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ContainsKey Method

Dictionary(TKey, TValue).ContainsKey Method

Determines whether the Dictionary(TKey, TValue) contains the specified key.

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

abstract ContainsKey : 
        key:'TKey -> bool  
override ContainsKey : 
        key:'TKey -> bool


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.


IDictionary(TKey, TValue).ContainsKey(TKey)
IReadOnlyDictionary(TKey, TValue).ContainsKey(TKey)


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

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).

No code example is currently available or this language may not be supported.

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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