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Collection(Of T) Class

Provides the base class for a generic collection.

Namespace:  System.Collections.ObjectModel
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
<SerializableAttribute> _
<ComVisibleAttribute(False)> _
Public Class Collection(Of T) _
	Implements IList(Of T), ICollection(Of T),  _
	IEnumerable(Of T), IList, ICollection, IEnumerable
'Usage
Dim instance As Collection(Of T)

Type Parameters

T

The type of elements in the collection.

The Collection(Of T) class can be used immediately by creating an instance of one of its constructed types; all you have to do is specify the type of object to be contained in the collection. In addition, you can derive your own collection type from any constructed type, or derive a generic collection type from the Collection(Of T) class itself.

The Collection(Of T) class provides protected methods that can be used to customize its behavior when adding and removing items, clearing the collection, or setting the value of an existing item.

Most Collection(Of T) objects can be modified. However, a Collection(Of T) object that is initialized with a read-only IList(Of T) object cannot be modified. See ReadOnlyCollection(Of T) for a read-only version of this class.

Elements in this collection can be accessed using an integer index. Indexes in this collection are zero-based.

Collection(Of T) accepts Nothing as a valid value for reference types and allows duplicate elements.

Notes to Implementers:

This base class is provided to make it easier for implementers to create a custom collection. Implementers are encouraged to extend this base class instead of creating their own.

This section contains two code examples. The first example demonstrates several properties and methods of the Collection(Of T) class. The second example shows how to derive a collection class from a constructed type of Collection(Of T), and how to override the protected methods of Collection(Of T) to provide custom behavior.

Example 1

The following code example demonstrates many of the properties and methods of Collection(Of T). The code example creates a collection of strings, uses the Add method to add several strings, displays the Count, and lists the strings. The example uses the IndexOf method to find the index of a string and the Contains method to determine whether a string is in the collection. The example inserts a string using the Insert method and retrieves and sets strings using the default Item property (the indexer in C#). The example removes strings by string identity using the Remove method and by index using the RemoveAt method. Finally, the Clear method is used to clear all strings from the collection.

Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel

Public Class Demo

    Public Shared Sub Main() 

        Dim dinosaurs As New Collection(Of String)

        dinosaurs.Add("Psitticosaurus")
        dinosaurs.Add("Caudipteryx")
        dinosaurs.Add("Compsognathus")
        dinosaurs.Add("Muttaburrasaurus")

        Console.WriteLine("{0} dinosaurs:", dinosaurs.Count)
        Display(dinosaurs)

        Console.WriteLine(vbLf & "IndexOf(""Muttaburrasaurus""): {0}", _
            dinosaurs.IndexOf("Muttaburrasaurus"))

        Console.WriteLine(vbLf & "Contains(""Caudipteryx""): {0}", _
            dinosaurs.Contains("Caudipteryx"))

        Console.WriteLine(vbLf & "Insert(2, ""Nanotyrannus"")")
        dinosaurs.Insert(2, "Nanotyrannus")
        Display(dinosaurs)

        Console.WriteLine(vbLf & "dinosaurs(2): {0}", dinosaurs(2))

        Console.WriteLine(vbLf & "dinosaurs(2) = ""Microraptor""")
        dinosaurs(2) = "Microraptor"
        Display(dinosaurs)

        Console.WriteLine(vbLf & "Remove(""Microraptor"")")
        dinosaurs.Remove("Microraptor")
        Display(dinosaurs)

        Console.WriteLine(vbLf & "RemoveAt(0)")
        dinosaurs.RemoveAt(0)
        Display(dinosaurs)

        Console.WriteLine(vbLf & "dinosaurs.Clear()")
        dinosaurs.Clear()
        Console.WriteLine("Count: {0}", dinosaurs.Count)

    End Sub 

    Private Shared Sub Display(ByVal cs As Collection(Of String)) 
        Console.WriteLine()
        For Each item As String In cs
            Console.WriteLine(item)
        Next item
    End Sub 
End Class 

' This code example produces the following output: 

'4 dinosaurs: 

'Psitticosaurus 
'Caudipteryx 
'Compsognathus 
'Muttaburrasaurus 

'IndexOf("Muttaburrasaurus"): 3 

'Contains("Caudipteryx"): True 

'Insert(2, "Nanotyrannus") 

