Extension Indexer Property (Visual Basic)

Provides access to individual elements in a collection.




Term Definition
object Required. A queryable collection. That is, a collection that implements IEnumerable<T> or IQueryable<T>.
( Required. Denotes the start of the indexer property.
index Required. An integer expression that specifies the zero-based position of an element of the collection.
) Required. Denotes the end of the indexer property.

Return Value

The object from the specified location in the collection, or Nothing if the index is out of range.


You can use the extension indexer property to access individual elements in a collection. This indexer property is typically used on the output of XML axis properties. The XML child and XML descendent axis properties return collections of XElement objects or an attribute value.

The Visual Basic compiler converts extension indexer properties to calls to the ElementAtOrDefault method. Unlike an array indexer, the ElementAtOrDefault method returns Nothing if the index is out of range. This behavior is useful when you cannot easily determine the number of elements in a collection.

This indexer property is like an extension property for collections that implement IEnumerable<T> or IQueryable<T>: it is used only if the collection does not have an indexer or a default property.

To access the value of the first element in a collection of XElement or XAttribute objects, you can use the XML Value property. For more information, see XML Value Property.


The following example shows how to use the extension indexer to access the second child node in a collection of XElement objects. The collection is accessed by using the child axis property, which gets all child elements named phone in the contact object.

Dim contact As XElement = 
        <name>Patrick Hines</name>
        <phone type="home">206-555-0144</phone>
        <phone type="work">425-555-0145</phone>

Console.WriteLine("Second phone number: " & contact.<phone>(1).Value)

This code displays the following text:

Second phone number: 425-555-0145

See Also

XML Axis Properties
XML Literals
Creating XML in Visual Basic
XML Value Property