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KeyedCollection(Of TKey, TItem) Class

Provides the abstract base class for a collection whose keys are embedded in the values.

System.Object
  System.Collections.ObjectModel.Collection(Of TItem)
    System.Collections.ObjectModel.KeyedCollection(Of TKey, TItem)
      More...

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

'Declaration
<SerializableAttribute> _
<ComVisibleAttribute(False)> _
Public MustInherit Class KeyedCollection(Of TKey, TItem) _
	Inherits Collection(Of TItem)

Type Parameters

TKey

The type of keys in the collection.

TItem

The type of items in the collection.

The KeyedCollection(Of TKey, TItem) type exposes the following members.

  NameDescription
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryKeyedCollection(Of TKey, TItem)Initializes a new instance of the KeyedCollection(Of TKey, TItem) class that uses the default equality comparer.
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryKeyedCollection(Of TKey, TItem)(IEqualityComparer(Of TKey))Initializes a new instance of the KeyedCollection(Of TKey, TItem) class that uses the specified equality comparer.
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryKeyedCollection(Of TKey, TItem)(IEqualityComparer(Of TKey), Int32)Initializes a new instance of the KeyedCollection(Of TKey, TItem) class that uses the specified equality comparer and creates a lookup dictionary when the specified threshold is exceeded.
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  NameDescription
Public propertySupported by the XNA FrameworkSupported by Portable Class LibraryComparerGets the generic equality comparer that is used to determine equality of keys in the collection.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibraryCountGets the number of elements actually contained in the Collection(Of T). (Inherited from Collection(Of T).)
Protected propertySupported by the XNA FrameworkSupported by Portable Class LibraryDictionaryGets the lookup dictionary of the KeyedCollection(Of TKey, TItem).
Public propertySupported by the XNA FrameworkSupported by Portable Class LibraryItem(TKey)Gets the element with the specified key.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibraryItem(Int32)Gets or sets the element at the specified index. (Inherited from Collection(Of T).)
Protected propertySupported by the XNA FrameworkSupported by Portable Class LibraryItemsGets a IList(Of T) wrapper around the Collection(Of T). (Inherited from Collection(Of T).)
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  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryAddAdds an object to the end of the Collection(Of T). (Inherited from Collection(Of T).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryChangeItemKeyChanges the key associated with the specified element in the lookup dictionary.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryClearRemoves all elements from the Collection(Of T). (Inherited from Collection(Of T).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryClearItemsRemoves all elements from the KeyedCollection(Of TKey, TItem). (Overrides Collection(Of T).ClearItems.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryContains(T)Determines whether an element is in the Collection(Of T). (Inherited from Collection(Of T).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryContains(TKey)Determines whether the collection contains an element with the specified key.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryCopyToCopies the entire Collection(Of T) to a compatible one-dimensional Array, starting at the specified index of the target array. (Inherited from Collection(Of T).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetEnumeratorReturns an enumerator that iterates through the Collection(Of T). (Inherited from Collection(Of T).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetKeyForItemWhen implemented in a derived class, extracts the key from the specified element.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryIndexOfSearches for the specified object and returns the zero-based index of the first occurrence within the entire Collection(Of T). (Inherited from Collection(Of T).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryInsertInserts an element into the Collection(Of T) at the specified index. (Inherited from Collection(Of T).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryInsertItemInserts an element into the KeyedCollection(Of TKey, TItem) at the specified index. (Overrides Collection(Of T).InsertItem(Int32, T).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryRemove(T)Removes the first occurrence of a specific object from the Collection(Of T). (Inherited from Collection(Of T).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryRemove(TKey)Removes the element with the specified key from the KeyedCollection(Of TKey, TItem).
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryRemoveAtRemoves the element at the specified index of the Collection(Of T). (Inherited from Collection(Of T).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryRemoveItemRemoves the element at the specified index of the KeyedCollection(Of TKey, TItem). (Overrides Collection(Of T).RemoveItem(Int32).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibrarySetItemReplaces the item at the specified index with the specified item. (Overrides Collection(Of T).SetItem(Int32, T).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryToStringReturns a string that represents the current object. (Inherited from Object.)
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  NameDescription
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryICollection.CopyToCopies the elements of the ICollection to an Array, starting at a particular Array index. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate propertySupported by the XNA FrameworkSupported by Portable Class LibraryICollection(Of T).IsReadOnlyGets a value indicating whether the ICollection(Of T) is read-only. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate propertySupported by the XNA FrameworkSupported by Portable Class LibraryICollection.IsSynchronizedGets a value indicating whether access to the ICollection is synchronized (thread safe). (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate propertySupported by the XNA FrameworkSupported by Portable Class LibraryICollection.SyncRootGets an object that can be used to synchronize access to the ICollection. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIEnumerable.GetEnumeratorReturns an enumerator that iterates through a collection. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIList.AddAdds an item to the IList. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIList.ContainsDetermines whether the IList contains a specific value. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIList.IndexOfDetermines the index of a specific item in the IList. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIList.InsertInserts an item into the IList at the specified index. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate propertySupported by the XNA FrameworkSupported by Portable Class LibraryIList.IsFixedSizeGets a value indicating whether the IList has a fixed size. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate propertySupported by the XNA FrameworkSupported by Portable Class LibraryIList.IsReadOnlyGets a value indicating whether the IList is read-only. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate propertySupported by the XNA FrameworkSupported by Portable Class LibraryIList.ItemGets or sets the element at the specified index. (Inherited from Collection(Of T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIList.RemoveRemoves the first occurrence of a specific object from the IList. (Inherited from Collection(Of T).)
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The KeyedCollection(Of TKey, TItem) class provides both O(1) indexed retrieval and keyed retrieval that approaches O(1). It is an abstract type, or more accurately an infinite set of abstract types, because each of its constructed generic types is an abstract base class. To use KeyedCollection(Of TKey, TItem), derive your collection type from the appropriate constructed type.

