• Article

Dictionary<TKey, TValue>.Add Method

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Adds the specified key and value to the dictionary.

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

Syntax

'Declaration
Public Sub Add ( _
    key As TKey, _
    value As TValue _
)

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 nulla null reference (Nothing in Visual Basic) for reference types.

Implements

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

Exceptions

Exception Condition
ArgumentNullException

key is nulla null reference (Nothing in Visual Basic).
ArgumentException

An element with the same key already exists in the Dictionary<TKey, TValue>.

Remarks

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 nulla 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.

Examples

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.
'
Dim openWith As New Dictionary(Of 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
   outputBlock.Text &= "An element with Key = ""txt"" already exists." & vbCrLf
End Try

// 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)
{
   outputBlock.Text += "An element with Key = \"txt\" already exists." + "\n";
}

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.