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KeyedCollection(TKey, TItem) Class

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

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

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

[<AbstractClass>]
[<SerializableAttribute>]
[<ComVisibleAttribute(false)>]
type KeyedCollection<'TKey, 'TItem> =  
    class
        inherit Collection<'TItem>
    end

Type Parameters

TKey

The type of keys in the collection.

TItem

The type of items in the collection.

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

  NameDescription
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryKeyedCollection(TKey, TItem)()Initializes a new instance of the KeyedCollection(TKey, TItem) class that uses the default equality comparer.
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryKeyedCollection(TKey, TItem)(IEqualityComparer(TKey))Initializes a new instance of the KeyedCollection(TKey, TItem) class that uses the specified equality comparer.
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryKeyedCollection(TKey, TItem)(IEqualityComparer(TKey), Int32)Initializes a new instance of the KeyedCollection(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(T). (Inherited from Collection(T).)
Protected propertySupported by the XNA FrameworkSupported by Portable Class LibraryDictionaryGets the lookup dictionary of the KeyedCollection(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(T).)
Protected propertySupported by the XNA FrameworkSupported by Portable Class LibraryItemsGets a IList(T) wrapper around the Collection(T). (Inherited from Collection(T).)
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  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryAddAdds an object to the end of the Collection(T). (Inherited from Collection(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(T). (Inherited from Collection(T).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryClearItemsRemoves all elements from the KeyedCollection(TKey, TItem). (Overrides Collection(T).ClearItems().)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryContains(T)Determines whether an element is in the Collection(T). (Inherited from Collection(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(T) to a compatible one-dimensional Array, starting at the specified index of the target array. (Inherited from Collection(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(T). (Inherited from Collection(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(T). (Inherited from Collection(T).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryInsertInserts an element into the Collection(T) at the specified index. (Inherited from Collection(T).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryInsertItemInserts an element into the KeyedCollection(TKey, TItem) at the specified index. (Overrides Collection(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(T). (Inherited from Collection(T).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryRemove(TKey)Removes the element with the specified key from the KeyedCollection(TKey, TItem).
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryRemoveAtRemoves the element at the specified index of the Collection(T). (Inherited from Collection(T).)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryRemoveItemRemoves the element at the specified index of the KeyedCollection(TKey, TItem). (Overrides Collection(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(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(T).)
Explicit interface implemetationPrivate propertySupported by the XNA FrameworkSupported by Portable Class LibraryICollection(T).IsReadOnlyGets a value indicating whether the ICollection(T) is read-only. (Inherited from Collection(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(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(T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIEnumerable.GetEnumeratorReturns an enumerator that iterates through a collection. (Inherited from Collection(T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIList.AddAdds an item to the IList. (Inherited from Collection(T).)
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibraryIList.ContainsDetermines whether the IList contains a specific value. (Inherited from Collection(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(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(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(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(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(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(T).)
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The KeyedCollection(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(TKey, TItem), derive your collection type from the appropriate constructed type.

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

Unlike dictionaries, an element of KeyedCollection(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(TKey, TItem) includes a lookup dictionary that you can obtain with the Dictionary property. When an item is added to the KeyedCollection(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(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(TKey, TItem), and demonstrates many of the inherited methods. The second example shows how to override the protected methods of KeyedCollection(TKey, TItem) to provide custom behavior.

Example 1

This code example shows the minimum code necessary to derive a collection class from KeyedCollection(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(TKey, TItem) and Collection(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(TKey, TItem). For a code example that uses mutable keys, see ChangeItemKey.

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

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(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.

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

.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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