The KeyedCollection(Of TKey, TItem) class is a hybrid between a collection based on the IList(Of T) generic interface and a collection based on the IDictionary(Of TKey, TValue) generic interface. Like collections based on the IList(Of T) generic interface, KeyedCollection(Of TKey, TItem) is an indexed list of items. Like collections based on the IDictionary(Of TKey, TValue) generic interface, KeyedCollection(Of TKey, TItem) has a key associated with each element.

Unlike dictionaries, an element of KeyedCollection(Of TKey, TItem) is not a key/value pair; instead, the entire element is the value and the key is embedded within the value. For example, an element of a collection derived from KeyedCollection<String,String> (KeyedCollection(Of String, String) in Visual Basic) might be "John Doe Jr." where the value is "John Doe Jr." and the key is "Doe"; or a collection of employee records containing integer keys could be derived from KeyedCollection<int,Employee>. The abstract GetKeyForItem method extracts the key from the element.

By default, the KeyedCollection(Of TKey, TItem) includes a lookup dictionary that you can obtain with the Dictionary property. When an item is added to the KeyedCollection(Of TKey, TItem), the item's key is extracted once and saved in the lookup dictionary for faster searches. This behavior is overridden by specifying a dictionary creation threshold when you create the KeyedCollection(Of TKey, TItem). The lookup dictionary is created the first time the number of elements exceeds that threshold. If you specify –1 as the threshold, the lookup dictionary is never created.

NoteNote

When the internal lookup dictionary is used, it contains references to all the items in the collection if TItem is a reference type, or copies of all the items in the collection if TItem is a value type. Thus, using the lookup dictionary may not be appropriate if TItem is a value type.

You can access an item by its index or key by using the Item property. You can add items without a key, but these items can subsequently be accessed only by index.

This section contains two code examples. The first example shows the minimum code required to derive from KeyedCollection(Of TKey, TItem), and demonstrates many of the inherited methods. The second example shows how to override the protected methods of KeyedCollection(Of TKey, TItem) to provide custom behavior.

Example 1

This code example shows the minimum code necessary to derive a collection class from KeyedCollection(Of TKey, TItem): overriding the GetKeyForItem method and providing a public constructor that delegates to a base class constructor. The code example also demonstrates many of the properties and methods inherited from KeyedCollection(Of TKey, TItem) and Collection(Of T) classes.

