Add Method
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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


Type: System.Object

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

Type: System.Object

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


key is null.


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 null.

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 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

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