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Dictionary(Of TKey, TValue) Class

Represents a collection of keys and values.

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

<SerializableAttribute> _
<ComVisibleAttribute(False)> _
Public Class Dictionary(Of TKey, TValue) _
	Implements IDictionary(Of TKey, TValue), ICollection(Of KeyValuePair(Of TKey, TValue)),  _
	IEnumerable(Of KeyValuePair(Of TKey, TValue)), IDictionary,  _
	ICollection, IEnumerable, ISerializable, IDeserializationCallback
Dim instance As Dictionary(Of TKey, TValue)

Type Parameters


The type of the keys in the dictionary.


The type of the values in the dictionary.

The Dictionary(Of TKey, TValue) generic class provides a mapping from a set of keys to a set of values. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is very fast, close to O(1), because the Dictionary(Of TKey, TValue) class is implemented as a hash table.


The speed of retrieval depends on the quality of the hashing algorithm of the type specified for TKey.

As long as an object is used as a key in the Dictionary(Of TKey, TValue), it must not change in any way that affects its hash value. Every key in a Dictionary(Of TKey, TValue) must be unique according to the dictionary's equality comparer. A key cannot be Nothing, but a value can be, if the value type TValue is a reference type.

Dictionary(Of TKey, TValue) requires an equality implementation to determine whether keys are equal. You can specify an implementation of the IEqualityComparer(Of T) generic interface by using a constructor that accepts a comparer parameter; if you do not specify an implementation, the default generic equality comparer EqualityComparer(Of T).Default is used. If type TKey implements the System.IEquatable(Of T) generic interface, the default equality comparer uses that implementation.


For example, you can use the case-insensitive string comparers provided by the StringComparer class to create dictionaries with case-insensitive string keys.

The capacity of a Dictionary(Of TKey, TValue) is the number of elements the Dictionary(Of TKey, TValue) can hold. As elements are added to a Dictionary(Of TKey, TValue), the capacity is automatically increased as required by reallocating the internal array.

For purposes of enumeration, each item in the dictionary is treated as a KeyValuePair(Of TKey, TValue) structure representing a value and its key. The order in which the items are returned is undefined.

The foreach statement of the C# language (for each in C++, For Each in Visual Basic) requires the type of each element in the collection. Since the Dictionary(Of TKey, TValue) is a collection of keys and values, the element type is not the type of the key or the type of the value. Instead, the element type is a KeyValuePair(Of TKey, TValue) of the key type and the value type. For example:

For Each kvp As KeyValuePair(Of Integer, String) In myDictionary
Next kvp

The foreach statement is a wrapper around the enumerator, which allows only reading from the collection, not writing to it.


Because keys can be inherited and their behavior changed, their absolute uniqueness cannot be guaranteed by comparisons using the Equals method.

The following code example creates an empty Dictionary(Of TKey, TValue) of strings with string keys and uses the Add method to add some elements. The example demonstrates that the Add method throws an ArgumentException when attempting to add a duplicate key.

The example uses the Item property (the indexer in C#) to retrieve values, demonstrating that a KeyNotFoundException is thrown when a requested key is not present, and showing that the value associated with a key can be replaced.

The example shows how to use the TryGetValue method as a more efficient way to retrieve values if a program often must try key values that are not in the dictionary, and it shows how to use the ContainsKey method to test whether a key exists before calling the Add method.

The example shows how to enumerate the keys and values in the dictionary and how to enumerate the keys and values alone using the Keys property and the Values property.

Finally, the example demonstrates the Remove method.

Imports System
Imports System.Collections.Generic

Public Class Example

    Public Shared Sub Main() 

        ' Create a new dictionary of strings, with string keys. 
        Dim openWith As New Dictionary(Of String, String)

        ' Add some elements to the dictionary. There are no  
        ' duplicate keys, but some of the values are duplicates.
        openWith.Add("txt", "notepad.exe")
        openWith.Add("bmp", "paint.exe")
        openWith.Add("dib", "paint.exe")
        openWith.Add("rtf", "wordpad.exe")

        ' The Add method throws an exception if the new key is  
        ' already in the dictionary. 
            openWith.Add("txt", "winword.exe")
            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}.", _

        ' 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}.", _

        ' 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 throws an exception if the requested 
        ' key is not in the dictionary. 
            Console.WriteLine("For key = ""tif"", value = {0}.", _
            Console.WriteLine("Key = ""tif"" is not found.")
        End Try 

        ' When a program often has to try keys that turn out not to 
        ' be in the dictionary, TryGetValue can be a more efficient  
        ' way to retrieve values. 
        Dim value As String = "" 
        If openWith.TryGetValue("tif", value) Then
            Console.WriteLine("For key = ""tif"", value = {0}.", value)
            Console.WriteLine("Key = ""tif"" is not found.")
        End If 

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

        ' When you use foreach to enumerate dictionary elements, 
        ' the elements are retrieved as KeyValuePair objects.
        For Each kvp As KeyValuePair(Of String, String) In openWith
            Console.WriteLine("Key = {0}, Value = {1}", _
                kvp.Key, kvp.Value)
        Next kvp

        ' To get the values alone, use the Values property. 
        Dim valueColl As _
            Dictionary(Of String, String).ValueCollection = _

        ' The elements of the ValueCollection are strongly typed 
        ' with the type that was specified for dictionary values.
        For Each s As String In  valueColl
            Console.WriteLine("Value = {0}", s)
        Next s

        ' To get the keys alone, use the Keys property. 
        Dim keyColl As _
            Dictionary(Of String, String).KeyCollection = _

        ' The elements of the KeyCollection are strongly typed 
        ' with the type that was specified for dictionary keys.
        For Each s As String In  keyColl
            Console.WriteLine("Key = {0}", s)
        Next s

        ' Use the Remove method to remove a key/value pair.
        Console.WriteLine(vbLf + "Remove(""doc"")")

        If Not openWith.ContainsKey("doc") Then
            Console.WriteLine("Key ""doc"" is not found.")
        End If 

    End Sub 

End Class 

' This code example produces the following output: 

'An element with Key = "txt" already exists. 
'For key = "rtf", value = wordpad.exe. 
'For key = "rtf", value = winword.exe. 
'Key = "tif" is not found. 
'Key = "tif" is not found. 
'Value added for key = "ht": hypertrm.exe 

'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 

'Key "doc" is not found. 

  System.Collections.Generic.Dictionary(Of TKey, TValue)

Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

A Dictionary(Of TKey, TValue) can support multiple readers concurrently, as long as the collection is not modified. Even so, enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with write accesses, the collection must be locked during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0