KeyedCollection<TKey,TItem>.ChangeItemKey(TItem, TKey) Method

protected:
 void ChangeItemKey(TItem item, TKey newKey);

protected void ChangeItemKey (TItem item, TKey newKey);

member this.ChangeItemKey : 'Item * 'Key -> unit

Protected Sub ChangeItemKey (item As TItem, newKey As TKey)

Parameters

Exceptions

Examples

The following code example shows how to override the protected ChangeItemKey method to support mutable keys, and how to override the protected InsertItem, RemoveItem, ClearItems, and SetItem methods to maintain the integrity of the keys and the collection.

The code example creates the MutableKeys collection, which derives from KeyedCollection<TKey,TItem>, and the MutableKey class. The MutableKey class has a settable Key property. When a new key is assigned to the property, the property setter calls the internal ChangeKey method of the collection to test whether the new key would conflict with an existing key. If so, an exception is thrown and the property value is not changed.

In order to maintain the connection between a MutableKey object and the MutableKeys collection and to prevent an object from being inserted into two collections, the MutableKey class has an internal Collection field. This field is maintained by the protected methods that provide custom behavior for adding and removing items from the collection, such as the InsertItem method. The field is set when the item is added to a collection and cleared when the item is removed.

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;

// This class demonstrates one way to use the ChangeItemKey
// method to store objects with keys that can be changed. The
// ChangeItemKey method is used to keep the internal lookup
// Dictionary in sync with the keys of the stored objects.
//
// MutableKeys stores MutableKey objects, which have an Integer
// Key property that can be set. Therefore, MutableKeys inherits
// KeyedCollection(Of Integer, MutableKey).
//
public class MutableKeys : KeyedCollection<int, MutableKey>
{
    // This parameterless constructor delegates to the base class
    // constructor that specifies a dictionary threshold. A
    // threshold of 0 means the internal Dictionary is created
    // the first time an object is added.
    //
    public MutableKeys() : base(null, 0) {}

    protected override int GetKeyForItem(MutableKey item)
    {
        // The key is MutableKey.Key.
        return item.Key;
    }

    protected override void InsertItem(int index, MutableKey newItem)
    {
        if (newItem.Collection != null)
            throw new ArgumentException("The item already belongs to a collection.");

        base.InsertItem(index, newItem);
        newItem.Collection = this;
    }

    protected override void SetItem(int index, MutableKey newItem)
    {
        MutableKey replaced = Items[index];

        if (newItem.Collection != null)
            throw new ArgumentException("The item already belongs to a collection.");

        base.SetItem(index, newItem);
        newItem.Collection = this;
        replaced.Collection = null;
    }

    protected override void RemoveItem(int index)
    {
        MutableKey removedItem = Items[index];

        base.RemoveItem(index);
        removedItem.Collection = null;
    }

    protected override void ClearItems()
    {
        foreach( MutableKey mk in Items )
        {
            mk.Collection = null;
        }

        base.ClearItems();
    }

    internal void ChangeKey(MutableKey item, int newKey)
    {
        base.ChangeItemKey(item, newKey);
    }

    public void Dump()
    {
        Console.WriteLine("\nDUMP:");
        if (Dictionary == null)
        {
            Console.WriteLine("    The dictionary has not been created.");
        }
        else
        {
            Console.WriteLine("    Dictionary entries");
            Console.WriteLine("    ------------------");

            foreach( KeyValuePair<int, MutableKey> kvp in Dictionary )
            {
                Console.WriteLine("    {0} : {1}", kvp.Key, kvp.Value);
            }
        }

        Console.WriteLine("\n    List of items");
        Console.WriteLine("    -------------");

        foreach( MutableKey mk in Items )
        {
            Console.WriteLine("    {0}", mk);
        }
    }
}

public class Demo
{
    public static void Main()
    {
        MutableKeys mkeys = new MutableKeys();

        // The Add method is inherited from Collection.
        //
        mkeys.Add(new MutableKey(110072674, "Widget"));
        mkeys.Add(new MutableKey(110072675, "Sprocket"));

        mkeys.Dump();

        Console.WriteLine("\nCreate and insert a new item:");
        MutableKey test = new MutableKey(110072684, "Gear");
        mkeys.Insert(1, test);

        mkeys.Dump();

        try
        {
            Console.WriteLine("\nTry to insert the item again:");
            mkeys.Insert(1, test);
        }
        catch(ArgumentException ex)
        {
            Console.WriteLine("Error: {0}", ex.Message);
        }