'Psitticosaurus 
'Caudipteryx 
'Nanotyrannus 
'Compsognathus 
'Muttaburrasaurus 

'dinosaurs(2): Nanotyrannus 

'dinosaurs(2) = "Microraptor" 

'Psitticosaurus 
'Caudipteryx 
'Microraptor 
'Compsognathus 
'Muttaburrasaurus 

'Remove("Microraptor") 

'Psitticosaurus 
'Caudipteryx 
'Compsognathus 
'Muttaburrasaurus 

'RemoveAt(0) 

'Caudipteryx 
'Compsognathus 
'Muttaburrasaurus 

'dinosaurs.Clear() 
'Count: 0

Example 2

The following code example shows how to derive a collection class from a constructed type of the Collection(Of T) generic class, and how to override the protected InsertItem, RemoveItem, ClearItems, and SetItem methods to provide custom behavior for the Add, Insert, Remove, and Clear methods, and for setting the Item property.

The custom behavior provided by this example is a Changed notification event that is raised at the end of each of the protected methods. The Dinosaurs class inherits Collection<string> (Collection(Of String) in Visual Basic) and defines the Changed event, which uses a DinosaursChangedEventArgs class for the event information, and an enumeration to identify the kind of change.

The code example calls several properties and methods of Collection(Of T) to demonstrate the custom event.

Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel

Public Class Dinosaurs
    Inherits Collection(Of String)

    Public Event Changed As EventHandler(Of DinosaursChangedEventArgs)

    Protected Overrides Sub InsertItem( _
        ByVal index As Integer, ByVal newItem As String)

        MyBase.InsertItem(index, newItem)

        RaiseEvent Changed(Me, New DinosaursChangedEventArgs( _
            ChangeType.Added, newItem, Nothing))
    End Sub 

    Protected Overrides Sub SetItem(ByVal index As Integer, _
        ByVal newItem As String)

        Dim replaced As String = Items(index)
        MyBase.SetItem(index, newItem)

        RaiseEvent Changed(Me, New DinosaursChangedEventArgs( _
            ChangeType.Replaced, replaced, newItem))
    End Sub 

    Protected Overrides Sub RemoveItem(ByVal index As Integer)

        Dim removedItem As String = Items(index)
        MyBase.RemoveItem(index)

        RaiseEvent Changed(Me, New DinosaursChangedEventArgs( _
            ChangeType.Removed, removedItem, Nothing))
    End Sub 

    Protected Overrides Sub ClearItems()
        MyBase.ClearItems()

        RaiseEvent Changed(Me, New DinosaursChangedEventArgs( _
            ChangeType.Cleared, Nothing, Nothing))
    End Sub 

End Class 

' Event argument for the Changed event. 

Public Class DinosaursChangedEventArgs
    Inherits EventArgs

    Public ReadOnly ChangedItem As String 
    Public ReadOnly ChangeType As ChangeType
    Public ReadOnly ReplacedWith As String 

    Public Sub New(ByVal change As ChangeType, ByVal item As String, _
        ByVal replacement As String)

        ChangeType = change
        ChangedItem = item
        ReplacedWith = replacement
    End Sub 
End Class 

Public Enum ChangeType
    Added
    Removed
    Replaced
    Cleared
End Enum 

Public Class Demo

    Public Shared Sub Main() 

        Dim dinosaurs As New Dinosaurs

        AddHandler dinosaurs.Changed, AddressOf ChangedHandler

        dinosaurs.Add("Psitticosaurus")
        dinosaurs.Add("Caudipteryx")
        dinosaurs.Add("Compsognathus")
        dinosaurs.Add("Muttaburrasaurus")

        Display(dinosaurs)

        Console.WriteLine(vbLf & "IndexOf(""Muttaburrasaurus""): {0}", _
            dinosaurs.IndexOf("Muttaburrasaurus"))

        Console.WriteLine(vbLf & "Contains(""Caudipteryx""): {0}", _
            dinosaurs.Contains("Caudipteryx"))

        Console.WriteLine(vbLf & "Insert(2, ""Nanotyrannus"")")
        dinosaurs.Insert(2, "Nanotyrannus")

        Console.WriteLine(vbLf & "dinosaurs(2): {0}", dinosaurs(2))

        Console.WriteLine(vbLf & "dinosaurs(2) = ""Microraptor""")
        dinosaurs(2) = "Microraptor"

