One" /> Before starting to learn how WPF works, you should analyze in more detail the XAML syntax, which is part of the WPF technology. ■ Note Especially if you have already worked in the past with XAML, you may find this introduction repetitive. This is not because I had the intention of writing a verbose introduction but because the XAML technology and syntax are unique across different technologies; so if you ever worked in the past with a XAML technology like Silverlight or Workflow Foundation, you may already understand the material in the next sections. The XAML Syntax The XAML language is a complex and very flexible markup language applied to different technologies by using different references in the XAML file so that you can refer to different objects that provide different XAML elements and attributes. Like any other XML file, a XAML file must have a valid structure and must follow some specific rules; one of these rules is the presence of a valid root element. Namespaces and Root Elements In XAML you define a root element as the root of your XAML document; this is a mandatory requirement for the XAML file and for the XML validation. In a WPF context, the root element of a XAML file will be a Page, UserControl, or Window if you are creating a UI container or a simple ResourceDictionary if you are creating a collection of resources for your application (which is another valid root element for a WPF application and which is also a standard file created by the Visual Studio project). These root elements are only some of the available options for a WPF XAML file. ■ Note If you are creating a XAML file for another technology like Silverlight or Workflow, you will probably use different root elements provided by these technologies. Listing 1-2 shows the structure of a Window object, which constitutes a normal window UI in WPF and is represented by the root element Window. Listing 1-2. XAML Root Namespaces The root element in the Listing 1-2 contains the attributes xmlns and xmlns:x. These attributes indicate to a XAML processor which XAML namespaces contain the type. The xmlns attribute specifically indicates the default XAML namespace. The xmlns:x attribute indicates an additional XAML namespace, which maps the XAML language namespace (which is The namespace declaration always applies to all the descendant element of a XAML document, just as they do for an XML document; for this reason, you won’t need to declare the same namespace twice in a XAML document in order to use its types, but you will need to declare those types preceded by the corresponding element prefix. You may also need to reference a custom type provided by a namespace that you have previously created and that is already referenced in your XAML application; to do that, you should use the keyword xmlns followed by your custom prefix and then add a reference to the namespace containing the classes you want to use. Listing 1-3. XAML file with custom references Listing 1-3 declared a new custom prefix called custom, which references a namespace that is referenced in the example project. Inside this namespace is a class called CustomPrefix that is available using the prefix custom. Objects and Common Elements Like when using XML, if you want to declare a new type in XAML, you should encapsulate the type in an element object that is denoted by an opening bracket (<) followed by the type name and then a closing bracket (>). The element can be closed by an additional element of the same type, starting with the bracket ( at the end of the element declaration. The property element syntax is represented by the name of the parent element, followed by a dot that is followed by the name of the property. In this way, the property element becomes a complete XAML element that can include a child structure. You can use the same technique to declare a collection of values. If the property element you are working with is of type collection, you can declare a set of sequential child elements inside it, like in the following code, which is declaring a set of child items for a ListBox element: