second);
firstToSecond.Remove(first);
}
#endregion
#region Try methods
/// <summary>
/// Tries to add the pair to the dictionary.
/// Returns false if either element is already in the dictionary
/// </summary>
/// <param name="first"></param>
/// <param name="second"></param>
/// <returns>true if successfully added, false if either element are already in the dictionary</returns>
public Boolean TryAdd(TFirst first, TSecond second)
{
if (firstToSecond.ContainsKey(first) || secondToFirst.ContainsKey(second))
return false;
firstToSecond.Add(first, second);
secondToFirst.Add(second, first);
return true;
}
/// <summary>
/// Find the TSecond corresponding to the TFirst first.
/// Returns false if first is not in the dictionary.
/// </summary>
/// <param name="first">the key to search for</param>
/// <param name="second">the corresponding value</param>
/// <returns>true if first is in the dictionary, false otherwise</returns>
public Boolean TryGetByFirst(TFirst first, out TSecond second)
{
return firstToSecond.TryGetValue(first, out second);
}
/// <summary>
/// Find the TFirst corresponding to the TSecond second.
/// Returns false if second is not in the dictionary.
/// </summary>
/// <param name="second">the key to search for</param>
/// <param name="first">the corresponding value</param>
/// <returns>true if second is in the dictionary, false otherwise</returns>
public Boolean TryGetBySecond(TSecond second, out TFirst first)
{
return secondToFirst.TryGetValue(second, out first);
}
/// <summary>
/// Remove the record containing first, if there is one.
/// </summary>
/// <param name="first"></param>
/// <returns> If first is not in the dictionary, returns false, otherwise true</returns>
public Boolean TryRemoveByFirst(TFirst first)
{
TSecond second;
if (!firstToSecond.TryGetValue(first, out second))
return false;
firstToSecond.Remove(first);
secondToFirst.Remove(second);
return true;
}
/// <summary>
/// Remove the record containing second, if there is one.
/// </summary>
/// <param name="second"></param>
/// <returns> If second is not in the dictionary, returns false, otherwise true</returns>
public Boolean TryRemoveBySecond(TSecond second)
{
TFirst first;
if (!secondToFirst.TryGetValue(second, out first))
return false;
secondToFirst.Remove(second);
firstToSecond.Remove(first);
return true;
}
#endregion
/// <summary>
/// The number of pairs stored in the dictionary
/// </summary>
public Int32 Count
{
get { return firstToSecond.Count; }
}
/// <summary>
/// Removes all items from the dictionary.
/// </summary>
public void Clear()
{
firstToSecond.Clear();
secondToFirst.Clear();
}
}

