Lazy Load XAML content from External File and Vice Versa

By | March 11, 2011

XAML is the most flexible language built ever. More I see XAML, more I know about it. Today while tweaking around with XAML code, I found XAML could be loaded dynamically from any XML string. That means if you have a xaml in a xml file you can probably load the part of the XAML into your ContentControl or to any control element you want and the UI will appear instantly.
Once you compile a XAML it produces BAML. BAML is in binary format for the XML, so if you can pass a BAML into the UI separately somehow during runtime,you would be seeing the content instantly in the Window. In this post I am going to discuss how easily you could load a Dynamic content of XAML file into a normal WPF ContentControl just like what we do for normal htmls.
What is XamlReader and XamlWriter?
If you look into the implementation of these classes you could wonder how flexible these are. They are highly capable of parsing the whole content of the file. It uses a XAMLDictionary which holds all the XAML elements that a XAML can see. The Reader parses the Xml content very cautiously to ensure it makes the XAML file to contain no reference of outside. Thus the class is used to Refactor the XAML from outside and hence allows you to put the content anywhere such that everything will be applied on that instantly.
XamlReader exposes methods like Load / Parse which allows you to parse a file content into BAML. Hence each of them returns an binary object which you can put into the Content.

The sample application implements these features to store the Xaml content into an external file and later on it loads the same content from the file to show up the content again.


The UI looks straight forward, you have a Button telling you to OpenFile. When you click and open a file with content :

<StackPanel Orientation="Vertical" xmlns="">
<Button>This is My Button</Button>
<Border Width="525" Height="250">
<GradientStop Color="#FF008000" Offset="0" />
<GradientStop Color="#FF0000FF" Offset="0.5" />
<GradientStop Color="#FFFF0000" Offset="1" />
<EventTrigger RoutedEvent="FrameworkElement.Loaded">
<ColorAnimation From="#FF008000" To="#FFFFFF00" AutoReverse="True" BeginTime="00:00:00" Duration="00:00:05" Storyboard.TargetProperty="(Panel.Background).(GradientBrush.GradientStops)[0].(GradientStop.Color)" />
<ColorAnimation From="#FF0000FF" To="#FFFF0000" AutoReverse="True" BeginTime="00:00:00" Duration="00:00:05" Storyboard.TargetProperty="(Panel.Background).(GradientBrush.GradientStops)[1].(GradientStop.Color)" />
<ColorAnimation From="#FFFF0000" To="#FF008000" AutoReverse="True" BeginTime="00:00:00" Duration="00:00:05" Storyboard.TargetProperty="(Panel.Background).(GradientBrush.GradientStops)[2].(GradientStop.Color)" />

It will eventually load a Button and the Border element with ColorAnimation running.


On the other hand say If I create the same code for the ContentControl and click on the SaveContent Button, it will produce the reverse.
For this purpose I have created  a simple utility class :

public class XamlUtility

public string FilePath { get; set; }
public object Content { get; set; }

public void LoadContent()
OpenFileDialog dlg = new OpenFileDialog();
dlg.Filter = "XAML Files|*.xaml";
bool? retval = dlg.ShowDialog();

if (retval.HasValue && retval.Value)
this.FilePath = dlg.FileName;
using (FileStream stream = new FileStream(dlg.FileName, FileMode.Open))
object content = XamlReader.Load(stream);
this.Content = content;

public void SaveContent()
SaveFileDialog dlg = new SaveFileDialog();
dlg.Filter = "XAML Files|*.xaml";

bool? retval = dlg.ShowDialog();
if (retval.HasValue && retval.Value)
using (FileStream stream = new FileStream(dlg.FileName, FileMode.Create))
XamlWriter.Save(this.Content, stream);
this.Content = null;

The class exposes few methods like LoadContent which itself calls the XamlReader.Load to load the Stream into Content. When the XamlLoads the Xaml Parser takes care of the whole Xml content and load only the element which are meaningful to the compiler. Hence if you put arbitrary attributes for say Button, it will not load the attribute in XAML content.
On the other hand, while SaveContent, the Parser automatically parses the content based on the Dictionary and writes only the content that doesn’t hold information for outside. Hence if you have Triggers enabled for your Border element, as we do have for the sample, the RoutedEvent will automatically changed from Border to FrameworkElement, also the reference to the TargetProperty is used to be a Panel rather than the Border itself. Another good thing that I found is, it automatically removes any external eventhandlers from the written Xaml output.
For each WPF application, the XamlReader and XamlWriter are the main component to load the UI. Other than normal loading the XamlContent these classes also exposes methods to define your custom  ParserContext which allows you to define the metadata for the Xaml.
Both Reader and Writer also exposes methods to load Xaml asynchronously. The methods lile LoadAsync, CancelAsync may come very handy for loading Large Xaml files into the UI.