Add Method
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IDictionary.Add Method (Object, Object)


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.

Exception Condition

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);

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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