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IDictionary.Item Property

Gets or sets the element with the specified key.

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

Object this [
	Object key
] { get; set; }
/** @property */
Object get_Item (Object key)

/** @property */
void set_Item (Object key, Object value)

Not applicable.



The key of the element to get or set.

Property Value

The element with the specified key.

Exception typeCondition


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


The property is set and the IDictionary object is read-only.


The property is set, key does not exist in the collection, and the IDictionary has a fixed size.

This property provides the ability to access a specific element in the collection by using the following syntax: myCollection[key].

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 C# language uses the this keyword to define the indexers instead of implementing the Item property. Visual Basic implements Item as a default property, which provides the same indexing functionality.

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

public object this[object key]
        // If this key is in the dictionary, return its value.
        Int32 index;
        if (TryGetIndexOfKey(key, out index))
            // The key was found; return its value.
            return items[index].Value;
            // The key was not found; return null.
            return null;

        // If this key is in the dictionary, change its value. 
        Int32 index;
        if (TryGetIndexOfKey(key, out index))
            // The key was found; change its value.
            items[index].Value = value;
            // This key is not in the dictionary; add this key/value pair.
            Add(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
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