A simple class that uses our custom type converter in VB.NET

Create QR Code JIS X 0510 in VB.NET A simple class that uses our custom type converter

public class TestClass : Control { [TypeConverter(typeof(BorderTypeConverter))] public Border Border { get { return (Border)GetValue(BorderProperty); } set { SetValue(BorderProperty, value); } }

public static readonly DependencyProperty BorderProperty = DependencyProperty.Register("Border", typeof(Border), typeof(TestClass), null); }

The TypeConverterAttribute that specifies the type converter to use for this specific property in this class is shown in listing 2.17. The attribute is applied to the public property because that s what s used by XAML. The converter is declared on the single property so it ll apply only there and not to all instances of the Border type. It s also important to note that the border isn t actually used for anything other than illustrating how to use a type converter. Finally, listing 2.18 shows the type converter implicitly in use in XAML.

<UserControl x:Class="TypeConverterExample.MainPage" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="clr-namespace:TypeConverterExample"> <Grid x:Name="LayoutRoot"> <local:TestClass Border="Red 5" /> </Grid> </UserControl>

Because we used the XamlReader.Load method, we could easily use any valid color string such as "LemonCream" or "#C830019F". Bonus points if you caught the Star Wars reference in listing 2.18.

You may have given the color string #C830019F a double-take if you re used to sixdigit HTML hex colors. Colors in Silverlight are typically expressed as eight-digit hex numbers, the first pair representing the alpha component and the remaining three pairs the red, green, and blue components in that order. In the color #C830019F, the values are Alpha: 0xC8, Red: 0x30, Green: 0x01, and Blue: 0x9F. The alpha component is optional, so you may use an HTML-style hex color if you wish. For consistency across the application, I recommend you specify the alpha value and use all eight digits without any shortcuts.

Type converters are a great way to extend the flexibility of XAML to include types you create yourself or new representations of existing types. We ve used them in projects to provide serialization support for legacy format strings stored in databases and to extend the known representations of existing types. Now that we understand the basics of XAML and have seen a simple example of dynamically loading XAML to parse a color string, let s take that a bit further and look at runtime loading or more complex content.

In listing 2.16, we saw a brief example of loading XAML at runtime using XamlReader.Load. Let s expand on that to do more than just some basic color conversion.

You can use dynamically loaded XAML to create entire sections of the object tree at runtime. This could be useful for rendering user-generated content such as shapes drawn on a screen and saved in a database or for creating highly dynamic controls. The process of loading XAML at runtime is incredibly easy. You only need to rely on the XamlReader class, which belongs to the System.Windows.Markup namespace. This class empowers you to parse XAML and convert it into an in-memory object. This object can be created by a statically visible method called Load. This method takes a string of XAML and converts it to the appropriate object. Then you can insert this object into another UIElement. Listing 2.19 shows this entire process in action.

Result:

XAML:

<UserControl x:Class="XamlReaderExample.MainPage" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Width="400" Height="300"> <Grid x:Name="LayoutRoot"> </Grid> </UserControl>

public MainPage() { InitializeComponent(); Loaded += new RoutedEventHandler(MainPage_Loaded); } void MainPage_Loaded(object sender, RoutedEventArgs e) { var element = CreateRectangle(); LayoutRoot.Children.Add(element); } private Rectangle CreateRectangle() { StringBuilder xaml = new StringBuilder(); string ns = "http://schemas.microsoft.com/winfx/2006/xaml/presentation"; xaml.Append("<Rectangle "); xaml.Append(string.Format("xmlns='{0}'", ns)); xaml.Append(" Margin='5 10 5 15'");

xaml.Append(" Fill='Orange'"); xaml.Append(" Stroke='Black' />"); var rectangle = (Rectangle) XamlReader.Load(xaml.ToString()); return rectangle; }

This example dynamically creates a rectangle and adds it to the object tree. The code in CreateRectangle simply builds up a string with XAML similar to what we d have inside a regular .xaml file. Note that we need to specify the namespaces used for any segment of XAML we ll pass into XamlReader.Load. The code that adds the generated XAML to the object tree can be seen inside the loaded event. You can of course do more with the element than just add it to the LayoutRoot. Listing 2.20 illustrates how we can take the XAML and integrate it with the managed code representations of XAML constructs to create multiple instances of the rectangle.