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IDictionary.Item Property

Gets or sets the element with the specified key.

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

'Declaration
Property Item ( _
	key As Object _
) As Object

Parameters

key
Type: System.Object

The key of the element to get or set.

Property Value

Type: System.Object
The element with the specified key, or Nothing if the key does not exist.

ExceptionCondition
ArgumentNullException

key is Nothing.

NotSupportedException

The property is set and the IDictionary object is read-only.

-or-

The property is set, key does not exist in the collection, and the IDictionary has a fixed size.

This property provides the ability to access a specific element in the collection by using the following syntax: myCollection[key].

You can also use the Item property to add new elements by setting the value of a key that does not exist in the dictionary (for example, myCollection["myNonexistentKey"] = myValue). However, if the specified key already exists in the dictionary, setting the Item property overwrites the old value. In contrast, the Add method does not modify existing elements.

Implementations can vary in whether they allow the key to be Nothing.

The C# language uses the thisthis (C# Reference) keyword to define the indexers instead of implementing the Item property. Visual Basic implements Item as a default property, which provides the same indexing functionality.

The following code example demonstrates how to implement the Item property. This code example is part of a larger example provided for the IDictionary class.

Public Property Item(ByVal key As Object) As Object Implements IDictionary.Item
    Get 

        ' If this key is in the dictionary, return its value. 
        Dim index As Integer 
        If TryGetIndexOfKey(key, index) Then 

            ' The key was found return its value. 
            Return items(index).Value
        Else 

            ' The key was not found return null. 
            Return Nothing 
        End If 
    End Get 

    Set(ByVal value As Object)
        ' If this key is in the dictionary, change its value.  
        Dim index As Integer 
        If TryGetIndexOfKey(key, index) Then 

            ' The key was found change its value.
            items(index).Value = value
        Else 

            ' This key is not in the dictionary add this key/value pair.
            Add(key, value)
        End If 
    End Set 
End Property

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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