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KeyedCollection.ClearItems Method

Removes all elements from the KeyedCollection.

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

virtual void ClearItems () override
protected void ClearItems ()
protected override function ClearItems ()
Not applicable.

If there is a lookup dictionary, this method clears it but does not delete it.

If the number of elements has exceeded the dictionary creation threshold and the KeyedCollection is using a lookup dictionary, it will continue to use a lookup dictionary even though the number of elements is again under the threshold.

This method is an O(n) operation, where n is Count.

Notes for Implementers

Override this method to provide customized behavior for the Clear method, inherited from the Collection generic class.

Call the base class implementation of this method to clear the underlying collection and to clear the lookup dictionary.

This section contains two code examples that demonstrate overriding the ClearItems method to provide custom behavior for clearing all objects from the collection. The first example adds a custom notification event and the second provides support for a collection of objects with mutable keys.

Example 1

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 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.

using namespace System;
using namespace System::Collections::Generic;
using namespace System::Collections::ObjectModel;

public enum class ChangeTypes

ref class SimpleOrderChangedEventArgs; 

// This class represents a simple line item in an order. All the 
// values are immutable except quantity.
public ref class OrderItem
    int _quantity;

    initonly int PartNumber;
    initonly String^ Description;
    initonly double UnitPrice;
    OrderItem(int partNumber, String^ description, int quantity, 
        double unitPrice)
        this->PartNumber = partNumber;
        this->Description = description;
        this->Quantity = quantity;
        this->UnitPrice = unitPrice;
    property int Quantity    
        int get() { return _quantity; };
        void set(int value)
            if (value < 0)
                throw gcnew ArgumentException("Quantity cannot be negative.");
            _quantity = value;
    virtual String^ ToString() override 
        return String::Format(
            "{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}", 
            PartNumber, _quantity, Description, UnitPrice, 
            UnitPrice * _quantity);

// Event argument for the Changed event.
public ref class SimpleOrderChangedEventArgs : EventArgs
    OrderItem^ ChangedItem;
    initonly ChangeTypes ChangeType;
    OrderItem^ ReplacedWith;

    SimpleOrderChangedEventArgs(ChangeTypes change, 
        OrderItem^ item, OrderItem^ replacement)
        this->ChangeType = change;
        this->ChangedItem = item;
        this->ReplacedWith = replacement;

// 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<int, OrderItem>.
// (Note that the key of OrderItem cannot be changed; if it could 
// be changed, SimpleOrder would have to override ChangeItemKey.)
public ref class SimpleOrder : KeyedCollection<int, OrderItem^>
    event EventHandler<SimpleOrderChangedEventArgs^>^ Changed;

    // 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.
    SimpleOrder() : KeyedCollection<int, OrderItem^>(nullptr, 0) {};
    // This is the only method that absolutely must be overridden,
    // because without it the KeyedCollection cannot extract the
    // keys from the items. 
    virtual int GetKeyForItem(OrderItem^ item) override
        // In this example, the key is the part number.
        return item->PartNumber;

    virtual void InsertItem(int index, OrderItem^ newItem) override 
        __super::InsertItem(index, newItem);

        Changed(this, gcnew SimpleOrderChangedEventArgs(
            ChangeTypes::Added, newItem, nullptr));

    virtual void SetItem(int index, OrderItem^ newItem) override 
        OrderItem^ replaced = this->Items[index];
        __super::SetItem(index, newItem);

        Changed(this, gcnew SimpleOrderChangedEventArgs(
            ChangeTypes::Replaced, replaced, newItem));

    virtual void RemoveItem(int index) override 
        OrderItem^ removedItem = Items[index];

        Changed(this, gcnew SimpleOrderChangedEventArgs(
            ChangeTypes::Removed, removedItem, nullptr));

    virtual void ClearItems() override 

        Changed(this, gcnew SimpleOrderChangedEventArgs(
            ChangeTypes::Cleared, nullptr, nullptr));

    // This method uses the internal reference to the dictionary
    // to test fo
    void AddOrMerge(OrderItem^ newItem)

        int key = this->GetKeyForItem(newItem);
        OrderItem^ existingItem = nullptr;

