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IDictionary<TKey, TValue>.ContainsKey Method

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
)

Parameters

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.

ExceptionCondition
ArgumentNullException

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}", 
        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", out 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: 4.5.2, 4.5.1, 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 8, Silverlight 8.1

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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