Dictionary<TKey,TValue>.IDictionary.Item[Object] Property

Definition

Namespace:

System.Collections.Generic

Assembly:

System.Collections.dll

Source:

Dictionary.cs

Gets or sets the value with the specified key.

property System::Object ^ System::Collections::IDictionary::Item[System::Object ^] { System::Object ^ get(System::Object ^ key); void set(System::Object ^ key, System::Object ^ value); };

object? System.Collections.IDictionary.Item[object key] { get; set; }

member this.System.Collections.IDictionary.Item(obj) : obj with get, set

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

Parameters

key

Object

The key of the value to get.

Property Value

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<TKey,TValue>.

Implements

Item[Object]

Exceptions

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<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<TKey,TValue>.

Examples

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<TKey,TValue>, and ways the property differs from the Dictionary<TKey,TValue>.Item[] property.

The example shows that, like the Dictionary<TKey,TValue>.Item[] property, the Dictionary<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<TKey,TValue>.Item[] property, the Dictionary<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<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.

using System;
using System.Collections;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        // Create a new dictionary of strings, with string keys,
        // and access it using the IDictionary interface.
        //
        IDictionary openWith = new Dictionary<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");

open System
open System.Collections
open System.Collections.Generic

// Create a new dictionary of strings, with string keys,
// and access it using the IDictionary interface.
let openWith: IDictionary = Dictionary<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")

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 another name for the indexer, so you
// can omit its name when accessing elements.
Console.WriteLine("For key = \"rtf\", value = {0}.",
    openWith["rtf"]);

// The indexer 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 indexer for that key
// adds a new key/value pair.
openWith["doc"] = "winword.exe";

// The indexer returns null if the key is of the wrong data
// type.
Console.WriteLine("The indexer returns null"
    + " if the key is of the wrong type:");
Console.WriteLine("For key = 2, value = {0}.",
    openWith[2]);

// The indexer 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 (ArgumentException)
{
    Console.WriteLine("A key of the wrong type was specified"
        + " when assigning to the indexer.");
}

// The Item property is another name for the indexer, so you
// can omit its name when accessing elements.
printfn $"""For key = "rtf", value = {openWith.["rtf"]}."""

// The indexer can be used to change the value associated
// with a key.
openWith["rtf"] <- "winword.exe"
printfn $"""For key = "rtf", value = {openWith.["rtf"]}."""

// If a key does not exist, setting the indexer for that key
// adds a new key/value pair.
openWith["doc"] <- "winword.exe"

// The indexer returns null if the key is of the wrong data
// type.
printfn "The indexer returns null if the key is of the wrong type:"
printfn $"""For key = 2, value = {openWith.[2]}."""

// The indexer throws an exception when setting a value
// if the key is of the wrong data type.
try
    openWith[2] <- "This does not get added."
with :? ArgumentException ->
    printfn "A key of the wrong type was specified when assigning to the indexer."

' 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"]);

// 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.
printfn $"""For key = "tif", value = {openWith.["tif"]}."""

' 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

Remarks

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.

See also

• Add(Object, Object)