Question

I am trying to apply a custom style to a control whenever it is in a particular state, this style can be set on the object as a style. However, when setting a trigger to do this, the style property cannot be set again:

<Style TargetType="{x:Type ContentPresenter}">
  <Style.Triggers>
    <Trigger Property="ContentTemplate" Value="{x:Null}">
      <Setter Property="Style" Value="{Binding MouseOverGroupStyle, RelativeSource={RelativeSource TemplatedParent}}" />
    </Trigger>
  </Style.Triggers>
</Style>

Style object is not allowed to affect the Style property of the object to which it applies.

Which makes sense, however, what is the alternative? I cannot bind to the setters list, because it is readonly.

Answer 1

Your solution here is to use a StyleSelector, which takes the ContentPresenter and checks the ContentTemplate.

    internal class ContentTemplateStyleSelector : StyleSelector
    {
        public Style NullStyle { get; set; }
        public Style DefaultStyle { get; set; }

        public override Style SelectStyle(object item, DependencyObject container)
        {
            var cp = container as ContentPresenter;

            if (cp == null)
                return null;

            if (cp.ContentTemplate == null)
                return NullStyle;

            return DefaultStyle;
        }
    }

Unfortunately, ContentPresenter doesn’t have a StyleSelector property to which you can bind a StaticResource of an instance of your ContentTemplateStyleSelector so you may need to cast from ContentPresenter to something which does.

Alternatively, there is the option of using a DataTemplateSelector.