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What is a ContentPresenter?

05.12.2010
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In WPF there is an element called ContentPresenter, that is often used inside control templates, as well as inside the root application markup. The concept of ContentPresenter is quite simple – it is a placeholder for any XAML content and it can be used to insert content at runtime.

What I personally find it useful for is dynamic user interface, where there is no need to define a static UI structure at design time, but at runtime there should be some components that are either defined in the application XAML or in an external resource.

A content presenter is used via the ContentPresenter element: <ContentPresenter></ContentPresenter>. The content inside this element can be partially (or completely, although its value is lost in this case) defined inside the <ContentPresenter.Content></ContentPresenter.Content> element.

If you consider using the ContentPresenter element by defining the content from scratch, there is no need to include the Content element in the base XAML – it cannot be empty and running it so will cause an exception to be thrown. Once the content is set at runtime, the proper tag will be added automatically.

Let’s take a look at an example. Here is the base XAML for a window:

<Window x:Class="WPF_Test.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="380" Width="568" Loaded="Window_Loaded">

<Grid>
<ContentPresenter Name="MyContent">

</ContentPresenter>
</Grid>
</Window>

I am also setting a name for my ContentPresenter, so that I can access it from the code-behind.  I did not define any default content, but if I would want to, I would use the following XAML structure:

<ContentPresenter Name="MyContent">
<ContentPresenter.Content>
<Button>Click Me</Button>
</ContentPresenter.Content>
</ContentPresenter>

Here is some sample XAML content to test the basic capabilities:

<Button xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">Click</Button>

The namespace is required here. Although you can use namespace-less elements when explicitly defining the content directly in the application, a namespace indicator is required when the content is dynamically placed.

To load the content above, in the code behind I can use this sample code:

StringReader reader = new StringReader(File.ReadAllText("D:\\Temporary\\button.txt"));
XmlReader xmlReader = XmlReader.Create(reader);
MyContent.Content = (Button)XamlReader.Load(xmlReader);

In this specific example, I am loading the content shown above from a text file. I am then reading it as XML (since XAML is based on XML and the formatting is preserved). The XAML data is then assigned to the Content property for the specified ContentPlaceholder.

I am parsing it via the XamlReader, that will read the elements and assign them properly. Notice the fact that I am explicitly converting the XAML content to a Button. I need to do this so that the content is correctly rendered. Otherwise, an exception will be thrown.

You might be wondering – well, with the same success you could be using a stackpanel or a grid. But this is not always the case. When control templates are built, there is a well-defined element hierarchy and sometimes using a defined container is not a choice. A content presenter can be a holder for any controls and can be bound to the content from various elements via TemplateBinding, if used in a template.

Comments

Richard Mccutchen replied on Fri, 2010/05/14 - 5:20pm

Excellent read. I'm starting to dive into WPF and this will definitely come in handy as I get further into it

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