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Generics are new feature provided with version 2.0 of the Microsoft.Net framework. Generic classes and methods combine re-usability, type safety and efficiency in a way that there non-generics counterparts do not/cannot.
In this part some features of ArrayLists and there shortcomings/limitations will be discussed. The code in this article has been written in Visual Studio 2005 in VB.Net.
Earlier ArrayLists used to server the purpose, but that in a certain limit. Moreover, using ArrayLists to store different types came as a good performance cost. At the client end it shows our desired type that we are storing in the ArrayList but internally there's much more that goes on.
Sample the code below:-
Dim myList As New System.Collections.ArrayList()
myList.Add(22)
myList.Add("VB.Net Generics")
myList.Add(22.45)
The usage convenience that is perceived above comes at a cost. Any reference or value type that is stored or added in the ArrayList (myList) is implicitly upcasted to System.Object type. And while retrieval, the reverse happens - downcasting to the appropriate type takes place.
Moreover, there is compromise on the Type Safety front also. Consider the code below:-
Dim item As Integer = 0
'This will cause an InvalidCastException
Dim x As Integer
For Each x In myList
Item = item + x
Next x
The reason is evident.
In .Net 1.x type-safety was achieved by writing your own typed ArrayList. But again, in that case re-usability was a major issue. Consider the following code:-
Imports System
Imports System.Collections
Imports System.Collections.Generic
Imports System.Text
Namespace ArrayListSample
Class Person
Private Name As [String]
Private Age As Int32
Private Address As [String]
Public Sub New()
End Sub 'New
Public Sub New(ByVal Name As [String], ByVal Age As Int32, ByVal Address As [String])
Name = Name
Age = Age
Address = Address
End Sub 'New
Public Sub New(ByVal Name As [String], ByVal Age As Int32)
Name = Name
Age = Age
Address = [String].Empty
End Sub 'New
Public Sub New(ByVal Name As [String], ByVal Address As [String])
Name = Name
Age = 0
Address = Address
End Sub 'New
Public Sub New(ByVal Name As [String])
Name = Name
Age = 0
Address = [String].Empty
End Sub 'New
End Class 'Person
Class PeopleCollection
Inherits System.Collections.IEnumerable
Private arPeople As New ArrayList()
Public Sub New()
End Sub 'New
Public Sub AddPeople(ByVal p As Person)
arPeople.Add(p)
End Sub 'AddPeople
Public Sub ClearPeople()
arPeople.Clear()
End Sub 'ClearPeople
Private Count As Integer
Public ReadOnly Property PeopleCount() As Integer
Get
Return Count
End Get
End Property
Public Function GetEnumerator() As System.Collections.IEnumerator
Return arPeople.GetEnumerator()
End Function 'GetEnumerator
End Class 'PeopleCollection
Class Client
Public Shared Sub Main()
Dim myPeople As New PeopleCollection()
myPeople.AddPeople(New Person("Saurabh", 24, "Gurgaon"))
myPeople.AddPeople(New Person("Manu"))
Dim Person As Person
For Each Person In myPeople
Console.WriteLine(Person)
Next Person
Console.ReadLine()
End Sub 'Main
End Class 'Client
End Namespace 'ArrayListSample
The above code does achieve type safety but then we will have to write an almost identical custom collection for each type we wish to contain. Because:-
myPeople.AddPeople(New Car())
would be a compile time error, since the code/approach above achieves type safety. So at the end of the day it will be a big nightmare !!
So above we find some of the limitations of ArrayLists. In the next article we will find how Generics solve the issues discussed above.
NOTE: THIS ARTICLE IS CONVERTED FROM C# TO VB.NET USING A CONVERSION TOOL. ORIGINAL ARTICLE CAN BE FOUND ON C# CORNER (http://www.c-sharpcorner.com/).
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Saurabh
Saurabh comes with almost 3years of experience in .Net. He specializes in C#, ASP.Net and is working with Open Solutions Software Services Pvt Ltd, Gurgaon. Besides being an active poster and author on C# Corner, Saurabh frequently blogs on, howtodotnet.blogspot.com, on technicals in .Net.
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