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Dictionary<TKey, TValue>::ContainsKey Method

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

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

public:
virtual bool ContainsKey(
	TKey key
) sealed

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)
IReadOnlyDictionary<TKey, TValue>::ContainsKey(TKey)

ExceptionCondition
ArgumentNullException

key is nullptr.

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.

// 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}",
        openWith["ht"]);
}


...


// 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", value))
{
    Console::WriteLine("For key = \"tif\", value = {0}.", value);
}
else
{
    Console::WriteLine("Key = \"tif\" is not found.");
}


...


// The indexer throws an exception if the requested key is 
// not in the dictionary. 
try
{
    Console::WriteLine("For key = \"tif\", value = {0}.",
        openWith["tif"]);
}
catch (KeyNotFoundException^)
{
    Console::WriteLine("Key = \"tif\" is not found.");
}

.NET Framework

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

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

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

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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