[Note: This topic is pre-release documentation and is subject to change in future releases. Blank topics are included as placeholders.]
Represents a collection of keys and values.
(in mscorlib.dll)
Visual Basic (Declaration)
<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
Dim instance As Dictionary(Of TKey, TValue)
[ComVisibleAttribute(false)]
public class Dictionary<TKey, TValue> : IDictionary<TKey, TValue>,
ICollection<KeyValuePair<TKey, TValue>>, IEnumerable<KeyValuePair<TKey, TValue>>,
IDictionary, ICollection, IEnumerable
Type Parameters
- TKey
The type of the keys in the dictionary.
- TValue
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.
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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 nullNothingnullptra null reference (Nothing in Visual Basic), 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.
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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:
foreach (KeyValuePair<int, string> kvp in myDictionary) {...}
for each (KeyValuePair<int, String^> kvp in myDictionary) {...}
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.
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Because keys can be inherited and their behavior changed, their absolute uniqueness cannot be guaranteed by comparisons using the Equals method. |
There are two examples for this class. This first example uses a Dictionary<(Of <(TKey, TValue>)>) object to contain cast member information for the play "Othello". The names of the characters are the keys of the dictionary and the names of the actors are the values of the dictionary. This dictionary (OthelloDict) is contained in a class (OthelloCast) that is derived from a List<(Of <(T>)>) class. The constructor for the OthelloCast object populates its list with the keys of the OthelloDict dictionary. The OthelloCast object is bound to a ListBox control. The binding in this example is one-way; see Data Binding for a discussion of other binding options and scenarios.
The following code shows the OthelloCast class. Add this class to the partial Page class of your project.
Public Class OthelloCast
Inherits List(Of String)
Public OthelloDict As New Dictionary(Of String, String)
Public Sub New()
' Add data to the dictionary.
OthelloDict.Add("Bianca", "Gretchen Rivas")
OthelloDict.Add("Brabantio", "Carlos Lacerda")
OthelloDict.Add("Cassio", "Steve Masters")
OthelloDict.Add("Clown", "Michael Ludwig")
OthelloDict.Add("Desdemona", "Catherine Autier Miconi")
OthelloDict.Add("Duke of Venice", "Ken Circeo")
OthelloDict.Add("Emilia", "Eva Valverde")
OthelloDict.Add("Gratiano", "Akos Kozari")
OthelloDict.Add("Iago", "Darius Stasevicius")
OthelloDict.Add("Lodovico", "Fernando Souza")
OthelloDict.Add("Montano", "Jeff Hay")
OthelloDict.Add("Othello", "Marco Tanara")
OthelloDict.Add("Roderigo", "Pedro Ruivo")
' Populate the list with character names.
For Each kvp As KeyValuePair(Of String, String) In OthelloDict
Me.Add(kvp.Key)
Next
End Sub
End Class
// Make sure this class is
// within the C# namespace.
public class OthelloCast : List<string>
{
// Use a dictionary to contain
// cast names (key) and actor names (value).
public Dictionary<string, string> OthelloDict =
new Dictionary<string, string>();
public OthelloCast()
{
// Add data to the dictionary.
OthelloDict.Add("Bianca", "Gretchen Rivas");
OthelloDict.Add("Brabantio", "Carlos Lacerda");
OthelloDict.Add("Cassio", "Steve Masters");
OthelloDict.Add("Clown", "Michael Ludwig");
OthelloDict.Add("Desdemona", "Catherine Autier Miconi");
OthelloDict.Add("Duke of Venice", "Ken Circeo");
OthelloDict.Add("Emilia", "Eva Valverde");
OthelloDict.Add("Gratiano", "Akos Kozari");
OthelloDict.Add("Iago", "Darius Stasevicius");
OthelloDict.Add("Lodovico", "Fernando Souza");
OthelloDict.Add("Montano", "Jeff Hay");
OthelloDict.Add("Othello", "Marco Tanara");
OthelloDict.Add("Roderigo", "Pedro Ruivo");
// Populate the list with character names.
foreach (KeyValuePair<string, string> kvp in OthelloDict)
{
this.Add(kvp.Key);
}
}
}
When you select a character in the ListBox control, the actor who plays that character is displayed in a TextBlock control. The following code shows how this is done by the event handler for the SelectionChanged event for the ListBox control. Add this handler to the partial Page class.
Private Sub ListCharacters_SelectionChanged(ByVal sender As System.Object, ByVal e As SelectionChangedEventArgs)
' Create an instance of OthelloCast to
' look up an actor by their character.
