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IDictionary<TKey, TValue>.ContainsKey Method

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Determines whether the IDictionary<TKey, TValue> contains an element with the specified key.

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

Syntax

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

bool ContainsKey(
    TKey key
)

Parameters

Return Value

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

Exceptions

Exception Condition
ArgumentNullException

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

Remarks

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

Implementations can vary in whether they allow key to be nulla null reference (Nothing in Visual Basic).

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, 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<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
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.