A more complete implementation of bidirectional dictionary:
Supports almost all interfaces of original Dictionary<TKey,TValue> (except infrastructure interfaces):
IDictionary<TKey, TValue>
IReadOnlyDictionary<TKey, TValue>
IDictionary
ICollection<KeyValuePair<TKey, TValue>> (this one and below are the base interfaces of the ones above)
ICollection
IReadOnlyCollection<KeyValuePair<TKey, TValue>>
IEnumerable<KeyValuePair<TKey, TValue>>
IEnumerable
Serialization using SerializableAttribute.
Debug view using DebuggerDisplayAttribute (with Count info) and DebuggerTypeProxyAttribute (for displaying key-value pairs in watches).
Reverse dictionary is available as IDictionary<TValue, TKey> Reverse property and also implements all interfaces mentioned above. All operations on either dictionaries modify both.
Usage:
var dic = new BiDictionary<int, string>();
dic.Add(1, "1");
dic[2] = "2";
dic.Reverse.Add("3", 3);
dic.Reverse["4"] = 4;
dic.Clear();
Code is available in my private framework on GitHub: BiDictionary(TFirst,TSecond).cs (permalink, search).
Copy:
[Serializable]
[DebuggerDisplay ("Count = {Count}"), DebuggerTypeProxy (typeof(DictionaryDebugView<,>))]
public class BiDictionary<TFirst, TSecond> : IDictionary<TFirst, TSecond>, IReadOnlyDictionary<TFirst, TSecond>, IDictionary
{
private readonly IDictionary<TFirst, TSecond> _firstToSecond = new Dictionary<TFirst, TSecond>();
[NonSerialized]
private readonly IDictionary<TSecond, TFirst> _secondToFirst = new Dictionary<TSecond, TFirst>();
[NonSerialized]
private readonly ReverseDictionary _reverseDictionary;
public BiDictionary ()
{
_reverseDictionary = new ReverseDictionary(this);
}
public IDictionary<TSecond, TFirst> Reverse
{
get { return _reverseDictionary; }
}
public int Count
{
get { return _firstToSecond.Count; }
}
object ICollection.SyncRoot
{
get { return ((ICollection)_firstToSecond).SyncRoot; }
}
bool ICollection.IsSynchronized
{
get { return ((ICollection)_firstToSecond).IsSynchronized; }
}
bool IDictionary.IsFixedSize
{
get { return ((IDictionary)_firstToSecond).IsFixedSize; }
}
public bool IsReadOnly
{
get { return _firstToSecond.IsReadOnly || _secondToFirst.IsReadOnly; }
}
public TSecond this [TFirst key]
{
get { return _firstToSecond[key]; }
set
{
_firstToSecond[key] = value;
_secondToFirst[value] = key;
}
}
object IDictionary.this [object key]
{
get { return ((IDictionary)_firstToSecond)[key]; }
set
{
((IDictionary)_firstToSecond)[key] = value;
((IDictionary)_secondToFirst)[value] = key;
}
}
public ICollection<TFirst> Keys
{
get { return _firstToSecond.Keys; }
}
ICollection IDictionary.Keys
{
get { return ((IDictionary)_firstToSecond).Keys; }
}
IEnumerable<TFirst> IReadOnlyDictionary<TFirst, TSecond>.Keys
{
get { return ((IReadOnlyDictionary<TFirst, TSecond>)_firstToSecond).Keys; }
}
public ICollection<TSecond> Values
{
get { return _firstToSecond.Values; }
}
ICollection IDictionary.Values
{
get { return ((IDictionary)_firstToSecond).Values; }
}
IEnumerable<TSecond> IReadOnlyDictionary<TFirst, TSecond>.Values
{
get { return ((IReadOnlyDictionary<TFirst, TSecond>)_firstToSecond).Values; }
}
public IEnumerator<KeyValuePair<TFirst, TSecond>> GetEnumerator ()
{
return _firstToSecond.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator ()
{
return GetEnumerator();
}
IDictionaryEnumerator IDictionary.GetEnumerator ()
{
return ((IDictionary)_firstToSecond).GetEnumerator();
}
public void Add (TFirst key, TSecond value)
{
_firstToSecond.Add(key, value);
_secondToFirst.Add(value, key);
}
void IDictionary.Add (object key, object value)
{
((IDictionary)_firstToSecond).Add(key, value);
((IDictionary)_secondToFirst).Add(value, key);
}
public void Add (KeyValuePair<TFirst, TSecond> item)
{
_firstToSecond.Add(item);
_secondToFirst.Add(item.Reverse());
}
public bool ContainsKey (TFirst key)
{
return _firstToSecond.ContainsKey(key);
}
public bool Contains (KeyValuePair<TFirst, TSecond> item)
{
return _firstToSecond.Contains(item);
}
public bool TryGetValue (TFirst key, out TSecond value)
{
return _firstToSecond.TryGetValue(key, out value);
}
public bool Remove (TFirst key)
{
TSecond value;
if (_firstToSecond.TryGetValue(key, out value)) {
_firstToSecond.Remove(key);
_secondToFirst.Remove(value);
return true;
}
else
return false;
}
void IDictionary.Remove (object key)
{
var firstToSecond = (IDictionary)_firstToSecond;
if (!firstToSecond.Contains(key))
return;
var value = firstToSecond[key];
firstToSecond.Remove(key);
((IDictionary)_secondToFirst).Remove(value);
}
public bool Remove (KeyValuePair<TFirst, TSecond> item)
{
return _firstToSecond.Remove(item);
}
public bool Contains (object key)
{
return ((IDictionary)_firstToSecond).Contains(key);
}
public void Clear ()
{
_firstToSecond.Clear();
_secondToFirst.Clear();
}
public void CopyTo (KeyValuePair<TFirst, TSecond>[] array, int arrayIndex)
{
_firstToSecond.CopyTo(array, arrayIndex);
}
void ICollection.CopyTo (Array array, int index)
{
((IDictionary)_firstToSecond).CopyTo(array, index);
}
[OnDeserialized]
internal void OnDeserialized (StreamingContext context)
{
_secondToFirst.Clear();
foreach (var item in _firstToSecond)
_secondToFirst.Add(item.Value, item.Key);
}
private class ReverseDictionary : IDictionary<TSecond, TFirst>, IReadOnlyDictionary<TSecond, TFirst>, IDictionary
{
private readonly BiDictionary<TFirst, TSecond> _owner;
public ReverseDictionary (BiDictionary<TFirst, TSecond> owner)
{
_owner = owner;
}
public int Count
{
get { return _owner._secondToFirst.Count; }
}
object ICollection.SyncRoot
{
get { return ((ICollection)_owner._secondToFirst).SyncRoot; }
}
bool ICollection.IsSynchronized
{
get { return ((ICollection)_owner._secondToFirst).IsSynchronized; }
}
bool IDictionary.IsFixedSize
{
get { return ((IDictionary)_owner._secondToFirst).IsFixedSize; }
}
public bool IsReadOnly
{
get { return _owner._secondToFirst.IsReadOnly || _owner._firstToSecond.IsReadOnly; }
}
public TFirst this [TSecond key]
{
get { return _owner._secondToFirst[key]; }
set
{
_owner._secondToFirst[key] = value;
_owner._firstToSecond[value] = key;
}
}
object IDictionary.this [object key]
{
get { return ((IDictionary)_owner._secondToFirst)[key]; }
set
{
((IDictionary)_owner._secondToFirst)[key] = value;
((IDictionary)_owner._firstToSecond)[value] = key;
}
}
public ICollection<TSecond> Keys
{
get { return _owner._secondToFirst.Keys; }
}
ICollection IDictionary.Keys
{
get { return ((IDictionary)_owner._secondToFirst).Keys; }
}
IEnumerable<TSecond> IReadOnlyDictionary<TSecond, TFirst>.Keys
{
get { return ((IReadOnlyDictionary<TSecond, TFirst>)_owner._secondToFirst).Keys; }
}
public ICollection<TFirst> Values
{
get { return _owner._secondToFirst.Values; }
}
ICollection IDictionary.Values
{
get { return ((IDictionary)_owner._secondToFirst).Values; }
}
IEnumerable<TFirst> IReadOnlyDictionary<TSecond, TFirst>.Values
{
get { return ((IReadOnlyDictionary<TSecond, TFirst>)_owner._secondToFirst).Values; }
}
public IEnumerator<KeyValuePair<TSecond, TFirst>> GetEnumerator ()
{
return _owner._secondToFirst.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator ()
{
return GetEnumerator();
}
IDictionaryEnumerator IDictionary.GetEnumerator ()
{
return ((IDictionary)_owner._secondToFirst).GetEnumerator();
}
public void Add (TSecond key, TFirst value)
{
_owner._secondToFirst.Add(key, value);
_owner._firstToSecond.Add(value, key);
}
void IDictionary.Add (object key, object value)
{
((IDictionary)_owner._secondToFirst).Add(key, value);
((IDictionary)_owner._firstToSecond).Add(value, key);
}
public void Add (KeyValuePair<TSecond, TFirst> item)
{
_owner._secondToFirst.Add(item);
_owner._firstToSecond.Add(item.Reverse());
}
public bool ContainsKey (TSecond key)
{
return _owner._secondToFirst.ContainsKey(key);
}
public bool Contains (KeyValuePair<TSecond, TFirst> item)
{
return _owner._secondToFirst.Contains(item);
}
public bool TryGetValue (TSecond key, out TFirst value)
{
return _owner._secondToFirst.TryGetValue(key, out value);
}
public bool Remove (TSecond key)
{
TFirst value;
if (_owner._secondToFirst.TryGetValue(key, out value)) {
_owner._secondToFirst.Remove(key);
_owner._firstToSecond.Remove(value);
return true;
}
else
return false;
}
void IDictionary.Remove (object key)
{
var firstToSecond = (IDictionary)_owner._secondToFirst;
if (!firstToSecond.Contains(key))
return;
var value = firstToSecond[key];
firstToSecond.Remove(key);
((IDictionary)_owner._firstToSecond).Remove(value);
}
public bool Remove (KeyValuePair<TSecond, TFirst> item)
{
return _owner._secondToFirst.Remove(item);
}
public bool Contains (object key)
{
return ((IDictionary)_owner._secondToFirst).Contains(key);
}
public void Clear ()
{
_owner._secondToFirst.Clear();
_owner._firstToSecond.Clear();
}
public void CopyTo (KeyValuePair<TSecond, TFirst>[] array, int arrayIndex)
{
_owner._secondToFirst.CopyTo(array, arrayIndex);
}
void ICollection.CopyTo (Array array, int index)
{
((IDictionary)_owner._secondToFirst).CopyTo(array, index);
}
}
}
internal class DictionaryDebugView<TKey, TValue>
{
private readonly IDictionary<TKey, TValue> _dictionary;
[DebuggerBrowsable (DebuggerBrowsableState.RootHidden)]
public KeyValuePair<TKey, TValue>[] Items
{
get
{
var array = new KeyValuePair<TKey, TValue>[_dictionary.Count];
_dictionary.CopyTo(array, 0);
return array;
}
}
public DictionaryDebugView (IDictionary<TKey, TValue> dictionary)
{
if (dictionary == null)
throw new ArgumentNullException("dictionary");
_dictionary = dictionary;
}
}
public static class KeyValuePairExts
{
public static KeyValuePair<TValue, TKey> Reverse<TKey, TValue> (this KeyValuePair<TKey, TValue> #this)
{
return new KeyValuePair<TValue, TKey>(#this.Value, #this.Key);
}
}

