Generic.List

This type of collection represents a collection on keys and values
provides access by index numbers

System.Collections.Generic.List 

Fixed Size

Allows modification of elements but does not allow the addition or removal of elements.

mycollection = new List<string>(3); 
mycollection.Add("one"); 
mycollection.Add("two"); 
mycollection.Add("three"); 

IsFixed property

IsReadOnly property

Variable Size

Allows addition, removal and modification of elements.

Where

GenericList.Add = how to check of its already been added ? 

Distinct

GenericList.Distinct.ToList 

ToArray