Article
11/18/2011

Dictionary<TKey, TValue>.ContainsKey Method

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Determines whether the Dictionary<TKey, TValue> contains the specified key.

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

Syntax

'Declaration
Public Function ContainsKey ( _
    key As TKey _
) As Boolean

public bool ContainsKey(
    TKey key
)

Parameters

key
Type: TKey
The key to locate in the Dictionary<TKey, TValue>.

Return Value

Type: System.Boolean
true if the Dictionary<TKey, TValue> contains an element with the specified key; otherwise, false.

Implements

IDictionary<TKey, TValue>.ContainsKey(TKey)

Exceptions

Exception	Condition
ArgumentNullException	key is nulla null reference (Nothing in Visual Basic).

Remarks

This method approaches an O(1) operation.

Examples

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<TKey, TValue> class.

' 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
Else
   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.
Try
   outputBlock.Text &= String.Format("For key = ""tif"", value = {0}.", _
       openWith("tif")) & vbCrLf
Catch
   outputBlock.Text &= "Key = ""tif"" is not found." & vbCrLf
End Try

// 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 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";
}


...


// 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";
}

Version Information

Silverlight

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

Platforms

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

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