The SimpleOrder class is a very simple requisition list that contains OrderItem objects, each of which represents a line item in the order. The key of OrderItem is immutable, an important consideration for classes that derive from KeyedCollection(Of TKey, TItem). For a code example that uses mutable keys, see ChangeItemKey.


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

' This class represents a very simple keyed list of OrderItems,
' inheriting most of its behavior from the KeyedCollection and 
' Collection classes. The immediate base class is the constructed
' type KeyedCollection(Of Integer, OrderItem). When you inherit
' from KeyedCollection, the second generic type argument is the 
' type that you want to store in the collection -- in this case
' OrderItem. The first generic argument is the type that you want
' to use as a key. Its values must be calculated from OrderItem; 
' in this case it is the Integer field PartNumber, so SimpleOrder
' inherits KeyedCollection(Of Integer, OrderItem).
'
Public Class SimpleOrder
    Inherits KeyedCollection(Of Integer, OrderItem)

    ' The parameterless constructor of the base class creates a 
    ' KeyedCollection with an internal dictionary. For this code 
    ' example, no other constructors are exposed.
    '
    Public Sub New()
        MyBase.New()
    End Sub

    ' This is the only method that absolutely must be overridden,
    ' because without it the KeyedCollection cannot extract the
    ' keys from the items. The input parameter type is the 
    ' second generic type argument, in this case OrderItem, and 
    ' the return value type is the first generic type argument,
    ' in this case Integer.
    '
    Protected Overrides Function GetKeyForItem( _
        ByVal item As OrderItem) As Integer

        ' In this example, the key is the part number.
        Return item.PartNumber   
    End Function

End Class

Public Class Demo

    Public Shared Sub Main() 
        Dim weekly As New SimpleOrder()

        ' The Add method, inherited from Collection, takes OrderItem.
        '
        weekly.Add(New OrderItem(110072674, "Widget", 400, 45.17))
        weekly.Add(New OrderItem(110072675, "Sprocket", 27, 5.3))
        weekly.Add(New OrderItem(101030411, "Motor", 10, 237.5))
        weekly.Add(New OrderItem(110072684, "Gear", 175, 5.17))

        Display(weekly)

        ' The Contains method of KeyedCollection takes TKey.
        '
        Console.WriteLine(vbLf & "Contains(101030411): {0}", _
            weekly.Contains(101030411))

        ' The default Item property of KeyedCollection takes the key
        ' type, Integer.
        '
        Console.WriteLine(vbLf & "weekly(101030411).Description: {0}", _
            weekly(101030411).Description)

        ' The Remove method of KeyedCollection takes a key.
        '
        Console.WriteLine(vbLf & "Remove(101030411)")
        weekly.Remove(101030411)
        Display(weekly)

        ' The Insert method, inherited from Collection, takes an 
        ' index and an OrderItem.
        '
        Console.WriteLine(vbLf & "Insert(2, New OrderItem(...))")
        weekly.Insert(2, New OrderItem(111033401, "Nut", 10, .5))
        Display(weekly)

        ' The default Item property is overloaded. One overload comes
        ' from KeyedCollection(Of Integer, OrderItem); that overload
        ' is read-only, and takes Integer because it retrieves by key. 
        ' The other overload comes from Collection(Of OrderItem), the 
        ' base class of KeyedCollection(Of Integer, OrderItem); it 
        ' retrieves by index, so it also takes an Integer. The compiler
        ' uses the most-derived overload, from KeyedCollection, so the
        ' only way to access SimpleOrder by index is to cast it to
        ' Collection(Of OrderItem). Otherwise the index is interpreted
        ' as a key, and KeyNotFoundException is thrown.
        '
        Dim coweekly As Collection(Of OrderItem) = weekly
        Console.WriteLine(vbLf & "coweekly(2).Description: {0}", _
            coweekly(2).Description)

        Console.WriteLine(vbLf & "coweekly(2) = New OrderItem(...)")
        coweekly(2) = New OrderItem(127700026, "Crank", 27, 5.98)

        Dim temp As OrderItem = coweekly(2)

