自定义控件中Style类型的DependencyProperty

Question

In a custom control I'd like to use a Style (given as DependencyProperty) for a TextBlock in my Template.

MyControl.cs

public static DependencyProperty HeadingStyleProperty =
    DependencyProperty.Register("HeadingStyle",
                                typeof (Style),
                                typeof (MyControlElement),
                                new PropertyMetadata(new Style(typeof(TextBlock))));

public Style HeadingStyle {
    get { return (Style) GetValue(HeadingStyleProperty); }
    set { SetValue(HeadingStyleProperty, value); }
}

MyControl.xaml

<ResourceDictionary ...>
    <Style TargetType="local:MyControl">
        <Style.Resources>
            <!-- Getting error on BasedOn="TemplateBinding -->
            <Style TargetType="TextBlock" BasedOn="{TemplateBinding HeadingStyle}" x:Key="Heading" />
        </Style.Resources>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate>
                    <TextBlock Style="{StaticResource Heading}" Text="StyledHeading" />
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

I get compiler error 'HeadingStyle' member is not valid because it does not have a qualifying type name.

Do I have to modify the DataType of my DP from Style to a more specific one? Or what causes this error. The initial value of DP is set to Style for TargetType TextBlock...

Answer 1

First of all, Style does not derive from DependencyObject , so you cannot set any binding on it.

If you want your TextBlock (which is part of the template) to be styled by your property, simply set the TemplateBinding on its Style property directly (what would be the purpose of the style you defined as a resource anyway?). This can be done in two ways. One is to use fully qualified property name:

<ControlTemplate>
    <TextBlock Style="{TemplateBinding local:MyControl.HeadingStyle}" (...) />
</ControlTemplate>

The other way, more commonly used, is to use simplified property name, but it requires the ControlTemplate.TargetType to be specified:

<ControlTemplate TargetType="{x:Type local:MyControl}">
    <TextBlock Style="{TemplateBinding HeadingStyle}" (...) />
</ControlTemplate>

The second method works because the XAML parser is smart enough to know how to resolve the HeadingStyle property - if you didn't specify ControlTemplate.TargetType , you'd get the exact same error you're getting right now.

You could of course trick the compiler to compile your code by using fully qualified property name:

<Style x:Key="Heading" BasedOn="{TemplateBinding local:MyControl.HeadingStyle}" (...) />

but that would result in a runtime error saying that TemplateBindingExpression cannot be converted to Style .