The question you refer to also shows a one-to-one implementation in this answer. Adding RemoveByFirst and RemoveBySecond would be trivial - as would implementing extra interfaces etc.

This is same as accepted answer, but I provided Update methods as well, and over all little more fleshed out:
public class BiDictionary<TKey1, TKey2> : IEnumerable<Tuple<TKey1, TKey2>>
{
Dictionary<TKey1, TKey2> _forwards;
Dictionary<TKey2, TKey1> _reverses;
public int Count
{
get
{
if (_forwards.Count != _reverses.Count)
throw new Exception("somewhere logic went wrong and your data got corrupt");
return _forwards.Count;
}
}
public ICollection<TKey1> Key1s
{
get { return _forwards.Keys; }
}
public ICollection<TKey2> Key2s
{
get { return _reverses.Keys; }
}
public BiDictionary(IEqualityComparer<TKey1> comparer1 = null, IEqualityComparer<TKey2> comparer2 = null)
{
_forwards = new Dictionary<TKey1, TKey2>(comparer1);
_reverses = new Dictionary<TKey2, TKey1>(comparer2);
}
public bool ContainsKey1(TKey1 key)
{
return ContainsKey(key, _forwards);
}
private static bool ContainsKey<S, T>(S key, Dictionary<S, T> dict)
{
return dict.ContainsKey(key);
}
public bool ContainsKey2(TKey2 key)
{
return ContainsKey(key, _reverses);
}
public TKey2 GetValueByKey1(TKey1 key)
{
return GetValueByKey(key, _forwards);
}
private static T GetValueByKey<S, T>(S key, Dictionary<S, T> dict)
{
return dict[key];
}
public TKey1 GetValueByKey2(TKey2 key)
{
return GetValueByKey(key, _reverses);
}
public bool TryGetValueByKey1(TKey1 key, out TKey2 value)
{
return TryGetValue(key, _forwards, out value);
}
private static bool TryGetValue<S, T>(S key, Dictionary<S, T> dict, out T value)
{
return dict.TryGetValue(key, out value);
}
public bool TryGetValueByKey2(TKey2 key, out TKey1 value)
{
return TryGetValue(key, _reverses, out value);
}
public bool Add(TKey1 key1, TKey2 key2)
{
if (ContainsKey1(key1) || ContainsKey2(key2)) // very important
return false;
AddOrUpdate(key1, key2);
return true;
}
public void AddOrUpdateByKey1(TKey1 key1, TKey2 key2)
{
if (!UpdateByKey1(key1, key2))
AddOrUpdate(key1, key2);
}
// dont make this public; a dangerous method used cautiously in this class
private void AddOrUpdate(TKey1 key1, TKey2 key2)
{
_forwards[key1] = key2;
_reverses[key2] = key1;
}
public void AddOrUpdateKeyByKey2(TKey2 key2, TKey1 key1)
{
if (!UpdateByKey2(key2, key1))
AddOrUpdate(key1, key2);
}
public bool UpdateKey1(TKey1 oldKey, TKey1 newKey)
{
return UpdateKey(oldKey, _forwards, newKey, (key1, key2) => AddOrUpdate(key1, key2));
}
private static bool UpdateKey<S, T>(S oldKey, Dictionary<S, T> dict, S newKey, Action<S, T> updater)
{
T otherKey;
if (!TryGetValue(oldKey, dict, out otherKey) || ContainsKey(newKey, dict))
return false;
Remove(oldKey, dict);
updater(newKey, otherKey);
return true;
}
public bool UpdateKey2(TKey2 oldKey, TKey2 newKey)
{
return UpdateKey(oldKey, _reverses, newKey, (key1, key2) => AddOrUpdate(key2, key1));
}
public bool UpdateByKey1(TKey1 key1, TKey2 key2)
{
return UpdateByKey(key1, _forwards, _reverses, key2, (k1, k2) => AddOrUpdate(k1, k2));
}
private static bool UpdateByKey<S, T>(S key1, Dictionary<S, T> forwards, Dictionary<T, S> reverses, T key2,
Action<S, T> updater)
{
T otherKey;
if (!TryGetValue(key1, forwards, out otherKey) || ContainsKey(key2, reverses))
return false;
if (!Remove(otherKey, reverses))
throw new Exception("somewhere logic went wrong and your data got corrupt");
updater(key1, key2);
return true;
}
public bool UpdateByKey2(TKey2 key2, TKey1 key1)
{
return UpdateByKey(key2, _reverses, _forwards, key1, (k1, k2) => AddOrUpdate(k2, k1));
}
public bool RemoveByKey1(TKey1 key)
{
return RemoveByKey(key, _forwards, _reverses);
}
private static bool RemoveByKey<S, T>(S key, Dictionary<S, T> keyDict, Dictionary<T, S> valueDict)
{
T otherKey;
if (!TryGetValue(key, keyDict, out otherKey))
return false;
if (!Remove(key, keyDict) || !Remove(otherKey, valueDict))
throw new Exception("somewhere logic went wrong and your data got corrupt");
return true;
}
private static bool Remove<S, T>(S key, Dictionary<S, T> dict)
{
return dict.Remove(key);
}
public bool RemoveByKey2(TKey2 key)
{
return RemoveByKey(key, _reverses, _forwards);
}
public void Clear()
{
_forwards.Clear();
_reverses.Clear();
}
public IEnumerator<Tuple<TKey1, TKey2>> GetEnumerator()
{
if (_forwards.Count != _reverses.Count)
throw new Exception("somewhere logic went wrong and your data got corrupt");
foreach (var item in _forwards)
yield return Tuple.Create(item.Key, item.Value);
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
Similar to my answer here
Few things to note:
I have implemented only IEnumerable<>. I don't think ICollection<> makes sense here since the method names all could be way different for this special collection structure. Up to you to decide what should go inside IEnumerable<>. So now you have collection initializer syntax too, like
var p = new BiDictionary<int, string> { 1, "a" }, { 2, "b" } };
I have attempted for some weird exceptions to be thrown here and there - just for data integrity. Just to be on the safer side so that you know if ever my code has bugs.
Performance: You can lookup for Value with either of the Keys, which means Get and Contains method require just 1 lookup (O(1)). Add requires 2 lookups and 2 adds. Update requires 1 lookup and 2 adds. Remove takes 3 lookups. All similar to accepted answer.

