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

Reference

Definition

Namespace:: System.Collections.Generic

Assembly:: System.Collections.dll

Assembly:: mscorlib.dll

Assembly:: netstandard.dll

Source:: Dictionary.cs

Source:: Dictionary.cs

Source:: Dictionary.cs

Important

Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

Adds the specified key and value to the dictionary.

public:
 virtual void Add(TKey key, TValue value);

public void Add (TKey key, TValue value);

abstract member Add : 'Key * 'Value -> unit
override this.Add : 'Key * 'Value -> unit

Public Sub Add (key As TKey, value As TValue)

Parameters

key: TKey

The key of the element to add.

value: TValue

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

Implements

Add(TKey, TValue)

Exceptions

ArgumentNullException

key is null.

ArgumentException

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

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.
//
Dictionary<String^, String^>^ openWith =
    gcnew 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.");
}

// 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.");
}

// Create a new dictionary of strings, with string keys.
let openWith = 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")
with :? ArgumentException ->
    printfn "An element with Key = \"txt\" already exists."

' 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 
    Console.WriteLine("An element with Key = ""txt"" already exists.")
End Try

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 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.

Applies to

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