        ' The IndexOf method, inherited from Collection(Of OrderItem), 
        ' takes an OrderItem instead of a key.
        ' 
        Console.WriteLine(vbLf & "IndexOf(temp): {0}", _
            weekly.IndexOf(temp))

        ' The inherited Remove method also takes an OrderItem.
        '
        Console.WriteLine(vbLf & "Remove(temp)")
        weekly.Remove(temp)
        Display(weekly)

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

    End Sub

    Private Shared Sub Display(ByVal order As SimpleOrder) 
        Console.WriteLine()
        For Each item As OrderItem In  order
            Console.WriteLine(item)
        Next item
    End Sub
End Class

' This class represents a simple line item in an order. All the 
' values are immutable except quantity.
' 
Public Class OrderItem
    Public ReadOnly PartNumber As Integer
    Public ReadOnly Description As String
    Public ReadOnly UnitPrice As Double

    Private _quantity As Integer = 0

    Public Sub New(ByVal partNumber As Integer, _
                   ByVal description As String, _
                   ByVal quantity As Integer, _
                   ByVal unitPrice As Double) 
        Me.PartNumber = partNumber
        Me.Description = description
        Me.Quantity = quantity
        Me.UnitPrice = unitPrice
    End Sub 'New

    Public Property Quantity() As Integer 
        Get
            Return _quantity
        End Get
        Set
            If value < 0 Then
                Throw New ArgumentException("Quantity cannot be negative.")
            End If
            _quantity = value
        End Set
    End Property

    Public Overrides Function ToString() As String 
        Return String.Format( _
            "{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}", _
            PartNumber, _quantity, Description, UnitPrice, _
            UnitPrice * _quantity)
    End Function
End Class

' This code example produces the following output:
'
'110072674    400 Widget       at    45.17 =  18,068.00
'110072675     27 Sprocket     at     5.30 =     143.10
'101030411     10 Motor        at   237.50 =   2,375.00
'110072684    175 Gear         at     5.17 =     904.75
'
'Contains(101030411): True
'
'weekly(101030411).Description: Motor
'
'Remove(101030411)
'
'110072674    400 Widget       at    45.17 =  18,068.00
'110072675     27 Sprocket     at     5.30 =     143.10
'110072684    175 Gear         at     5.17 =     904.75
'
'Insert(2, New OrderItem(...))
'
'110072674    400 Widget       at    45.17 =  18,068.00
'110072675     27 Sprocket     at     5.30 =     143.10
'111033401     10 Nut          at      .50 =       5.00
'110072684    175 Gear         at     5.17 =     904.75
'
'coweekly(2).Description: Nut
'
'coweekly(2) = New OrderItem(...)
'
'IndexOf(temp): 2
'
'Remove(temp)
'
'110072674    400 Widget       at    45.17 =  18,068.00
'110072675     27 Sprocket     at     5.30 =     143.10
'110072684    175 Gear         at     5.17 =     904.75
'
'RemoveAt(0)
'
'110072675     27 Sprocket     at     5.30 =     143.10
'110072684    175 Gear         at     5.17 =     904.75


Example 2

The following code example shows how to override the protected InsertItem, RemoveItem, ClearItems, and SetItem methods, to provide custom behavior for the Add, Remove, and Clear methods, and for setting the default Item property (the indexer in C#). The custom behavior provided in this example is a notification event named Changed, which is raised at the end of each of the overridden methods.

The code example creates the SimpleOrder class, which derives from KeyedCollection(Of TKey, TItem) and represents a simple order form. The order form contains OrderItem objects representing items ordered. The code example also creates a SimpleOrderChangedEventArgs class to contain the event information, and an enumeration to identify the type of change.

The code example demonstrates the custom behavior by calling the properties and methods of the derived class, in the Main method of the Demo class.

This code example uses objects with immutable keys. For a code example that uses mutable keys, see ChangeItemKey.


