What are Special (Specialized) Collections in .Net?

The System.Collections.Specialized namespace contains specialized and strongly-typed collections. Specialized Collections are the customized .Net collection classes which are used for specific purposes.

Four most important special collection classes which are commonly used are as follows:

1. CollectionsUtil – CollectionsUtil class helps to Creates collections that ignore the case in strings. The methods of the CollectionsUtil class generate a case-insensitive instance of the collection using case-insensitive implementations of the hash code provider and the comparer. The resulting instance can be used like any other instances of that class, although it may behave differently.
   Example:

Hashtable ObjHash = CollectionsUtil.CreateCaseInsensitiveHashtable();
ObjHash.Add("mohan","he is a software developer");
string str = (string) ObjHash["MOHAN"];
MessageBox.Show(str);

2. ListDictionary – ListDictionary implements IDictionary by using a singly linked list. It is smaller and faster than a Hashtable if the number of elements is 10 or less. So, it is recommended for collections that typically contain 10 items or less.
   Example:

ListDictionary ObjDic = new ListDictionary();
ObjDic.Add("manoj", "he is a software developer");
ObjDic.Add("ramesh", "he is a software developer");

3. HybridDictionary – HybridDictionary implements IDictionary by using a ListDictionary while the collection is small, and then switching to a Hashtable when the collection gets large. It should be used in cases where the number of elements in a dictionary is unknown.
   If the initial size of the collection is greater than the optimal size for a ListDictionary, the collection is stored in a Hashtable right away to avoid the overhead of copying elements from the ListDictionary to a Hashtable.
   Example:

HybridDictionary ObjHybrid = new HybridDictionary();
ObjHybrid.Add("mohan", "he is a software developer");
ObjDic.Add("Raja", "he is a network administrator");
ObjDic.Add("ramesh", "he is a hardware engineer");

4. StringCollection – StringCollection is a very special collection, which represents a collection of strings. StringCollection accepts a null reference (Nothing in Visual Basic) as a valid value and allows duplicate elements. String comparisons are case-sensitive. Indexes in this collection are zero-based.
   

   Example:
   
   // Creates and initializes a new StringCollection.

   StringCollection myCol = new StringCollection();
   
   // Adds a range of elements from an array to the end of the StringCollection.
   String[] myArr = new String[] { "RED", "orange", "yellow", "RED", "green", "blue", "RED", "indigo", "violet", "RED" };
   myCol.AddRange( myArr );
   
   // Adds one element to the end of the StringCollection and inserts another at index 3.
   myCol.Add( "* white" );
   myCol.Insert( 3, "* gray" );
   
   // Removes one element from the StringCollection.
   myCol.Remove( "yellow" );
   
   // Clears the entire collection.
   myCol.Clear();