Dictionary(Of TKey, TValue).ContainsKey Method (TKey)


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Determines whether the Dictionary(Of TKey, TValue) contains the specified key.

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

Public Function ContainsKey (
	key As TKey
) As Boolean


Type: TKey

The key to locate in the Dictionary(Of TKey, TValue).

Return Value

Type: System.Boolean

true if the Dictionary(Of TKey, TValue) contains an element with the specified key; otherwise, false.

Exception Condition

key is null.

This method approaches an O(1) operation.

The following code example shows how to use the ContainsKey method to test whether a key exists prior to calling the Add method. It also shows how to use the TryGetValue method to retrieve values, which is an efficient way to retrieve values when a program frequently tries keys that are not in the dictionary. Finally, it shows the least efficient way to test whether keys exist, by using the Item property (the indexer in C#).

This code example is part of a larger example provided for the Dictionary(Of TKey, TValue) class (openWith is the name of the Dictionary used in this example).

' 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 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
' 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

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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