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IDictionary.Add Method

Adds an element with the provided key and value to the IDictionary object.

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

void Add (
	Object key,
	Object value
void Add (
	Object key, 
	Object value
function Add (
	key : Object, 
	value : Object
Not applicable.



The Object to use as the key of the element to add.


The Object to use as the value of the element to add.

Exception typeCondition


key is a null reference (Nothing in Visual Basic).


An element with the same key already exists in the IDictionary object.


The IDictionary is read-only.


The IDictionary has a fixed size.

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). 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 whether they allow the key to be a null reference (Nothing in Visual Basic).

The following code example demonstrates how to implement the Add method. This code example is part of a larger example provided for the IDictionary class.

public void Add(object key, object value) 
    // Add the new key/value pair even if this key already exists in the dictionary.
    if (ItemsInUse == items.Length)
        throw new InvalidOperationException("The dictionary cannot hold any more items.");
    items[ItemsInUse++] = new DictionaryEntry(key, value);

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

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0

XNA Framework

Supported in: 1.0