Windows Presentation Foundation 4.5 Cookbook
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Creating custom type instances in XAML

Sometimes there's a need to create instances of your own types, or other .NET Framework, non-WPF types within XAML. A classic example is a data binding value converter (which we'll explore in Chapter 6, Data Binding, but other scenarios might call for it).

Getting ready

Make sure you have Visual Studio 2010 up and running.

How to do it...

We'll create a simple application that creates an instance of a custom type in XAML to demonstrate the entire procedure:

  1. Create a new WPF Application project named CH01.CustomTypes.
  2. Let's create a custom type named Book. In the Solution Explorer window, right-click on the project node and select Add and then Class…:
    How to do it...
  3. Type Book in the Name box and click on Add:
    How to do it...
  4. Add four simple properties to the resulting class: 
    class Book {
   public string Name { get; set; }
   public string Author { get; set; }
   public decimal Price { get; set; }
   public int YearPublished { get; set; }
 }

    Tip

    Downloading the example code

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  5. Open the MainWindw.xaml file (using the Solution Explorer), which was created automatically by the project wizard. We would like to create an instance of the Book class. As a Book is not an element (does not derive from UIElement), we cannot simply create it inside our Grid. But, we can make it the Content property (that can be anything, as its type is Object) of a ContentControl-derived type, such as Button. Add a button control to the existing grid, as follows: 
    <Grid>
    <Button FontSize="20">
    </Button>    
</Grid>
  6. To create an instance of Book, we first need to map the .NET namespace (and assembly) where Book is defined to an XML namespace that can be used by the XAML compiler. Let's add a mapping at the top of the XAML near the default mappings added by the application wizard: 
    <Window x:Class="CH01.CustomTypes.MainWindow"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:local="clr-namespace:CH01.CustomTypes"
  7. This says that anything prefixed by the string local (we can select anything here), should be looked up in the CH01.CustomTypes namespace (in the current assembly).
  8. Now, we can finally create a Book instance. Add the following inside the Button tag: 
    <Button FontSize="20">
 <local:Book Name="Windows Internals" 
 Author="Mark Russinovich" Price="40" 
 YearPublished="2009" />
</Button>
  9. That's it. We can verify this by adding a suitable ToString implementation to the Book type, and running the application: 
    public override string ToString() {
return string.Format("{0} by {1}\nPublished {2}", Name, 
      Author, earPublished); 

}
    How to do it...

How it works...

The XAML compiler needs to be able to resolve type names such as Button or Book. A simple name like Button is not necessarily unique, not in the XML sense and certainly not in the .NET sense (there are at least four Button types in .NET, naturally in diffrent namespaces) .

A mapping is required between an XML namespace and a .NET namespace, so that the XAML compiler can reference the correct type. By default, two XML namespaces are declared by a typical XAML file: the first, which is the default XML namespace, is mapped to the normal WPF namespaces (System.Windows, System.Windows.Controls, and so on). The other, typically with the x prefix, is mapped to the XAML namespace (System.Windows.Markup).

For our own types, we need to do similar mapping (but with a different syntax) means map the XML namespace prefix local to the .NET namespace CH01.CustomTypes. The following line:

xmlns:local="clr-namespace:CH01.CustomTypes"

This allows our Book class to be recognized and used within the XAML.

If the type was defined in a referenced assembly (not our own assembly), then the mapping would continue to something like the following:

xmlns:local="clr-namespace:CH01.CustomTypes;assembly=MyAssembly"

For example, suppose we want the ability to create instances of the System.Random type. Here's how we'd map an XML namespace to the .NET namespace and assembly where Sys em.Random resides:

xmlns:sys="clr-namespace:System;assembly=mscorlib"

Now, we could create an instance of anything in the System namespace (that is XAML friendly) and the mscorlib assembly (such as Random):

 <sys:Random x:Key="rnd" />

In this case, it's hosted in a ResourceDictionary (which is a kind of dictionary, meaning every value requires a key; we'll discuss these in more detail in the next chapter).

There's more...

It's possible to map a single XML namespace to multiple .NET namespaces. This is the same technique used by the WPF assemblies itself: a single XML namespace maps to multiple WPF namespaces, such as System.Windows, System.Windows.Controls, and System.Wind ws.Media.

The trick is to use the XmlnsDefinition attribute within the assembly where the exported types reside. This only works for referenced assemblies; that is, it's typically used in class library assemblies.

For example, suppose we create a MyClassLibrary class library assembly, with a type like the Book introduced earlier:

namespace MyClassLibrary {
   public class Book {
      public string Name { get; set; }
      public string Author { get; set; }
      public decimal Price { get; set; }
      public int YearPublished { get; set; }
   }
}

We can make all types within this assembly and the MyClassLibrary namespace part of an XML namespace by adding the following attribute:

[assembly: XmlnsDefinition("http://mylibrary.com",
"MyClassLibrary")]

The first argument to the attribute is the XML namespace name. This can be anything, but is typically some form of fictitious URL (usually a variation on the company's URL), so as to lower chances of collisions (this is exactly the same idea used by WPF). The second string is the .NET namespace mapped by this XML namespace.

Now, suppose we have another .NET namespace within that same assembly with some types declared within it:

namespace MyClassLibrary.MyOtherTypes {
   public class MyType1 {
      //...
   }
 
   public class MyType2 {
      //...
   }
}

We can add another attribute to map the same XML namespace to the new .NET namespace:

[assembly: XmlnsDefinition("http://mylibrary.com", "MyClassLibrary.MyOtherTypes")]

This means that a single XML prefix (in some client application) can now map to multiple .NET namespaces:

xmlns:mylib="http://mylibrary.com"

This scheme can save multiple distinct XML prefix declarations. One consequence of this idea is that all public type names must be unique across the mapped .NET namespaces (as they are indeed within WPF itself).

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