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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)

public void Add(
	TKey key,
	TValue value
)

Parameters

key
Type: TKey

The key of the element to add.

value
Type: TValue

The value of the element to add. The value can be null for reference types.

Implements

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

ExceptionCondition
ArgumentNullException

key is null.

ArgumentException

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 null, 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.

// Create a new dictionary of strings, with string keys. 
//
Dictionary<string, string> openWith = 
    new Dictionary<string, string>();

// Add some elements to the dictionary. There are no  
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe");
openWith.Add("bmp", "paint.exe");
openWith.Add("dib", "paint.exe");
openWith.Add("rtf", "wordpad.exe");

// The Add method throws an exception if the new key is  
// already in the dictionary. 
try
{
    openWith.Add("txt", "winword.exe");
}
catch (ArgumentException)
{
    Console.WriteLine("An element with Key = \"txt\" already exists.");
}

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
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