IDictionary.Item Property (Object)

 
System_CAPS_noteNote

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Gets or sets the element with the specified key.

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

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 null if the key does not exist.

Exception Condition
ArgumentNullException

key is null.

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 null.

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

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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