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Dictionary(TKey, TValue) Class

Represents a collection of keys and values.

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

JScript does not support generic types or methods.

Type Parameters


The type of the keys in the dictionary.


The type of the values in the dictionary.

The Dictionary(TKey, TValue) generic class provides a mapping from a set of keys to a set of values. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is very fast, close to O(1), because the Dictionary(TKey, TValue) class is implemented as a hash table.


The speed of retrieval depends on the quality of the hashing algorithm of the type specified for TKey.

As long as an object is used as a key in the Dictionary(TKey, TValue), it must not change in any way that affects its hash value. Every key in a Dictionary(TKey, TValue) must be unique according to the dictionary's equality comparer. A key cannot be a null reference (Nothing in Visual Basic), but a value can be, if the value type TValue is a reference type.

Dictionary(TKey, TValue) requires an equality implementation to determine whether keys are equal. You can specify an implementation of the IEqualityComparer(T) generic interface by using a constructor that accepts a comparer parameter; if you do not specify an implementation, the default generic equality comparer EqualityComparer(T).Default is used. If type TKey implements the System.IEquatable(T) generic interface, the default equality comparer uses that implementation.


For example, you can use the case-insensitive string comparers provided by the StringComparer class to create dictionaries with case-insensitive string keys.

The capacity of a Dictionary(TKey, TValue) is the number of elements the Dictionary(TKey, TValue) can hold. As elements are added to a Dictionary(TKey, TValue), the capacity is automatically increased as required by reallocating the internal array.

For purposes of enumeration, each item in the dictionary is treated as a KeyValuePair(TKey, TValue) structure representing a value and its key. The order in which the items are returned is undefined.

The foreach statement of the C# language (for each in C++, For Each in Visual Basic) requires the type of each element in the collection. Since the Dictionary(TKey, TValue) is a collection of keys and values, the element type is not the type of the key or the type of the value. Instead, the element type is a KeyValuePair(TKey, TValue) of the key type and the value type. For example:

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

The foreach statement is a wrapper around the enumerator, which allows only reading from the collection, not writing to it.


Because keys can be inherited and their behavior changed, their absolute uniqueness cannot be guaranteed by comparisons using the Equals method.

The following code example creates an empty Dictionary(TKey, TValue) of strings with string keys and uses the Add method to add some elements. The example demonstrates that the Add method throws an ArgumentException when attempting to add a duplicate key.

The example uses the Item property (the indexer in C#) to retrieve values, demonstrating that a KeyNotFoundException is thrown when a requested key is not present, and showing that the value associated with a key can be replaced.

The example shows how to use the TryGetValue method as a more efficient way to retrieve values if a program often must try key values that are not in the dictionary, and it shows how to use the ContainsKey method to test whether a key exists before calling the Add method.

The example shows how to enumerate the keys and values in the dictionary and how to enumerate the keys and values alone using the Keys property and the Values property.

Finally, the example demonstrates the Remove method.

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

  System.Collections.Generic.Dictionary(TKey, TValue)

Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

A Dictionary(TKey, TValue) can support multiple readers concurrently, as long as the collection is not modified. Even so, enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with write accesses, the collection must be locked during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0