Dictionary<TKey, TValue>.ContainsKey Method (TKey)


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

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

public bool ContainsKey(
	TKey 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.

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<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 (!openWith.ContainsKey("ht"))
    openWith.Add("ht", "hypertrm.exe");
    Console.WriteLine("Value added for key = \"ht\": {0}", 
// 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))
    Console.WriteLine("For key = \"tif\", value = {0}.", value);
    Console.WriteLine("Key = \"tif\" is not found.");
// The indexer throws an exception if the requested key is
// not in the dictionary.
    Console.WriteLine("For key = \"tif\", value = {0}.", 
catch (KeyNotFoundException)
    Console.WriteLine("Key = \"tif\" is not found.");

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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