Dim othello As New OthelloCast()
' Get the selected character name (key).
Dim key As String = ListCharacters.SelectedItem.ToString()
' Get the key's value (actor name) and display it.
ShowActor.Text = othello.OthelloDict(key).ToString()
End Sub
private void ListCharacters_SelectionChanged(object sender,
SelectionChangedEventArgs e)
{
// Create an instance of OthelloCast to
// look up an actor by their character.
OthelloCast othello = new OthelloCast();
// Get the selected character name (key).
string key = ListCharacters.SelectedItem.ToString();
// Get the key's value (actor name) and display it.
ShowActor.Text = othello.OthelloDict[key].ToString();
}
The following XAML references and elements perform the data binding:
xmlns:my="clr-namespace:CastMembers"
This is a reference to the common language runtime (CLR) namespace, which is automatically declared within the assembly and exposes its public types. Replace CsatMembers with the name of your assembly (project name).
<my:OthelloCast x:Name="Characters"/>
This is a resource dictionary reference specified in the <Grid.Resources> element. It specifies the object to bind to (OthelloCast) and its name (Characters) is specified by controls, such as a ListBox control, that use this resource.
<ListBox Margin ="5,5,5,5" SelectionChanged="ListCharacters_SelectionChanged" Grid.Row="1" Grid.Column="0" x:Name="ListCharacters" ItemsSource="{Binding Source={StaticResource Characters}}"/>
This element defines a ListBox control with the ItemsSource property set to the bound object. Note that it includes the event handler defined for the SelectionChanged event.
The complete XAML is as follows.
<UserControl x:Class="CastMembers.Page"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:my="clr-namespace:CastMembers"
Width="400" Height="400">
<Grid x:Name="LayoutRoot" Background="White" ShowGridLines="False">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Grid.Resources>
<my:OthelloCast x:Name="Characters"/>
</Grid.Resources>
<TextBlock Margin ="5,5,5,5" Grid.Row="0" Grid.Column="0" Text="Othello Cast:" FontWeight="Bold" />
<ListBox Margin ="5,5,5,5" SelectionChanged="ListCharacters_SelectionChanged" Grid.Row="1" Grid.Column="0" x:Name="ListCharacters" ItemsSource="{Binding Source={StaticResource Characters}}"/>
<TextBlock Margin ="5,5,5,5" Grid.Row="0" Grid.Column="1" Text="Actor:" FontWeight="Bold" />
<TextBlock Margin ="5,5,5,5" Grid.Row="1" Grid.Column="1" x:Name="ShowActor" Text=""/>
</Grid>
</UserControl>
The next 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.Collections.Generic
Public Class Example
Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
' 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.
Try
openWith.Add("txt", "winword.exe")
Catch
outputBlock.Text &= "An element with Key = ""txt"" already exists." & vbCrLf
End Try
' The Item property is the default property, so you
' can omit its name when accessing elements.
outputBlock.Text &= String.Format("For key = ""rtf"", value = {0}.", _
openWith("rtf")) & vbCrLf
' The default Item property can be used to change the value
' associated with a key.
openWith("rtf") = "winword.exe"
outputBlock.Text &= String.Format("For key = ""rtf"", value = {0}.", _
openWith("rtf")) & vbCrLf
' 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.
Try
outputBlock.Text &= String.Format("For key = ""tif"", value = {0}.", _
openWith("tif")) & vbCrLf
Catch
outputBlock.Text &= "Key = ""tif"" is not found." & vbCrLf
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
outputBlock.Text &= String.Format("For key = ""tif"", value = {0}.", value) & vbCrLf
Else
outputBlock.Text &= "Key = ""tif"" is not found." & vbCrLf
End If
' ContainsKey can be used to test keys before inserting
' them.
If Not openWith.ContainsKey("ht") Then
openWith.Add("ht", "hypertrm.exe")
outputBlock.Text &= String.Format("Value added for key = ""ht"": {0}", _
openWith("ht")) & vbCrLf
End If
' When you use foreach to enumerate dictionary elements,
' the elements are retrieved as KeyValuePair objects.
outputBlock.Text &= vbCrLf
For Each kvp As KeyValuePair(Of String, String) In openWith
outputBlock.Text &= String.Format("Key = {0}, Value = {1}", _
kvp.Key, kvp.Value) & vbCrLf
Next kvp
' To get the values alone, use the Values property.
