XAML and property elements

In the previous post, we’ve started looking at same basic XAML. At the time, we’ve defined a simple button by using the following syntax:

<Button xmlns=
"http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" x:Name="bt" Content="Say hi" Click="bt_Click" />

In the previous snippet, we’re setting the Content property and the Click event. Btw, and in case you’re wondering, events are set up before the properties. This is the only thing you can take for granted (you can’t really depend on the order of the attributes in your XAML to influence the order by which properties are set). This makes sense because it means that eventual handlers will be called if an event is generated by setting a property to a specific value.

If you look at Content’s definition, you’ll see that it expects an object. That means we can, for instance, pass it an image instead of setting it to text:

<Button xmlns=
"http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" x:Name="bt" Click="bt_Click" Width="100"> <Button.Content> <Image Source="/p52.jpg" Stretch="UniformToFill" /> </Button.Content> </Button>

The previous snippet is showing the property element syntax. You should use it whenever you can’t use a simple string to define the value of a property. Property element syntax is really simple: you define a property by combining the name of the element with the name of the property. Property element syntax and attribute syntax are the two options you’ve got for setting the value of a property in XAML (sort of: we’ll leave content properties and collection properties to a future post).

In case you’re wondering, you can’t use something like this in Silverlight:

<Button xmlns=
"http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" x:Name="bt" Click="bt_Click" Width="100"> <Button.Content> Say Hi </Button.Content> </Button>

If you try loading the previous XAML, you should get an exception which says something like “Say Hi isn’t allowed as the button’s content”. In case you’re wondering, this does work in WPF. Unfortunately,in Silverlight you’ll have to use the attribute syntax for setting the value of the Content to a string.

In this cases,you do need to use another element. Here’s the code you can use for setting the Content’s property to text through the property element syntax:

<Button xmlns=
"http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" x:Name="bt" Click="bt_Click" Width="100"> <Button.Content> <TextBlock>Say Hi</TextBlock> </Button.Content> </Button>

Fair enough: this isn’t really similar to what we had before, but it’s really the only way to set the button’s Content property to something that looks like text (notice that in this case, the Content is set to another FrameworkElement and not to a simple string).

Understanding why something works in WPF and not in Silverlight is not complicated: you just need to keep in mind that all the features considered not essential were removed in order to control the size of the Silverlight framework. And it looks like this was one of them…

You must be wondering which syntax you should use when you want to set the value of a property. Sometimes, the answer is simple since we’ve seen a case where you can’t use both syntaxes for setting a specific value. When both approaches are supported, I’ll tend to use the one which is more compact (typically, this means going with the attribute syntax whenever possible).

And I guess this wraps it up for now. Stay tuned for more on Silverlight.

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~ by Luis Abreu on November 17, 2009.

2 Responses to “XAML and property elements”

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