Silverlight: Why can't designer see the dependency property? - silverlight

Question

I have to create Silverlight User Control with public properties that should be used in inner controls.
public partial class MyControl : UserControl
{
public static readonly DependencyProperty MyCustomProperty =
DependencyProperty.Register(
"MyCustom", typeof(string), typeof(MyControl),
new PropertyMetadata("defaultValue"));
public string MyCustom
{
...
}
I tried several ways to bind, but all fail - dependency property is not seen for some reason.
For example this straightforward binding fails:
<UserControl x:Class="...MyControl"
...
x:Name="mc"
>
<Grid VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
<Image Source="{Binding Path=MyCustom, Mode=OneWay, ElementName=mc}" />
</Grid>
</UserControl>
What I'am doing wrong?

Answer 1

What you are doing is not a good pattern. The UserControl does not really "own" the name property. If another UserControl or Page were to place an instanced of your MyControl in its Xaml, it can give it name other than "mc", at which point your code is broken.
Instead use this approach:-
<UserControl x:Class="...MyControl"
>
<Grid x:Name="LayoutRoot" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
<Image Source="{Binding Path=Parent.MyCustom, Mode=OneWay, ElementName=LayoutRoot}" />
</Grid>
</UserControl>
That out of the way you main problem is that the Image Source property is of type ImageSource not string. You get to use a string literal in Xaml because the Xaml parser does some parser magic the converts the string to an ImageSource. This doesn't happen when using binding.
Change you controls property to:-
public partial class MyControl : UserControl
{
public static readonly DependencyProperty MyCustomProperty =
DependencyProperty.Register(
"MyCustom", typeof(ImageSource), typeof(MyControl),
new PropertyMetadata(null));
[TypeConverter(typeof(ImageSourceConverter))]
public ImageSource MyCustom
{
...
}
Now in another UserControl or Page where you MyControl is hosted you can use a string to assign this MyCustom property. However in code you need to create an instance of something like BitmapImage to assign to this property.