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

' This class derives from KeyedCollection and shows how to override
' the protected ClearItems, InsertItem, RemoveItem, and SetItem 
' methods in order to change the behavior of the default Item 
' property and the Add, Clear, Insert, and Remove methods. The
' class implements a Changed event, which is raised by all the
' protected methods.
'
' SimpleOrder is a collection of OrderItem objects, and its key
' is the PartNumber field of OrderItem. PartNumber is an Integer,
' so SimpleOrder inherits KeyedCollection(Of Integer, OrderItem).
' (Note that the key of OrderItem cannot be changed; if it could 
' be changed, SimpleOrder would have to override ChangeItemKey.)
'
Public Class SimpleOrder
    Inherits KeyedCollection(Of Integer, OrderItem)

    Public Event Changed As EventHandler(Of SimpleOrderChangedEventArgs)

    ' This parameterless constructor calls the base class constructor
    ' that specifies a dictionary threshold of 0, so that the internal
    ' dictionary is created as soon as an item is added to the 
    ' collection.
    '
    Public Sub New()
        MyBase.New(Nothing, 0)
    End Sub

    ' This is the only method that absolutely must be overridden,
    ' because without it the KeyedCollection cannot extract the
    ' keys from the items. 
    '
    Protected Overrides Function GetKeyForItem( _
        ByVal item As OrderItem) As Integer

        ' In this example, the key is the part number.
        Return item.PartNumber   
    End Function

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

        MyBase.InsertItem(index, newItem)

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

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

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

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

    Protected Overrides Sub RemoveItem(ByVal index As Integer)

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

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

    Protected Overrides Sub ClearItems()
        MyBase.ClearItems()

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

End Class

' Event argument for the Changed event.
'
Public Class SimpleOrderChangedEventArgs
    Inherits EventArgs

    Private _changedItem As OrderItem
    Private _changeType As ChangeType
    Private _replacedWith As OrderItem

    Public ReadOnly Property ChangedItem As OrderItem
        Get
            Return _changedItem
        End Get
    End Property

    Public ReadOnly Property ChangeType As ChangeType
        Get
            Return _changeType
        End Get
    End Property

    Public ReadOnly Property ReplacedWith As OrderItem
        Get
            Return _replacedWith
        End Get
    End Property

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

        _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 weekly As New SimpleOrder()
        AddHandler weekly.Changed, AddressOf ChangedHandler

        ' The Add method, inherited from Collection, takes OrderItem.
        '
        weekly.Add(New OrderItem(110072674, "Widget", 400, 45.17))
        weekly.Add(New OrderItem(110072675, "Sprocket", 27, 5.3))
        weekly.Add(New OrderItem(101030411, "Motor", 10, 237.5))
        weekly.Add(New OrderItem(110072684, "Gear", 175, 5.17))

        Display(weekly)

        ' The Contains method of KeyedCollection takes TKey.
        '
        Console.WriteLine(vbLf & "Contains(101030411): {0}", _
            weekly.Contains(101030411))

        ' The default Item property of KeyedCollection takes the key
        ' type, Integer. The property is read-only.
        '
        Console.WriteLine(vbLf & "weekly(101030411).Description: {0}", _
            weekly(101030411).Description)

        ' The Remove method of KeyedCollection takes a key.
        '
        Console.WriteLine(vbLf & "Remove(101030411)")
        weekly.Remove(101030411)

        ' The Insert method, inherited from Collection, takes an 
        ' index and an OrderItem.
        '
        Console.WriteLine(vbLf & "Insert(2, New OrderItem(...))")
        weekly.Insert(2, New OrderItem(111033401, "Nut", 10, .5))

        ' The default Item property is overloaded. One overload comes
        ' from KeyedCollection(Of Integer, OrderItem); that overload
        ' is read-only, and takes Integer because it retrieves by key. 
        ' The other overload comes from Collection(Of OrderItem), the 
        ' base class of KeyedCollection(Of Integer, OrderItem); it 
        ' retrieves by index, so it also takes an Integer. The compiler
        ' uses the most-derived overload, from KeyedCollection, so the
        ' only way to access SimpleOrder by index is to cast it to
        ' Collection(Of OrderItem). Otherwise the index is interpreted
        ' as a key, and KeyNotFoundException is thrown.
        '
        Dim coweekly As Collection(Of OrderItem) = weekly
        Console.WriteLine(vbLf & "coweekly(2).Description: {0}", _
            coweekly(2).Description)

        Console.WriteLine(vbLf & "coweekly(2) = New OrderItem(...)")
        coweekly(2) = New OrderItem(127700026, "Crank", 27, 5.98)

