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Dictionary(Of TKey, TValue).IDictionary.Add Method

Adds the specified key and value to the dictionary.

Namespace:  System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)
'Declaration
Private Sub Add ( _
	key As Object, _
	value As Object _
) Implements IDictionary.Add

Parameters

key
Type: System.Object

The object to use as the key.

value
Type: System.Object

The object to use as the value.

Implements

IDictionary.Add(Object, Object)
ExceptionCondition
ArgumentNullException

key is Nothing.

ArgumentException

key is of a type that is not assignable to the key type TKey of the Dictionary(Of TKey, TValue).

-or-

value is of a type that is not assignable to TValue, the type of values in the Dictionary(Of TKey, TValue).

-or-

A value with the same key already exists in the Dictionary(Of TKey, TValue).

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 throws an exception if the specified key already exists.

If Count is less than the capacity, this method approaches an O(1) operation. If the capacity needs to be increased to accommodate the new element, this method becomes an O(n) operation, where n is Count.

The following code example shows how to access the Dictionary(Of TKey, TValue) class through the System.Collections.IDictionary interface. The code example creates an empty Dictionary(Of TKey, TValue) of strings with string keys and uses the IDictionary.Add method to add some elements. The example demonstrates that the IDictionary.Add method throws an ArgumentException when attempting to add a duplicate key, or when a key or value of the wrong data type is supplied.

The code example demonstrates the use of several other members of the System.Collections.IDictionary interface.

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")
        Try
            openWith.Add(42, New Example())
        Catch ex As ArgumentException
            Console.WriteLine("An exception was caught for " & _
                "IDictionary.Add. Exception message:" & vbLf _
                & vbTab & ex.Message & vbLf)
        End Try 

        ' The Add method throws an exception if the new key is  
        ' already in the dictionary. 
        Try
            openWith.Add("txt", "winword.exe")
        Catch 
            Console.WriteLine("An element with Key = ""txt"" already exists.")
        End Try 

        ' 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"))

        ' Contains can be used to test keys before inserting  
        ' them. 
        If Not openWith.Contains("ht") Then
            openWith.Add("ht", "hypertrm.exe")
            Console.WriteLine("Value added for key = ""ht"": {0}", _
                openWith("ht"))
        End If 

        ' IDictionary.Contains returns False if the wrong data  
        ' type is supplied.
        Console.WriteLine("openWith.Contains(29.7) returns {0}", _
            openWith.Contains(29.7))

        ' When you use foreach to enumerate dictionary elements 
        ' with the IDictionary interface, the elements are retrieved 
        ' as DictionaryEntry objects instead of KeyValuePair objects.
        Console.WriteLine()
        For Each de As DictionaryEntry In openWith
            Console.WriteLine("Key = {0}, Value = {1}", _
                de.Key, de.Value)
        Next  

        ' To get the values alone, use the Values property. 
        Dim icoll As ICollection = openWith.Values

        ' The elements of the collection are strongly typed 
        ' with the type that was specified for dictionary values, 
        ' even though the ICollection interface is not strongly 
        ' typed.
        Console.WriteLine()
        For Each s As String In  icoll
            Console.WriteLine("Value = {0}", s)
        Next s

        ' To get the keys alone, use the Keys property.
        icoll = openWith.Keys

        ' The elements of the collection are strongly typed 
        ' with the type that was specified for dictionary keys, 
        ' even though the ICollection interface is not strongly 
        ' typed.
        Console.WriteLine()
        For Each s As String In  icoll
            Console.WriteLine("Key = {0}", s)
        Next s

        ' Use the Remove method to remove a key/value pair. No 
        ' exception is thrown if the wrong data type is supplied.
        Console.WriteLine(vbLf + "Remove(""dib"")")
        openWith.Remove("dib")

        If Not openWith.Contains("dib") Then
            Console.WriteLine("Key ""dib"" is not found.")
        End If 

    End Sub 

End Class 

' This code example produces the following output: 

'An exception was caught for IDictionary.Add. Exception message: 
'        The value "42" is not of type "System.String" and cannot be used in this generic collection. 
'Parameter name: key 

'An element with Key = "txt" already exists. 
'For key = "rtf", value = wordpad.exe. 
'For key = "rtf", value = winword.exe. 
'The default Item property returns Nothing if the key is of the wrong type: 
'For key = 2, value = . 
'A key of the wrong type was specified when setting the default Item property. 
'For key = "tif", value = . 
'Value added for key = "ht": hypertrm.exe 
'openWith.Contains(29.7) returns False 

'Key = txt, Value = notepad.exe 
'Key = bmp, Value = paint.exe 
'Key = dib, Value = paint.exe 
'Key = rtf, Value = winword.exe 
'Key = doc, Value = winword.exe 
'Key = ht, Value = hypertrm.exe 

'Value = notepad.exe 
'Value = paint.exe 
'Value = paint.exe 
'Value = winword.exe 
'Value = winword.exe 
'Value = hypertrm.exe 

'Key = txt 
'Key = bmp 
'Key = dib 
'Key = rtf 
'Key = doc 
'Key = ht 

'Remove("dib") 
'Key "dib" is not found. 
' 

.NET Framework

Supported in: 4.5.1, 4.5, 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

.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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