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


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

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

bool ContainsKey(
	TKey key


Type: TKey

The key to locate in the IDictionary<TKey, TValue>.

Return Value

Type: System.Boolean

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

Exception Condition

key is null.

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 null.

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 (!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 4.5
.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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