Dictionaries in .Net

Namespace: System.Collections

Dictionaries are meant to store data in Key/Value pair.

Dictionary class supports the Key/Value pair collection.

Most basic Implementation is HashTable.

HashTable h = new HashTable();

h.add(“mubbasher@ymail.com”, “Mubbasher Mukhtar”);

h[“mubbasher@ymail.com”] = “Mukhtar, Mubbasher”;

Console.WriteLine(h[“mubbasher@ymail.com”]);

To iterate items in Dictionary:

foreach(DictionaryEntery entry in h)

                Console.WriteLine( entry.Value);

All Dictionary classes inherit from IDictionary interface. And intern IDictionary interface inherits from ICollection Interface.

Inheritance Sequence:

IEnumerator  ⇨ IEnumerable ⇨ ICollection ⇨ IDictionary

Important properties of the IDictionary interface:

Name	Description
Count	Gets the number of elements contained in the ICollection. (Inherited from ICollection.)
IsFixedSize	Gets a value indicating whether the IDictionary object has a fixed size.
IsReadOnly	Gets a value indicating whether the IDictionary object is read-only.
IsSynchronized	Gets a value indicating whether access to the ICollection is synchronized (thread safe). (Inherited from ICollection.)
Item	Gets or sets the element with the specified key.
Keys	Gets an ICollection object containing the keys of the IDictionary object.
SyncRoot	Gets an object that can be used to synchronize access to the ICollection. (Inherited from ICollection.)
Values	Gets an ICollection object containing the values in the IDictionary object.

IDictionary Methods:

Name	Description
Add	Adds an element with the provided key and value to the IDictionary object.
Clear	Removes all elements from the IDictionary object.
Contains	Determines whether the IDictionary object contains an element with the specified key.
CopyTo	Copies the elements of the ICollection to an Array, starting at a particular Array index. (Inherited from ICollection.)
GetEnumerator	Returns an IDictionaryEnumerator object for the IDictionary object.
GetEnumerator	Returns an enumerator that iterates through a collection. (Inherited from IEnumerable.)
Remove	Removes the element with the specified key from the IDictionary object.

HashTable also supports the following two methods:

ContainsKey	Determines whether the Hashtable contains a specific key.
ContainsValue	Determines whether the Hashtable contains a specific value.

Understanding Equality:

HashTable is a special type of Dictionary class that uses an integral value (a hash value) as its key. HahsTable uses hash value to speedup searching. Hash value is obtained by the GetHash() method of the object that return a unique hash value identifying the object.

If one is using an object as a key for the HashTable, Then one must override the GetHashCode() method that will return a Hash code to be used as key value.

public class Fish
{
    public string fname;
    public Fish(string pFishName)
    {
        fname=pFishName;
    }
   
    public override int GetHashCode()
    {
        return fname.GetHashCode();
    }
}

Fish fishA = new Fish(“blue fish”);
Fish fishB = new Fish(“blue fish”);
Hashtable[fishA] = “New fish”;
Hashtable[fishB] = “New fishB”;

But the Hashtable.Count() will return two; even we have overridden the GetHashCode method. We will also have to override the Equals method to uniquely identify the object.

public override bool Equals(object obj)
{
        return fname == ((Fish)obj).fname;
}

For Uniqueness of a key for a hash table, Hash code and Instances should not be equal.

Using IEqualtyComparer Interface:

                HashTable supports a constructor that accepts a IEqualityComparer interface. IEqualityComparaer contains two methods, GetHashCode and Equals. Following CaseInsensitveComparer can be used for HashTables to make case insensitive comparisons.

public class InsensitiveComparer: IEqualityComparer

{

                public int GetHashCode(object obj)

                {

                                return obj.ToString().ToLowerInverient().GetHashCode();

                }

                public new bool Equals(object a, object b)

                {

return a.ToString().ToLowerInvarient() ==  b.ToString().ToLowerInvarient();

                }

}

SortedList:

HashTable displays items by the order of HashCode. SortedList automatically sorts items on basis of key. When items are being added they are automatically sorted. It inherits from IDictionary.

Default Sort method used by SortedList sorts items in Ascending order. One can specify his own IComparer interface in one of the SortedList constructor to specify different sorting order.

SortedList Members:

Name	Description
Capacity	Gets or sets the capacity of a SortedList object.
Count	Gets the number of elements contained in a SortedList object.
IsFixedSize	Gets a value indicating whether a SortedList object has a fixed size.
IsReadOnly	Gets a value indicating whether a SortedList object is read-only.
IsSynchronized	Gets a value indicating whether access to a SortedList object is synchronized (thread safe).
Item	Gets and sets the value associated with a specific key in a SortedList object.
Keys	Gets the keys in a SortedList object.
SyncRoot	Gets an object that can be used to synchronize access to a SortedList object.
Values	Gets the values in a SortedList object.

SortedList Methods:

Name	Description
Contains	Determines whether a SortedList object contains a specific key.
ContainsKey	Determines whether a SortedList object contains a specific key.
ContainsValue	Determines whether a SortedList object contains a specific value.
GetByIndex	Gets the value at the specified index of a SortedList object.
GetKey	Gets the key at the specified index of a SortedList object.
GetKeyList	Gets the keys in a SortedList object.
GetValueList	Gets the values in a SortedList object.
IndexOfKey	Returns the zero-based index of the specified key in a SortedList object.
IndexOfValue	Returns the zero-based index of the first occurrence of the specified value in a SortedList object.
Remove	Removes the element with the specified key from a SortedList object.
RemoveAt	Removes the element at the specified index of a SortedList object.
SetByIndex	Replaces the value at a specific index in a SortedList object.
Synchronized	Returns a synchronized (thread-safe) wrapper for a SortedList object.
TrimToSize	Sets the capacity to the actual number of elements in a SortedList object.

List Dictionary:

At times we have smaller collection of nearly ten items, where HashTable have a bit overhead in performance. To bridge the gap .Net framework provides ListDictionary that have the same methods and Properties as HashTable but saves items in an array internally.

HybridDictionary:

At times we don’t know how large a collection could be, So we can use HybridDictionary, that stores items in an array initially and when collection, it automatically converts the collection into a HashTable. It also provides the same methods and properties as HashTable.

OrderedDictionary:

                It is much like the HashTable, but keeps the items in the order they are being added.

Extra properties provided by OrderedDictionary are:

Item: Access items by index.

Methods

Insert: Inserts Key/value pair at a specific location.

RemoveAt: Removes Item from a specific location.