        Dim temp As OrderItem = coweekly(2)

        ' The IndexOf method, inherited from Collection(Of OrderItem), 
        ' takes an OrderItem instead of a key.
        ' 
        Console.WriteLine(vbLf & "IndexOf(temp): {0}", _
            weekly.IndexOf(temp))

        ' The inherited Remove method also takes an OrderItem.
        '
        Console.WriteLine(vbLf & "Remove(temp)")
        weekly.Remove(temp)

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

        ' Increase the quantity for a line item.
        Console.WriteLine(vbLf & "coweekly(1) = New OrderItem(...)")
        coweekly(1) = New OrderItem(coweekly(1).PartNumber, _
            coweekly(1).Description, coweekly(1).Quantity + 1000, _
            coweekly(1).UnitPrice)

        Display(weekly)

        Console.WriteLine()
        weekly.Clear()
    End Sub

    Private Shared Sub Display(ByVal order As SimpleOrder) 
        Console.WriteLine()
        For Each item As OrderItem In  order
            Console.WriteLine(item)
        Next item
    End Sub

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

        Dim item As OrderItem = e.ChangedItem

        If e.ChangeType = ChangeType.Replaced Then
            Dim replacement As OrderItem = e.ReplacedWith

            Console.WriteLine("{0} (quantity {1}) was replaced " & _
                "by {2}, (quantity {3}).", item.Description, _
                item.Quantity, replacement.Description, replacement.Quantity)

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

        Else
            Console.WriteLine("{0} (quantity {1}) was {2}.", _
                item.Description, item.Quantity, e.ChangeType)
        End If
    End Sub
End Class

' This class represents a simple line item in an order. All the 
' values are immutable except quantity.
' 
Public Class OrderItem

    Private _partNumber As Integer
    Private _description As String
    Private _unitPrice As Double
    Private _quantity As Integer

    Public ReadOnly Property PartNumber As Integer
        Get
            Return _partNumber
        End Get
    End Property

    Public ReadOnly Property Description As String
        Get
            Return _description
        End Get
    End Property

    Public ReadOnly Property UnitPrice As Double
        Get
            Return _unitPrice
        End Get
    End Property

    Public ReadOnly Property Quantity() As Integer 
        Get
            Return _quantity
        End Get
    End Property

    Public Sub New(ByVal partNumber As Integer, _
                   ByVal description As String, _
                   ByVal quantity As Integer, _
                   ByVal unitPrice As Double) 
        _partNumber = partNumber
        _description = description
        _quantity = quantity
        _unitPrice = unitPrice
    End Sub 'New

    Public Overrides Function ToString() As String 
        Return String.Format( _
            "{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}", _
            PartNumber, _quantity, Description, UnitPrice, _
            UnitPrice * _quantity)
    End Function
End Class

' This code example produces the following output:
'
'Widget (quantity 400) was Added.
'Sprocket (quantity 27) was Added.
'Motor (quantity 10) was Added.
'Gear (quantity 175) was Added.
'
'110072674    400 Widget       at    45.17 =  18,068.00
'110072675     27 Sprocket     at     5.30 =     143.10
'101030411     10 Motor        at   237.50 =   2,375.00
'110072684    175 Gear         at     5.17 =     904.75
'
'Contains(101030411): True
'
'weekly(101030411).Description: Motor
'
'Remove(101030411)
'Motor (quantity 10) was Removed.
'
'Insert(2, New OrderItem(...))
'Nut (quantity 10) was Added.
'
'coweekly(2).Description: Nut
'
'coweekly(2) = New OrderItem(...)
'Nut (quantity 10) was replaced by Crank, (quantity 27).
'
'IndexOf(temp): 2
'
'Remove(temp)
'Crank (quantity 27) was Removed.
'
'RemoveAt(0)
'Widget (quantity 400) was Removed.
'
'coweekly(1) = New OrderItem(...)
'Gear (quantity 175) was replaced by Gear, (quantity 1175).
'
'110072675     27 Sprocket     at     5.30 =     143.10
'110072684   1175 Gear         at     5.17 =   6,074.75
'
'The order list was cleared.


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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

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

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