# Using xsi:type

If you have elements in your XML file whose type is a complex type, xsi:type support in the XML editor lets you choose between the complex type and any other complex types derived from it.

The XML Schema specification allows you to derive types by extension. For example, you have an XML schema and you create a complex type for it called `Address`. You then add some basic elements to `Address`, such as `streetName` and `city`.

After this, you derive (by extension) two new complex types from `Address` - `USAddress` and `UKAddress` You add a new element to `USAddress` called `state`, and also a new element to `UKAddress` called `postcode`.

After you have done this, you create two more elements - `billTo` and `shipTo` - as `Address` types `.`

When you create an XML instance document for an element such as `billTo` or `Address`, an xsi:type attribute will automatically be added to it. For example:

    <billTo xsi:type="ipo:Address">

The xsi:type attribute is used to identify derived complex types (as well as complex types that have been derived from).

In the Design view of the XML editor, a list will be available, letting you select the appropriate type definition (Address, USAddress, or UKAddress). The guided editing for the content model will reflect the type definition that you choose. For example, if you select USAddress , your `billTo` element can contain a `state` element, but it cannot contain a `postcode` element.

The XML example "Editing and validating XML files" demonstrates xsi:type support.

For more information about xsi:type, refer to the Using Derived Types in Instance Documents section in XML Schema Part 0: Primer.

For more information about validation semantics when xsi:type is used, refer to the Schema-Related Markup in Documents Being Validated section in XML Schema Part 1: Structures

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