        Console.WriteLine("\nChange the Key property of the item:");
        test.Key = 100000072;

        mkeys.Dump();

        try
        {
            Console.WriteLine("\nTry to set the Key property to an existing key:");
            test.Key = 110072674;
        }
        catch(ArgumentException ex)
        {
            Console.WriteLine("Error: {0}", ex.Message);
        }

        mkeys.Dump();
    }

    private static void Display(MutableKeys order)
    {
        Console.WriteLine();
        foreach( MutableKey item in order )
        {
            Console.WriteLine(item);
        }
    }
}

// This class has a key that can be changed.
//
public class MutableKey
{

    public MutableKey(int newKey, string newValue)
    {
        _key = newKey;
        Value = newValue;
    } //New

    public string Value;
    internal MutableKeys Collection;

    private int _key;
    public int Key
    {
        get
        {
            return _key;
        }
        set
        {
            if (Collection != null)
            {
                Collection.ChangeKey(this, value);
            }

            _key = value;
        }
    }

    public override string ToString()
    {
        return String.Format("{0,9} {1}", _key, Value);
    }
}

/* This code example produces the following output:

DUMP:
    Dictionary entries
    ------------------
    110072674 : 110072674 Widget
    110072675 : 110072675 Sprocket

    List of items
    -------------
    110072674 Widget
    110072675 Sprocket

Create and insert a new item:

DUMP:
    Dictionary entries
    ------------------
    110072674 : 110072674 Widget
    110072675 : 110072675 Sprocket
    110072684 : 110072684 Gear

    List of items
    -------------
    110072674 Widget
    110072684 Gear
    110072675 Sprocket

Try to insert the item again:
Error: The item already belongs to a collection.

Change the Key property of the item:

DUMP:
    Dictionary entries
    ------------------
    110072674 : 110072674 Widget
    110072675 : 110072675 Sprocket
    100000072 : 100000072 Gear

    List of items
    -------------
    110072674 Widget
    100000072 Gear
    110072675 Sprocket

Try to set the Key property to an existing key:
Error: An item with the same key has already been added.

DUMP:
    Dictionary entries
    ------------------
    110072674 : 110072674 Widget
    110072675 : 110072675 Sprocket
    100000072 : 100000072 Gear

    List of items
    -------------
    110072674 Widget
    100000072 Gear
    110072675 Sprocket
 */

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

' This class demonstrates one way to use the ChangeItemKey
' method to store objects with keys that can be changed. The 
' ChangeItemKey method is used to keep the internal lookup
' Dictionary in sync with the keys of the stored objects. 
'
' MutableKeys stores MutableKey objects, which have an Integer
' Key property that can be set. Therefore, MutableKeys inherits
' KeyedCollection(Of Integer, MutableKey).
'
Public Class MutableKeys
    Inherits KeyedCollection(Of Integer, MutableKey)

    ' This parameterless constructor delegates to the base class 
    ' constructor that specifies a dictionary threshold. A
    ' threshold of 0 means the internal Dictionary is created
    ' the first time an object is added.
    '
    Public Sub New()
        MyBase.New(Nothing, 0)
    End Sub
    
    Protected Overrides Function GetKeyForItem( _
        ByVal item As MutableKey) As Integer

        ' The key is MutableKey.Key.
        Return item.Key   
    End Function

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

        If newItem.Collection IsNot Nothing Then _
            Throw New ArgumentException("The item already belongs to a collection.")

        MyBase.InsertItem(index, newItem)
        newItem.Collection = Me
    End Sub

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

        Dim replaced As MutableKey = Items(index)

        If newItem.Collection IsNot Nothing Then _
            Throw New ArgumentException("The item already belongs to a collection.")