I have created such a class, using C5 collection classes.
public class Mapper<K,T> : IEnumerable<T>
{
C5.TreeDictionary<K,T> KToTMap = new TreeDictionary<K,T>();
C5.HashDictionary<T,K> TToKMap = new HashDictionary<T,K>();
/// <summary>
/// Initializes a new instance of the Mapper class.
/// </summary>
public Mapper()
{
KToTMap = new TreeDictionary<K,T>();
TToKMap = new HashDictionary<T,K>();
}
public void Add(K key, T value)
{
KToTMap.Add(key, value);
TToKMap.Add(value, key);
}
public bool ContainsKey(K key)
{
return KToTMap.Contains(key);
}
public int Count
{
get { return KToTMap.Count; }
}
public K this[T obj]
{
get
{
return TToKMap[obj];
}
}
public T this[K obj]
{
get
{
return KToTMap[obj];
}
}
public IEnumerator<T> GetEnumerator()
{
return KToTMap.Values.GetEnumerator();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return KToTMap.Values.GetEnumerator();
}
}

Another extension to the accepted answer. It implements IEnumerable so one can use foreach with that. I realize there are more answers with IEnumerable implementation but this one uses structs so it is garbage collector friendly.
This is especially usefull in Unity engine (checked with the profiler).
/// <summary>
/// This is a dictionary guaranteed to have only one of each value and key.
/// It may be searched either by TFirst or by TSecond, giving a unique answer because it is 1 to 1.
/// It implements garbage-collector-friendly IEnumerable.
/// </summary>
/// <typeparam name="TFirst">The type of the "key"</typeparam>
/// <typeparam name="TSecond">The type of the "value"</typeparam>
public class BiDictionary<TFirst, TSecond> : IEnumerable<BiDictionary<TFirst, TSecond>.Pair>
{
public struct Pair
{
public TFirst First;
public TSecond Second;
}
public struct Enumerator : IEnumerator<Pair>, IEnumerator
{
public Enumerator(Dictionary<TFirst, TSecond>.Enumerator dictEnumerator)
{
_dictEnumerator = dictEnumerator;
}
public Pair Current
{
get
{
Pair pair;
pair.First = _dictEnumerator.Current.Key;
pair.Second = _dictEnumerator.Current.Value;
return pair;
}
}
object IEnumerator.Current
{
get
{
return Current;
}
}
public void Dispose()
{
_dictEnumerator.Dispose();
}
public bool MoveNext()
{
return _dictEnumerator.MoveNext();
}
public void Reset()
{
throw new NotSupportedException();
}
private Dictionary<TFirst, TSecond>.Enumerator _dictEnumerator;
}
#region Exception throwing methods
/// <summary>
/// Tries to add the pair to the dictionary.
/// Throws an exception if either element is already in the dictionary
/// </summary>
/// <param name="first"></param>
/// <param name="second"></param>
public void Add(TFirst first, TSecond second)
{
if (_firstToSecond.ContainsKey(first) || _secondToFirst.ContainsKey(second))
throw new ArgumentException("Duplicate first or second");
_firstToSecond.Add(first, second);
_secondToFirst.Add(second, first);
}
/// <summary>
/// Find the TSecond corresponding to the TFirst first
/// Throws an exception if first is not in the dictionary.
/// </summary>
/// <param name="first">the key to search for</param>
/// <returns>the value corresponding to first</returns>
public TSecond GetByFirst(TFirst first)
{
TSecond second;
if (!_firstToSecond.TryGetValue(first, out second))
throw new ArgumentException("first");
return second;
}
/// <summary>
/// Find the TFirst corresponing to the Second second.
/// Throws an exception if second is not in the dictionary.
/// </summary>
/// <param name="second">the key to search for</param>
/// <returns>the value corresponding to second</returns>
public TFirst GetBySecond(TSecond second)
{
TFirst first;
if (!_secondToFirst.TryGetValue(second, out first))
throw new ArgumentException("second");
return first;
}
/// <summary>
/// Remove the record containing first.
/// If first is not in the dictionary, throws an Exception.
/// </summary>
/// <param name="first">the key of the record to delete</param>
public void RemoveByFirst(TFirst first)
{
TSecond second;
if (!_firstToSecond.TryGetValue(first, out second))
throw new ArgumentException("first");
_firstToSecond.Remove(first);
_secondToFirst.Remove(second);
}
/// <summary>
/// Remove the record containing second.
/// If second is not in the dictionary, throws an Exception.
/// </summary>
/// <param name="second">the key of the record to delete</param>
public void RemoveBySecond(TSecond second)
{
TFirst first;
if (!_secondToFirst.TryGetValue(second, out first))
throw new ArgumentException("second");
_secondToFirst.Remove(second);
_firstToSecond.Remove(first);
}
#endregion
#region Try methods
/// <summary>
/// Tries to add the pair to the dictionary.
/// Returns false if either element is already in the dictionary
/// </summary>
/// <param name="first"></param>
/// <param name="second"></param>
/// <returns>true if successfully added, false if either element are already in the dictionary</returns>
public bool TryAdd(TFirst first, TSecond second)
{
if (_firstToSecond.ContainsKey(first) || _secondToFirst.ContainsKey(second))
return false;
_firstToSecond.Add(first, second);
_secondToFirst.Add(second, first);
return true;
}
/// <summary>
/// Find the TSecond corresponding to the TFirst first.
/// Returns false if first is not in the dictionary.
/// </summary>
/// <param name="first">the key to search for</param>
/// <param name="second">the corresponding value</param>
/// <returns>true if first is in the dictionary, false otherwise</returns>
public bool TryGetByFirst(TFirst first, out TSecond second)
{
return _firstToSecond.TryGetValue(first, out second);
}
/// <summary>
/// Find the TFirst corresponding to the TSecond second.
/// Returns false if second is not in the dictionary.
/// </summary>
/// <param name="second">the key to search for</param>
/// <param name="first">the corresponding value</param>
/// <returns>true if second is in the dictionary, false otherwise</returns>
public bool TryGetBySecond(TSecond second, out TFirst first)
{
return _secondToFirst.TryGetValue(second, out first);
}
/// <summary>
/// Remove the record containing first, if there is one.
/// </summary>
/// <param name="first"></param>
/// <returns> If first is not in the dictionary, returns false, otherwise true</returns>
public bool TryRemoveByFirst(TFirst first)
{
TSecond second;
if (!_firstToSecond.TryGetValue(first, out second))
return false;
_firstToSecond.Remove(first);
_secondToFirst.Remove(second);
return true;
}
/// <summary>
/// Remove the record containing second, if there is one.
/// </summary>
/// <param name="second"></param>
/// <returns> If second is not in the dictionary, returns false, otherwise true</returns>
public bool TryRemoveBySecond(TSecond second)
{
TFirst first;
if (!_secondToFirst.TryGetValue(second, out first))
return false;
_secondToFirst.Remove(second);
_firstToSecond.Remove(first);
return true;
}
#endregion
/// <summary>
/// The number of pairs stored in the dictionary
/// </summary>
public Int32 Count
{
get { return _firstToSecond.Count; }
}
/// <summary>
/// Removes all items from the dictionary.
/// </summary>
public void Clear()
{
_firstToSecond.Clear();
_secondToFirst.Clear();
}
public Enumerator GetEnumerator()
{
//enumerator.Reset(firstToSecond.GetEnumerator());
return new Enumerator(_firstToSecond.GetEnumerator());
}
IEnumerator<Pair> IEnumerable<Pair>.GetEnumerator()
{
return GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
private Dictionary<TFirst, TSecond> _firstToSecond = new Dictionary<TFirst, TSecond>();
private Dictionary<TSecond, TFirst> _secondToFirst = new Dictionary<TSecond, TFirst>();
}