        Console.WriteLine(vbLf & "Remove(""Microraptor"")")
        dinosaurs.Remove("Microraptor")

        Console.WriteLine(vbLf & "RemoveAt(0)")
        dinosaurs.RemoveAt(0)

        Display(dinosaurs)

    End Sub 

    Private Shared Sub Display(ByVal cs As Collection(Of String)) 
        Console.WriteLine()
        For Each item As String In cs
            Console.WriteLine(item)
        Next item
    End Sub 

    Private Shared Sub ChangedHandler(ByVal source As Object, _
        ByVal e As DinosaursChangedEventArgs)

        If e.ChangeType = ChangeType.Replaced Then
            Console.WriteLine("{0} was replaced with {1}", _
                e.ChangedItem, e.ReplacedWith)

        ElseIf e.ChangeType = ChangeType.Cleared Then
            Console.WriteLine("The dinosaur list was cleared.")

        Else
            Console.WriteLine("{0} was {1}.", _
                e.ChangedItem, e.ChangeType)
        End If 
    End Sub 

End Class 

' This code example produces the following output: 

'Psitticosaurus was Added. 
'Caudipteryx was Added. 
'Compsognathus was Added. 
'Muttaburrasaurus was Added. 

'Psitticosaurus 
'Caudipteryx 
'Compsognathus 
'Muttaburrasaurus 

'IndexOf("Muttaburrasaurus"): 3 

'Contains("Caudipteryx"): True 

'Insert(2, "Nanotyrannus") 
'Nanotyrannus was Added. 

'dinosaurs(2): Nanotyrannus 

'dinosaurs(2) = "Microraptor" 
'Nanotyrannus was replaced with Microraptor 

'Remove("Microraptor") 
'Microraptor was Removed. 

'RemoveAt(0) 
'Psitticosaurus was Removed. 

'Caudipteryx 
'Compsognathus 
'Muttaburrasaurus

System.Object
  System.Collections.ObjectModel.Collection(Of T)
    System.Collections.ObjectModel.KeyedCollection(Of TKey, TItem)
    System.Collections.ObjectModel.ObservableCollection(Of T)
    System.ComponentModel.BindingList(Of T)
    System.ComponentModel.SortDescriptionCollection
    System.Net.IPEndPointCollection
    System.Net.Mail.AlternateViewCollection
    System.Net.Mail.AttachmentCollection
    System.Net.Mail.LinkedResourceCollection
    System.Net.Mail.MailAddressCollection
    System.Net.PeerToPeer.CloudCollection
    System.Net.PeerToPeer.Collaboration.PeerApplicationCollection
    System.Net.PeerToPeer.Collaboration.PeerContactCollection
    System.Net.PeerToPeer.Collaboration.PeerEndPointCollection
    System.Net.PeerToPeer.Collaboration.PeerNearMeCollection
    System.Net.PeerToPeer.Collaboration.PeerObjectCollection
    System.Net.PeerToPeer.PeerNameRecordCollection
    System.Security.Cryptography.CngPropertyCollection
    System.ServiceModel.Channels.BindingElementCollection
    System.ServiceModel.Channels.ChannelParameterCollection
    System.ServiceModel.Description.FaultDescriptionCollection
    System.ServiceModel.Description.MessageDescriptionCollection
    System.ServiceModel.Description.OperationDescriptionCollection
    System.ServiceModel.Description.PolicyAssertionCollection
    System.ServiceModel.Description.ServiceEndpointCollection
    System.ServiceModel.Syndication.SyndicationElementExtensionCollection
    System.Web.UI.ScriptReferenceCollection
    System.Web.UI.ServiceReferenceCollection
    System.Web.UI.UpdatePanelTriggerCollection
    System.Windows.ConditionCollection
    System.Windows.Ink.StrokeCollection
    System.Windows.Input.StylusPlugIns.StylusPlugInCollection
    System.Windows.Input.StylusPointCollection
    System.Windows.SetterBaseCollection
    System.Windows.TriggerCollection
    System.Workflow.ComponentModel.Compiler.ValidationErrorCollection

Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

A Collection(Of T) can support multiple readers concurrently, as long as the collection is not modified. Even so, enumerating through a collection is intrinsically not a thread-safe procedure. To guarantee thread safety during enumeration, you can lock the collection during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

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