Dictionary.Add Method

public void Add (
	TKey key,
	TValue value
)

public final void Add (
	TKey key, 
	TValue value
)

public final function Add (
	key : TKey, 
	value : TValue
)

Not applicable.

Parameters

key

The key of the element to add.

value

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

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; for example, myCollection[myKey] = myValue (in Visual Basic, myCollection(myKey) = myValue). However, if the specified key already exists in the Dictionary, 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 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 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 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 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

.NET Compact Framework

XNA Framework