A bit late, but here's an implementation I wrote a while back. It handles a few interesting edge cases, such as when the key overrides the equality check to perform partial equality. This results in the main dictionary storing A => 1 but the inverse storing 1 => A'.
You access the inverse dictionary via the Inverse property.
var map = new BidirectionalDictionary<int, int>();
map.Add(1, 2);
var result = map.Inverse[2]; // result is 1
//
// BidirectionalDictionary.cs
//
// Author:
// Chris Chilvers <chilversc#googlemail.com>
//
// Copyright (c) 2009 Chris Chilvers
//
// Permission is hereby granted, free of charge, to any person obtaining
// a copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//
using System;
using System.Collections;
using System.Collections.Generic;
namespace Cadenza.Collections
{
public class BidirectionalDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
private readonly IEqualityComparer<TKey> keyComparer;
private readonly IEqualityComparer<TValue> valueComparer;
private readonly Dictionary<TKey, TValue> keysToValues;
private readonly Dictionary<TValue, TKey> valuesToKeys;
private readonly BidirectionalDictionary<TValue, TKey> inverse;
public BidirectionalDictionary () : this (10, null, null) {}
public BidirectionalDictionary (int capacity) : this (capacity, null, null) {}
public BidirectionalDictionary (IEqualityComparer<TKey> keyComparer, IEqualityComparer<TValue> valueComparer)
: this (10, keyComparer, valueComparer)
{
}
public BidirectionalDictionary (int capacity, IEqualityComparer<TKey> keyComparer, IEqualityComparer<TValue> valueComparer)
{
if (capacity < 0)
throw new ArgumentOutOfRangeException ("capacity", capacity, "capacity cannot be less than 0");
this.keyComparer = keyComparer ?? EqualityComparer<TKey>.Default;
this.valueComparer = valueComparer ?? EqualityComparer<TValue>.Default;
keysToValues = new Dictionary<TKey, TValue> (capacity, this.keyComparer);
valuesToKeys = new Dictionary<TValue, TKey> (capacity, this.valueComparer);
inverse = new BidirectionalDictionary<TValue, TKey> (this);
}
private BidirectionalDictionary (BidirectionalDictionary<TValue, TKey> inverse)
{
this.inverse = inverse;
keyComparer = inverse.valueComparer;
valueComparer = inverse.keyComparer;
valuesToKeys = inverse.keysToValues;
keysToValues = inverse.valuesToKeys;
}
public BidirectionalDictionary<TValue, TKey> Inverse {
get { return inverse; }
}
public ICollection<TKey> Keys {
get { return keysToValues.Keys; }
}
public ICollection<TValue> Values {
get { return keysToValues.Values; }
}
public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator ()
{
return keysToValues.GetEnumerator ();
}
IEnumerator IEnumerable.GetEnumerator ()
{
return GetEnumerator ();
}
void ICollection<KeyValuePair<TKey, TValue>>.CopyTo (KeyValuePair<TKey, TValue>[] array, int arrayIndex)
{
((ICollection<KeyValuePair<TKey, TValue>>) keysToValues).CopyTo (array, arrayIndex);
}
public bool ContainsKey (TKey key)
{
if (key == null)
throw new ArgumentNullException ("key");
return keysToValues.ContainsKey (key);
}
public bool ContainsValue (TValue value)
{
if (value == null)
throw new ArgumentNullException ("value");
return valuesToKeys.ContainsKey (value);
}
bool ICollection<KeyValuePair<TKey, TValue>>.Contains (KeyValuePair<TKey, TValue> item)
{
return ((ICollection<KeyValuePair<TKey, TValue>>) keysToValues).Contains (item);
}
public bool TryGetKey (TValue value, out TKey key)
{
if (value == null)
throw new ArgumentNullException ("value");
return valuesToKeys.TryGetValue (value, out key);
}
public bool TryGetValue (TKey key, out TValue value)
{
if (key == null)
throw new ArgumentNullException ("key");
return keysToValues.TryGetValue (key, out value);
}
public TValue this[TKey key] {
get { return keysToValues [key]; }
set {
if (key == null)
throw new ArgumentNullException ("key");
if (value == null)
throw new ArgumentNullException ("value");
//foo[5] = "bar"; foo[6] = "bar"; should not be valid
//as it would have to remove foo[5], which is unexpected.
if (ValueBelongsToOtherKey (key, value))
throw new ArgumentException ("Value already exists", "value");
TValue oldValue;
if (keysToValues.TryGetValue (key, out oldValue)) {
// Use the current key for this value to stay consistent
// with Dictionary<TKey, TValue> which does not alter
// the key if it exists.
TKey oldKey = valuesToKeys [oldValue];
keysToValues [oldKey] = value;
valuesToKeys.Remove (oldValue);
valuesToKeys [value] = oldKey;
} else {
keysToValues [key] = value;
valuesToKeys [value] = key;
}
}
}
public int Count {
get { return keysToValues.Count; }
}
bool ICollection<KeyValuePair<TKey, TValue>>.IsReadOnly {
get { return false; }
}
public void Add (TKey key, TValue value)
{
if (key == null)
throw new ArgumentNullException ("key");
if (value == null)
throw new ArgumentNullException ("value");
if (keysToValues.ContainsKey (key))
throw new ArgumentException ("Key already exists", "key");
if (valuesToKeys.ContainsKey (value))
throw new ArgumentException ("Value already exists", "value");
keysToValues.Add (key, value);
valuesToKeys.Add (value, key);
}
public void Replace (TKey key, TValue value)
{
if (key == null)
throw new ArgumentNullException ("key");
if (value == null)
throw new ArgumentNullException ("value");
// replaces a key value pair, if the key or value already exists those mappings will be replaced.
// e.g. you have; a -> b, b -> a; c -> d, d -> c
// you add the mapping; a -> d, d -> a
// this will remove both of the original mappings
Remove (key);
inverse.Remove (value);
Add (key, value);
}
void ICollection<KeyValuePair<TKey, TValue>>.Add (KeyValuePair<TKey, TValue> item)
{
Add (item.Key, item.Value);
}
public bool Remove (TKey key)
{
if (key == null)
throw new ArgumentNullException ("key");
TValue value;
if (keysToValues.TryGetValue (key, out value)) {
keysToValues.Remove (key);
valuesToKeys.Remove (value);
return true;
}
else {
return false;
}
}
bool ICollection<KeyValuePair<TKey, TValue>>.Remove (KeyValuePair<TKey, TValue> item)
{
bool removed = ((ICollection<KeyValuePair<TKey, TValue>>) keysToValues).Remove (item);
if (removed)
valuesToKeys.Remove (item.Value);
return removed;
}
public void Clear ()
{
keysToValues.Clear ();
valuesToKeys.Clear ();
}
private bool ValueBelongsToOtherKey (TKey key, TValue value)
{
TKey otherKey;
if (valuesToKeys.TryGetValue (value, out otherKey))
// if the keys are not equal the value belongs to another key
return !keyComparer.Equals (key, otherKey);
else
// value doesn't exist in map, thus it cannot belong to another key
return false;
}
}
}
Original source and tests on github.

