IDictionary(Of TKey, TValue).ContainsKey Method

Determines whether the IDictionary(Of TKey, TValue) contains an element with the specified key.

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

Function ContainsKey ( _
	key As TKey _
) As Boolean


Type: TKey
The key to locate in the IDictionary(Of TKey, TValue).

Return Value

Type: System.Boolean
true if the IDictionary(Of TKey, TValue) contains an element with the key; otherwise, false.


key is Nothing.

Implementations can vary in how they determine equality of objects; for example, the List(Of T) class uses Comparer(Of T).Default, whereas the Dictionary(Of TKey, TValue) class allows the user to specify the IComparer(Of T) implementation to use for comparing keys.

Implementations can vary in whether they allow key to be Nothing.

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, which can be a more efficient way to retrieve values if a program frequently tries key values that are not in the dictionary. Finally, it shows how to insert items using Item property (the indexer in C#).

This code is part of a larger example that can be compiled and executed. See System.Collections.Generic.IDictionary(Of TKey, TValue).

' 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 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
   outputBlock.Text &= "Key = ""tif"" is not found." & vbCrLf
End If


' The default Item property throws an exception if the requested
' key is not in the dictionary.
   outputBlock.Text &= String.Format("For key = ""tif"", value = {0}.", _
       openWith("tif")) & vbCrLf
   outputBlock.Text &= "Key = ""tif"" is not found." & vbCrLf
End Try


Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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