        // The dictionary is not created until the first item is 
        // added, so it is necessary to test for null. Using 
        // AndAlso ensures that TryGetValue is not called if the
        // dictionary does not exist.
        if (this->Dictionary != nullptr && 
            this->Dictionary->TryGetValue(key, existingItem))
            existingItem->Quantity += newItem->Quantity;

public ref class Demo
    static void Main()
        SimpleOrder^ weekly = gcnew SimpleOrder();
        weekly->Changed += gcnew 

        // The Add method, inherited from Collection, takes OrderItem->
        weekly->Add(gcnew OrderItem(110072674, "Widget", 400, 45.17));
        weekly->Add(gcnew OrderItem(110072675, "Sprocket", 27, 5.3));
        weekly->Add(gcnew OrderItem(101030411, "Motor", 10, 237.5));
        weekly->Add(gcnew OrderItem(110072684, "Gear", 175, 5.17));

        // The Contains method of KeyedCollection takes TKey.
        Console::WriteLine("\nContains(101030411): {0}", 

        // The default Item property of KeyedCollection takes the key
        // type, Integer. The property is read-only.
        Console::WriteLine("\nweekly[101030411]->Description: {0}", 

        // The Remove method of KeyedCollection takes a key.

        // The Insert method, inherited from Collection, takes an 
        // index and an OrderItem.
        Console::WriteLine("\nInsert(2, gcnew OrderItem(...))");
        weekly->Insert(2, gcnew OrderItem(111033401, "Nut", 10, .5));
        // The default Item property is overloaded. One overload comes
        // from KeyedCollection<int, OrderItem>; that overload
        // is read-only, and takes Integer because it retrieves by key. 
        // The other overload comes from Collection<OrderItem>, the 
        // base class of KeyedCollection<int, 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<OrderItem>. Otherwise the index is interpreted
        // as a key, and KeyNotFoundException is thrown.
        Collection<OrderItem^>^ coweekly = weekly;
        Console::WriteLine("\ncoweekly[2].Description: {0}", 
        Console::WriteLine("\ncoweekly[2] = gcnew OrderItem(...)");
        coweekly[2] = gcnew OrderItem(127700026, "Crank", 27, 5.98);

        OrderItem^ temp = coweekly[2];

        // The IndexOf method, inherited from Collection<OrderItem>, 
        // takes an OrderItem instead of a key.
        Console::WriteLine("\nIndexOf(temp): {0}", weekly->IndexOf(temp));

        // The inherited Remove method also takes an OrderItem->


        weekly->AddOrMerge(gcnew OrderItem(110072684, "Gear", 1000, 5.17));


    static void Display(SimpleOrder^ order)
        for each( OrderItem^ item in order )

    static void ChangedHandler(Object^ source, 
        SimpleOrderChangedEventArgs^ e)
        OrderItem^ item = e->ChangedItem;

        if (e->ChangeType == ChangeTypes::Replaced)
            OrderItem^ replacement = e->ReplacedWith;

            Console::WriteLine("{0} (quantity {1}) was replaced " +
                "by {2}, (quantity {3}).", item->Description, 
                item->Quantity, replacement->Description, 
        else if(e->ChangeType == ChangeTypes::Cleared)
            Console::WriteLine("The order list was cleared.");
            Console::WriteLine("{0} (quantity {1}) was {2}.", 
                item->Description, item->Quantity, e->ChangeType);

void main()

/* 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

Motor (quantity 10) was Removed.

Insert(2, gcnew OrderItem(...))
Nut (quantity 10) was Added.

coweekly[2].Description: Nut

coweekly[2] = gcnew OrderItem(...)
Nut (quantity 10) was replaced by Crank, (quantity 27).

IndexOf(temp): 2

Crank (quantity 27) was Removed.

Widget (quantity 400) was Removed.

110072675     27 Sprocket     at     5.30 =     143.10
110072684   1175 Gear         at     5.17 =   6,074.75

The order list was cleared.

Example 2

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, 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 (Friend in Visual Basic) 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 (Friend in Visual Basic) 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.

No code example is currently available or this language may not be supported.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

.NET Compact Framework

Supported in: 2.0

XNA Framework

Supported in: 1.0