• Article

IDictionary.Item Property

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Gets or sets the element with the specified key.

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

Syntax

'Declaration
Property Item ( _
    key As Object _
) As Object

Object this[
    Object key
] { get; set; }

Parameters

Property Value

Type: System.Object
The element with the specified key.

Exceptions

Exception Condition
ArgumentNullException

key is nulla null reference (Nothing in Visual Basic).
NotSupportedException

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

-or-

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

Remarks

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 nulla 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.

Examples

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 Property Item(ByVal key As Object) As Object Implements IDictionary.Item
   Get

      ' If this key is in the dictionary, return its value.
      Dim index As Integer
      If TryGetIndexOfKey(key, index) Then

         ' The key was found return its value.
         Return items(index).Value
      Else

         ' The key was not found return null.
         Return Nothing
      End If
   End Get

   Set(ByVal value As Object)
      ' If this key is in the dictionary, change its value. 
      Dim index As Integer
      If TryGetIndexOfKey(key, index) Then

         ' The key was found change its value.
         items(index).Value = value
      Else

         ' This key is not in the dictionary add this key/value pair.
         Add(key, value)
      End If
   End Set
End Property

public object this[object key]
{
   get
   {
      // 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;
      }
      else
      {
         // The key was not found; return null.
         return null;
      }
   }

   set
   {
      // 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;
      }
      else
      {
         // This key is not in the dictionary; add this key/value pair.
         Add(key, value);
      }
   }
}

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.