c# dictionary How to add multiple values for single key?

I have created dictionary object
Dictionary<string, List<string>> dictionary =
new Dictionary<string,List<string>>();
I want to add string values to the list of string for a given single key.
If the key doesn't already exists then I have to add a new key. List<string> is not predefined, I mean I didn't create any list object and then supplied to dictionary.Add("key",Listname). How to create dynamically this list object in dictionary.Add("key",Listname) and then add strings to this list. If I have to add 100 keys then do I have to create 100 lists before executing dictionary.Add instruction and also do I have to pedefine the contents of this lists ?
Thank you.

Update: check for existence using TryGetValue to do only one lookup in the case where you have the list:
List<int> list;
if (!dictionary.TryGetValue("foo", out list))
{
list = new List<int>();
dictionary.Add("foo", list);
}
list.Add(2);
Original:
Check for existence and add once, then key into the dictionary to get the list and add to the list as normal:
var dictionary = new Dictionary<string, List<int>>();
if (!dictionary.ContainsKey("foo"))
dictionary.Add("foo", new List<int>());
dictionary["foo"].Add(42);
dictionary["foo"].AddRange(oneHundredInts);
Or List<string> as in your case.
As an aside, if you know how many items you are going to add to a dynamic collection such as List<T>, favour the constructor that takes the initial list capacity: new List<int>(100);.
This will grab the memory required to satisfy the specified capacity upfront, instead of grabbing small chunks every time it starts to fill up. You can do the same with dictionaries if you know you have 100 keys.

If I understood what you want:
dictionary.Add("key", new List<string>());
later...
dictionary["key"].Add("string to your list");

Dictionary<string, List<string>> dictionary = new Dictionary<string,List<string>>();
foreach(string key in keys) {
if(!dictionary.ContainsKey(key)) {
//add
dictionary.Add(key, new List<string>());
}
dictionary[key].Add("theString");
}
If the key doesn't exist, a new List is added (inside if). Else the key exists, so just add a new value to the List under that key.

You could use my implementation of a multimap, which derives from a Dictionary<K, List<V>>. It is not perfect, however it does a good job.
/// <summary>
/// Represents a collection of keys and values.
/// Multiple values can have the same key.
/// </summary>
/// <typeparam name="TKey">Type of the keys.</typeparam>
/// <typeparam name="TValue">Type of the values.</typeparam>
public class MultiMap<TKey, TValue> : Dictionary<TKey, List<TValue>>
{
public MultiMap()
: base()
{
}
public MultiMap(int capacity)
: base(capacity)
{
}
/// <summary>
/// Adds an element with the specified key and value into the MultiMap.
/// </summary>
/// <param name="key">The key of the element to add.</param>
/// <param name="value">The value of the element to add.</param>
public void Add(TKey key, TValue value)
{
List<TValue> valueList;
if (TryGetValue(key, out valueList)) {
valueList.Add(value);
} else {
valueList = new List<TValue>();
valueList.Add(value);
Add(key, valueList);
}
}
/// <summary>
/// Removes first occurence of an element with a specified key and value.
/// </summary>
/// <param name="key">The key of the element to remove.</param>
/// <param name="value">The value of the element to remove.</param>
/// <returns>true if the an element is removed;
/// false if the key or the value were not found.</returns>
public bool Remove(TKey key, TValue value)
{
List<TValue> valueList;
if (TryGetValue(key, out valueList)) {
if (valueList.Remove(value)) {
if (valueList.Count == 0) {
Remove(key);
}
return true;
}
}
return false;
}
/// <summary>
/// Removes all occurences of elements with a specified key and value.
/// </summary>
/// <param name="key">The key of the elements to remove.</param>
/// <param name="value">The value of the elements to remove.</param>
/// <returns>Number of elements removed.</returns>
public int RemoveAll(TKey key, TValue value)
{
List<TValue> valueList;
int n = 0;
if (TryGetValue(key, out valueList)) {
while (valueList.Remove(value)) {
n++;
}
if (valueList.Count == 0) {
Remove(key);
}
}
return n;
}
/// <summary>
/// Gets the total number of values contained in the MultiMap.
/// </summary>
public int CountAll
{
get
{
int n = 0;
foreach (List<TValue> valueList in Values) {
n += valueList.Count;
}
return n;
}
}
/// <summary>
/// Determines whether the MultiMap contains an element with a specific
/// key / value pair.
/// </summary>
/// <param name="key">Key of the element to search for.</param>
/// <param name="value">Value of the element to search for.</param>
/// <returns>true if the element was found; otherwise false.</returns>
public bool Contains(TKey key, TValue value)
{
List<TValue> valueList;
if (TryGetValue(key, out valueList)) {
return valueList.Contains(value);
}
return false;
}
/// <summary>
/// Determines whether the MultiMap contains an element with a specific value.
/// </summary>
/// <param name="value">Value of the element to search for.</param>
/// <returns>true if the element was found; otherwise false.</returns>
public bool Contains(TValue value)
{
foreach (List<TValue> valueList in Values) {
if (valueList.Contains(value)) {
return true;
}
}
return false;
}
}
Note that the Add method looks if a key is already present. If the key is new, a new list is created, the value is added to the list and the list is added to the dictionary. If the key was already present, the new value is added to the existing list.

