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Dictionary(TKey, TValue).Add Method

Adds the specified key and value to the dictionary.

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

abstract Add : 
        key:'TKey * 
        value:'TValue -> unit 
override Add : 
        key:'TKey * 
        value:'TValue -> unit 


Type: TKey
The key of the element to add.
Type: TValue
The value of the element to add. The value can be a null reference (Nothing in Visual Basic) for reference types.


IDictionary(TKey, TValue).Add(TKey, TValue)


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


An element with the same key already exists in the Dictionary(TKey, TValue).

You can also use the Item property to add new elements by setting the value of a key that does not exist in the Dictionary(TKey, TValue); for example, myCollection[myKey] = myValue (in Visual Basic, myCollection(myKey) = myValue). However, if the specified key already exists in the Dictionary(TKey, TValue), setting the Item property overwrites the old value. In contrast, the Add method throws an exception if a value with the specified key already exists.

If the Count property value already equals the capacity, the capacity of the Dictionary(TKey, TValue) is increased by automatically reallocating the internal array, and the existing elements are copied to the new array before the new element is added.

A key cannot be a null reference (Nothing in Visual Basic), but a value can be, if TValue is a reference type.

If Count is less than the capacity, this method approaches an O(1) operation. If the capacity must be increased to accommodate the new element, this method becomes an O(n) operation, where n is Count.

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.

This code example is part of a larger example provided for the Dictionary(TKey, TValue) class.

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

.NET Framework

Supported in: 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

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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