Article
11/18/2011

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

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Adds an element with the provided key and value to the IDictionary<TKey, TValue>.

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

Syntax

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

void Add(
    TKey key,
    TValue value
)

Parameters

key
Type: TKey
The object to use as the key of the element to add.

value
Type: TValue
The object to use as the value of the element to add.

Exceptions

Exception	Condition
ArgumentNullException	key is nulla null reference (Nothing in Visual Basic).
ArgumentException	An element with the same key already exists in the IDictionary<TKey, TValue>.
NotSupportedException	The IDictionary<TKey, TValue> is read-only.

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; for example, myCollection["myNonexistentKey"] = myValue in C# (myCollection("myNonexistentKey") = myValue in Visual Basic). However, if the specified key already exists in the dictionary, setting the Item property overwrites the old value. In contrast, the Add method does not modify existing elements.

Implementations can vary in how they determine equality of objects; for example, the List<T> class uses Comparer<T>.Default, whereas the Dictionary<TKey, TValue> class allows the user to specify the IComparer<T> implementation to use for comparing keys.

Implementations can vary in whether they allow key to be nulla null reference (Nothing in Visual Basic).

Examples

The following code example creates an empty Dictionary<TKey, TValue> of strings, with integer keys, and accesses it through the IDictionary<TKey, TValue> interface. The code example 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 is part of a larger example that can be compiled and executed. See System.Collections.Generic.IDictionary<TKey, TValue>.

' Create a new dictionary of strings, with string keys, 
' and access it through the IDictionary generic interface.
Dim openWith As IDictionary(Of String, String) = _
    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, 
// and access it through the IDictionary generic interface.
IDictionary<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.

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