Use NameValuedCollection.
Good starting point is here. Straight from the link.
System.Collections.Specialized.NameValueCollection myCollection
= new System.Collections.Specialized.NameValueCollection();
myCollection.Add(“Arcane”, “http://arcanecode.com”);
myCollection.Add(“PWOP”, “http://dotnetrocks.com”);
myCollection.Add(“PWOP”, “http://dnrtv.com”);
myCollection.Add(“PWOP”, “http://www.hanselminutes.com”);
myCollection.Add(“TWIT”, “http://www.twit.tv”);
myCollection.Add(“TWIT”, “http://www.twit.tv/SN”);

Though nearly the same as most of the other responses, I think this is the most efficient and concise way to implement it. Using TryGetValue is faster than using ContainsKey and reindexing into the dictionary as some other solutions have shown.
void Add(string key, string val)
{
List<string> list;
if (!dictionary.TryGetValue(someKey, out list))
{
values = new List<string>();
dictionary.Add(key, list);
}
list.Add(val);
}

When you add a string, do it differently depending on whether the key exists already or not. To add the string value for the key key:
List<string> list;
if (dictionary.ContainsKey(key)) {
list = dictionary[key];
} else {
list = new List<string>();
dictionary.Add(ley, list);
}
list.Add(value);

Instead of using a Dictionary, why not convert to an ILookup?
var myData = new[]{new {a=1,b="frog"}, new {a=1,b="cat"}, new {a=2,b="giraffe"}};
ILookup<int,string> lookup = myData.ToLookup(x => x.a, x => x.b);
IEnumerable<string> allOnes = lookup[1]; //enumerable of 2 items, frog and cat
An ILookup is an immutable data structure that allows multiple values per key. Probably not much use if you need to add items at different times, but if you have all your data up-front, this is definitely the way to go.

Here are many variations of the one answer :) My is another one and it uses extension mechanism as comfortable way to execute (handy):
public static void AddToList<T, U>(this IDictionary<T, List<U>> dict, T key, U elementToList)
{
List<U> list;
bool exists = dict.TryGetValue(key, out list);
if (exists)
{
dict[key].Add(elementToList);
}
else
{
dict[key] = new List<U>();
dict[key].Add(elementToList);
}
}
Then you use it as follows:
Dictionary<int, List<string>> dict = new Dictionary<int, List<string>>();
dict.AddToList(4, "test1");
dict.AddToList(4, "test2");
dict.AddToList(4, "test3");
dict.AddToList(5, "test4");

There is a NuGet package Microsoft Experimental Collections that contains a class MultiValueDictionary which does exactly what you need.
Here is a blog post of the creator of the package that describes it further.
Here is another blog post if you're feeling curious.
Example Usage:
MultiDictionary<string, int> myDictionary = new MultiDictionary<string, int>();
myDictionary.Add("key", 1);
myDictionary.Add("key", 2);
myDictionary.Add("key", 3);
//myDictionary["key"] now contains the values 1, 2, and 3

I was trying to add List to existing key in dictionary and reached the following solution:
Dictionary<string,List<string>> NewParent = new Dictionary<string,List<string>>();
child = new List<string> ();
child.Add('SomeData');
NewParent["item1"].AddRange(child);
It will not show any exception and won't replace previous values.

There is a 'one-command-line' way to do this using AddOrUpdate from ConcurrentDictionary:
using System.Linq;
using System.Collections.Generic;
using System.Collections.Concurrent;
...
var dictionary = new ConcurrentDictionary<string, IEnumerable<string>>();
var itemToAdd = "item to add to key-list";
dictionary.AddOrUpdate("key", new[]{itemToAdd}, (key,list) => list.Append(itemToAdd));
// If "key" doesn't exist, creates it with a list containing itemToAdd as value
// If "key" exists, adds item to already existent list (third parameter)

I'm improving on this answer with a couple of extension methods I've wrote. The first one is similar to how #Bronek wrote it, just a bit more concise. Simply put, if a key exists, it inserts into the already existing list (assuming it was initialized to begin with). Otherwise, it adds to the list.
public static void AddToList<K, V>(this Dictionary<K, List<V>> multiValueDictionary,
K key,
V value)
{
if (multiValueDictionary.TryGetValue(key, out List<V> lst))
lst.Add(value);
else
multiValueDictionary.Add(key, new List<V> { value });
}
This second function builds off the previous. In System.Linq, the extension method ToDictionary which can transform any IEnumerable into a Dictionary. But what if you have the above scenario where you want to store multiple values for a single key? Well then the below extension will work!
public static Dictionary<K, List<V>> ToDictionaryValueList<T, K, V>(this IEnumerable<T> values, Func<T, K> keySelector, Func<T, V> elementSelector)
{
var tmp = new Dictionary<K, List<V>>();
foreach (var val in values)
tmp.AddToList(keySelector(val), elementSelector(val));
return tmp;
// NOTE: You can also use the below IEnumerable extensions to accomplish the same goal, but the above might be (?) more efficient, not sure
// return values
// .GroupBy(v => keySelector(v))
// .ToDictionary(v => v.Key, v => v.ToList());
}
With the above two, its now easy to transform any IEnumerable into one of these dictionaries.
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
}
List<Person> test = new List<Person>
{
new Person { Name = "Bob", Age = 22 },
new Person { Name = "Bob", Age = 28 },
new Person { Name = "Sally", Age = 22 },
new Person { Name = "Sally", Age = 22 },
new Person { Name = "Jill", Age = 22 },
}
// Aggregate each person
Dictionary<string, List<int>> results = test.ToDictionaryValueList((p) => p.Name, (p) => p.Age);
// Use the AddToList extension method to add more values as neeeded
results.AddToList("Jill", 23);
One thing to consider is that duplicate values aren't handled, as expected from a standard List. You'll want to make extension methods for different collections like HashSet if you need it.