Dim valueColl As _
Dictionary(Of String, String).ValueCollection = _
openWith.Values
' The elements of the ValueCollection are strongly typed
' with the type that was specified for dictionary values.
outputBlock.Text &= vbCrLf
For Each s As String In valueColl
outputBlock.Text &= String.Format("Value = {0}", s) & vbCrLf
Next s
' To get the keys alone, use the Keys property.
Dim keyColl As _
Dictionary(Of String, String).KeyCollection = _
openWith.Keys
' The elements of the KeyCollection are strongly typed
' with the type that was specified for dictionary keys.
outputBlock.Text &= vbCrLf
For Each s As String In keyColl
outputBlock.Text &= String.Format("Key = {0}", s) & vbCrLf
Next s
' Use the Remove method to remove a key/value pair.
outputBlock.Text &= vbLf + "Remove(""doc"")" & vbCrLf
openWith.Remove("doc")
If Not openWith.ContainsKey("doc") Then
outputBlock.Text &= "Key ""doc"" is not found." & vbCrLf
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
'
'Remove("doc")
'Key "doc" is not found.
'
using System;
using System.Collections.Generic;
public class Example
{
public static void Demo(System.Windows.Controls.TextBlock outputBlock)
{
// Create a new dictionary of strings, with string keys.
//
Dictionary<string, string> openWith =
new Dictionary<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.
try
{
openWith.Add("txt", "winword.exe");
}
catch (ArgumentException)
{
outputBlock.Text += "An element with Key = \"txt\" already exists." + "\n";
}
// The Item property is another name for the indexer, so you
// can omit its name when accessing elements.
outputBlock.Text += String.Format("For key = \"rtf\", value = {0}.",
openWith["rtf"]) + "\n";
// The indexer can be used to change the value associated
// with a key.
openWith["rtf"] = "winword.exe";
outputBlock.Text += String.Format("For key = \"rtf\", value = {0}.",
openWith["rtf"]) + "\n";
// If a key does not exist, setting the indexer for that key
// adds a new key/value pair.
openWith["doc"] = "winword.exe";
// The indexer throws an exception if the requested key is
// not in the dictionary.
try
{
outputBlock.Text += String.Format("For key = \"tif\", value = {0}.",
openWith["tif"]) + "\n";
}
catch (KeyNotFoundException)
{
outputBlock.Text += "Key = \"tif\" is not found." + "\n";
}
// 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.
string value = "";
if (openWith.TryGetValue("tif", out value))
{
outputBlock.Text += String.Format("For key = \"tif\", value = {0}.", value) + "\n";
}
else
{
outputBlock.Text += "Key = \"tif\" is not found." + "\n";
}
// ContainsKey can be used to test keys before inserting
// them.
if (!openWith.ContainsKey("ht"))
{
openWith.Add("ht", "hypertrm.exe");
outputBlock.Text += String.Format("Value added for key = \"ht\": {0}",
openWith["ht"]) + "\n";
}
// When you use foreach to enumerate dictionary elements,
// the elements are retrieved as KeyValuePair objects.
outputBlock.Text += "\n";
foreach (KeyValuePair<string, string> kvp in openWith)
{
outputBlock.Text += String.Format("Key = {0}, Value = {1}",
kvp.Key, kvp.Value) + "\n";
}
// To get the values alone, use the Values property.
Dictionary<string, string>.ValueCollection valueColl =
openWith.Values;
// The elements of the ValueCollection are strongly typed
// with the type that was specified for dictionary values.
outputBlock.Text += "\n";
foreach (string s in valueColl)
{
outputBlock.Text += String.Format("Value = {0}", s) + "\n";
}
// To get the keys alone, use the Keys property.
Dictionary<string, string>.KeyCollection keyColl =
openWith.Keys;
// The elements of the KeyCollection are strongly typed
// with the type that was specified for dictionary keys.
outputBlock.Text += "\n";
foreach (string s in keyColl)
{
outputBlock.Text += String.Format("Key = {0}", s) + "\n";
}
// Use the Remove method to remove a key/value pair.
outputBlock.Text += "\nRemove(\"doc\")" + "\n";
openWith.Remove("doc");
if (!openWith.ContainsKey("doc"))
{
outputBlock.Text += "Key \"doc\" is not found." + "\n";
}
}
}
/* 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
Remove("doc")
Key "doc" is not found.
*/
System..::.Object
System.Collections.Generic..::.Dictionary<(Of <(TKey, TValue>)>)
System.Configuration..::.ConfigurationElement
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.
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.
Reference