Dictionary(Of TKey, TValue).IDictionary.Item Property (Object)

 

Gets or sets the value with the specified key.

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

Private Property Item (
	key As Object
) As Object Implements IDictionary.Item

Parameters

key
Type: System.Object

The key of the value to get.

Property Value

Type: System.Object

The value associated with the specified key, or null if key is not in the dictionary or key is of a type that is not assignable to the key type TKey of the Dictionary(Of TKey, TValue).

Exception Condition
ArgumentNullException

key is null.

ArgumentException

A value is being assigned, and key is of a type that is not assignable to the key type TKey of the Dictionary(Of TKey, TValue).

-or-

A value is being assigned, and value is of a type that is not assignable to the value type TValue of the Dictionary(Of TKey, TValue).

This property provides the ability to access a specific value in the collection by using the following C# syntax: myCollection[key] (myCollection(key) in Visual Basic).

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.

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

Getting or setting the value of this property approaches an O(1) operation.

The following code example shows how to use the IDictionary.Item property (the indexer in C#) of the System.Collections.IDictionary interface with a Dictionary(Of TKey, TValue), and ways the property differs from the Dictionary(Of TKey, TValue).Item property.

The example shows that, like the Dictionary(Of TKey, TValue).Item property, the Dictionary(Of TKey, TValue).IDictionary.Item property can change the value associated with an existing key and can be used to add a new key/value pair if the specified key is not in the dictionary. The example also shows that unlike the Dictionary(Of TKey, TValue).Item property, the Dictionary(Of TKey, TValue).IDictionary.Item property does not throw an exception if key is not in the dictionary, returning a null reference instead. Finally, the example demonstrates that getting the Dictionary(Of TKey, TValue).IDictionary.Item property returns a null reference if key is not the correct data type, and that setting the property throws an exception if key is not the correct data type.

The code example is part of a larger example, including output, provided for the IDictionary.Add method.

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

Public Class Example

    Public Shared Sub Main() 

        ' Create a new dictionary of strings, with string keys,
        ' and access it using the IDictionary interface.
        '
        Dim openWith As IDictionary = _
            New Dictionary(Of String, String)

        ' Add some elements to the dictionary. There are no 
        ' duplicate keys, but some of the values are duplicates.
        ' IDictionary.Add throws an exception if incorrect types
        ' are supplied for key or value.
        openWith.Add("txt", "notepad.exe")
        openWith.Add("bmp", "paint.exe")
        openWith.Add("dib", "paint.exe")
        openWith.Add("rtf", "wordpad.exe")
' The Item property is the default property, so you 
' can omit its name when accessing elements. 
Console.WriteLine("For key = ""rtf"", value = {0}.", _
    openWith("rtf"))

' The default Item property can be used to change the value
' associated with a key.
openWith("rtf") = "winword.exe"
Console.WriteLine("For key = ""rtf"", value = {0}.", _
    openWith("rtf"))

' If a key does not exist, setting the default Item property
' for that key adds a new key/value pair.
openWith("doc") = "winword.exe"

' The default Item property returns Nothing if the key
' is of the wrong data type.
Console.WriteLine("The default Item property returns Nothing" _
    & " if the key is of the wrong type:")
Console.WriteLine("For key = 2, value = {0}.", _
    openWith(2))

' The default Item property throws an exception when setting
' a value if the key is of the wrong data type.
Try
    openWith(2) = "This does not get added."
Catch 
    Console.WriteLine("A key of the wrong type was specified" _
        & " when setting the default Item property.")
End Try
' Unlike the default Item property on the Dictionary class
' itself, IDictionary.Item does not throw an exception
' if the requested key is not in the dictionary.
Console.WriteLine("For key = ""tif"", value = {0}.", _
    openWith("tif"))

    End Sub

End Class

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
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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