Can I use Linq's Except() with a lambda expression comparer?

I know I can call linq's Except and specify a custom IEqualityComparer, but implementing a new Comparer class for each data type seems like an overkill for this purpose. Can I use a lambda expression to provide the equality function, like when I use Where or other LINQ functions?
If I can't, is there an alternative?

For any one still looking; here's another way of implementing a custom lambda comparer.
public class LambdaComparer<T> : IEqualityComparer<T>
{
private readonly Func<T, T, bool> _expression;
public LambdaComparer(Func<T, T, bool> lambda)
{
_expression = lambda;
}
public bool Equals(T x, T y)
{
return _expression(x, y);
}
public int GetHashCode(T obj)
{
/*
If you just return 0 for the hash the Equals comparer will kick in.
The underlying evaluation checks the hash and then short circuits the evaluation if it is false.
Otherwise, it checks the Equals. If you force the hash to be true (by assuming 0 for both objects),
you will always fall through to the Equals check which is what we are always going for.
*/
return 0;
}
}
you can then create an extension for the linq Except an Intersect that take in lambda's
/// <summary>
/// Returns all items in the first collection except the ones in the second collection that match the lambda condition
/// </summary>
/// <typeparam name="T">The type</typeparam>
/// <param name="listA">The first list</param>
/// <param name="listB">The second list</param>
/// <param name="lambda">The filter expression</param>
/// <returns>The filtered list</returns>
public static IEnumerable<T> Except<T>(this IEnumerable<T> listA, IEnumerable<T> listB, Func<T, T, bool> lambda)
{
return listA.Except(listB, new LambdaComparer<T>(lambda));
}
/// <summary>
/// Returns all items in the first collection that intersect the ones in the second collection that match the lambda condition
/// </summary>
/// <typeparam name="T">The type</typeparam>
/// <param name="listA">The first list</param>
/// <param name="listB">The second list</param>
/// <param name="lambda">The filter expression</param>
/// <returns>The filtered list</returns>
public static IEnumerable<T> Intersect<T>(this IEnumerable<T> listA, IEnumerable<T> listB, Func<T, T, bool> lambda)
{
return listA.Intersect(listB, new LambdaComparer<T>(lambda));
}
Usage:
var availableItems = allItems.Except(filterItems, (p, p1) => p.Id== p1.Id);

Can you not use a .Where with a lambda that filters out your required values?
Example as requested:
static void Main(string[] args)
{
var firstCustomers = new[] { new Customer { Id = 1, Name = "Bob" }, new Customer { Id = 2, Name = "Steve" } };
var secondCustomers = new[] { new Customer { Id = 2, Name = "Steve" }, new Customer { Id = 3, Name = "John" } };
var customers = secondCustomers.Where(c => !firstCustomers.Select(fc => fc.Id).Contains(c.Id));
}
public class Customer
{
public int Id { get; set; }
public string Name { get; set; }
}

I don't think you can directly with the basic LINQ interfaces, but I've seen people implement a LambdaComparer class with extension methods which will help you do it.
Here's an example

Here's something simple I whipped up:
public class CustomComparer<TSource, TCompareType> : IEqualityComparer<TSource> where TSource : class
{
private readonly Func<TSource, TCompareType> getComparisonObject;
public CustomComparer(Func<TSource,TCompareType> getComparisonObject)
{
if (getComparisonObject == null) throw new ArgumentNullException("getComparisonObject");
this.getComparisonObject = getComparisonObject;
}
/// <summary>
/// Determines whether the specified objects are equal.
/// </summary>
/// <returns>
/// true if the specified objects are equal; otherwise, false.
/// </returns>
/// <param name="x">The first object of type <paramref name="T"/> to compare.
/// </param><param name="y">The second object of type <paramref name="T"/> to compare.
/// </param>
public bool Equals(TSource x, TSource y)
{
if (x == null)
{
return (y == null);
}
else if (y == null)
{
return false;
}
return EqualityComparer<TCompareType>.Default.Equals(getComparisonObject(x), getComparisonObject(y));
}
/// <summary>
/// Returns a hash code for the specified object.
/// </summary>
/// <returns>
/// A hash code for the specified object.
/// </returns>
/// <param name="obj">The <see cref="T:System.Object"/> for which a hash code is to be returned.
/// </param><exception cref="T:System.ArgumentNullException">The type of <paramref name="obj"/> is a reference type and <paramref name="obj"/> is null.
/// </exception>
public int GetHashCode(TSource obj)
{
return EqualityComparer<TCompareType>.Default.GetHashCode(getComparisonObject(obj));
}
}
Usage:
var myItems = allItems.Except(theirItems, new CustomComparer(item => item.Name));

Use an extension!
public static IEnumerable<T> Except<T, TKey>(this IEnumerable<T> items, IEnumerable<T> other,
Func<T, TKey> getKey)
{
return from item in items
join otherItem in other on getKey(item)
equals getKey(otherItem) into tempItems
from temp in tempItems.DefaultIfEmpty()
where ReferenceEquals(null, temp) || temp.Equals(default(T))
select item;
}
Source

Here' is l except r solution, based on a LINQ outer join technique:
from l in new[] { 1, 2, 3 }
join r in new[] { 2, 4, 5 }
on l equals r
into rr
where !rr.Any()
select l
Will yield: 1, 3

But you may try this. This is so much easy, I think code contains errors. Of course, the code is for small amounts, without LINQ translates to db etc.
public static IEnumerable<TSource> ExceptPredicate<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, Func<TSource, TSource, bool> compare) {
foreach (var itmFirst in first) {
if (!second.Any(itmsecond => compare(itmFirst, itmsecond))) {
yield return itmFirst;
}
}
yield break;
}

What is the generic version of a Hashtable?

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.

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.

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);
}

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;
}

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<TKey, TValue>"</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<TKey, TValue>"</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

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