        MyBase.SetItem(index, newItem)
        newItem.Collection = Me
        replaced.Collection = Nothing
    End Sub

    Protected Overrides Sub RemoveItem(ByVal index As Integer)
        Dim removedItem As MutableKey = Items(index)

        MyBase.RemoveItem(index)
        removedItem.Collection = Nothing
    End Sub

    Protected Overrides Sub ClearItems()
        For Each mk As MutableKey In Items
            mk.Collection = Nothing
        Next
        
        MyBase.ClearItems()
    End Sub

    Friend Sub ChangeKey(ByVal item As MutableKey, _
        ByVal newKey As Integer)

        MyBase.ChangeItemKey(item, newKey)
    End Sub
    
    Public Sub Dump()
        Console.WriteLine(vbLf & "DUMP:")
        If Dictionary Is Nothing Then
            Console.WriteLine("    The dictionary has not been created.")
        Else
            Console.WriteLine("    Dictionary entries")
            Console.WriteLine("    ------------------")

            For Each kvp As KeyValuePair(Of Integer, MutableKey) In Dictionary
                Console.WriteLine("    {0} : {1}", kvp.Key, kvp.Value)
            Next
        End If

        Console.WriteLine(vbLf & "    List of items")
        Console.WriteLine("    -------------")

        For Each mk As MutableKey In Items
            Console.WriteLine("    {0}", mk)
        Next
    End Sub

End Class

Public Class Demo
    
    Public Shared Sub Main() 

        Dim mkeys As New MutableKeys()

        ' The Add method is inherited from Collection.
        '
        mkeys.Add(New MutableKey(110072674, "Widget"))
        mkeys.Add(New MutableKey(110072675, "Sprocket"))

        mkeys.Dump() 
    
        Console.WriteLine(vbLf & "Create and insert a new item:")
        Dim test As New MutableKey(110072684, "Gear")
        mkeys.Insert(1, test)

        mkeys.Dump()

        Try
            Console.WriteLine(vbLf & "Try to insert the item again:")
            mkeys.Insert(1, test)
        Catch ex As ArgumentException
            Console.WriteLine("Error: {0}", ex.Message)
        End Try

        Console.WriteLine(vbLf & "Change the Key property of the item:")
        test.Key = 100000072

        mkeys.Dump()

        Try
            Console.WriteLine(vbLf & "Try to set the Key property to an existing key:")
            test.Key = 110072674
        Catch ex As ArgumentException
            Console.WriteLine("Error: {0}", ex.Message)
        End Try

        mkeys.Dump()

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

' This class has a key that can be changed.
' 
Public Class MutableKey

    Public Sub New(ByVal newKey As Integer, ByVal newValue As String)
        _key = newKey
        Value = newValue
    End Sub
    
    Public Value As String
    Friend Collection As MutableKeys
    
    Private _key As Integer
    Public Property Key As Integer 
        Get
            Return _key
        End Get
        Set
            If Collection IsNot Nothing Then
                Collection.ChangeKey(Me, value)
            End If

            _key = value
        End Set
    End Property

    Public Overrides Function ToString() As String 
        Return String.Format("{0,9} {1}", _key, Value)
    End Function
        
End Class

' This code example produces the following output:
'
'DUMP:
'    Dictionary entries
'    ------------------
'    110072674 : 110072674 Widget
'    110072675 : 110072675 Sprocket
'
'    List of items
'    -------------
'    110072674 Widget
'    110072675 Sprocket
'
'Create and insert a new item:
'
'DUMP:
'    Dictionary entries
'    ------------------
'    110072674 : 110072674 Widget
'    110072675 : 110072675 Sprocket
'    110072684 : 110072684 Gear
'
'    List of items
'    -------------
'    110072674 Widget
'    110072684 Gear
'    110072675 Sprocket
'
'Try to insert the item again:
'Error: The item already belongs to a collection.
'
'Change the Key property of the item:
'
'DUMP:
'    Dictionary entries
'    ------------------
'    110072674 : 110072674 Widget
'    110072675 : 110072675 Sprocket
'    100000072 : 100000072 Gear
'
'    List of items
'    -------------
'    110072674 Widget
'    100000072 Gear
'    110072675 Sprocket
'
'Try to set the Key property to an existing key:
'Error: An item with the same key has already been added.
'
'DUMP:
'    Dictionary entries
'    ------------------
'    110072674 : 110072674 Widget
'    110072675 : 110072675 Sprocket
'    100000072 : 100000072 Gear
'
'    List of items
'    -------------
'    110072674 Widget
'    100000072 Gear
'    110072675 Sprocket

Remarks

For more information about this API, see Supplemental API remarks for KeyedCollection<TKey,TItem>.ChangeItemKey.

Applies to

See also

• GetKeyForItem(TItem)
• Item[TKey]