Is there a way to use a style setter for properties of properties?

Question

Edit: In the original question i made some wrong assumptions about how setters work so i modified it to hopefully be more accurate and useful.

Answer 1

So after reading Meleak's answer and finding out that you can have a style within a style via resources what probably comes closest to doing this with a setter is an embedded style to access the icon's opacity. Here i assume the icon to be an image so i use that as the target type, the complete style hence looks like this:

<Style x:Key="MenuItemMouseOverStyle" TargetType="MenuItem">
    <Style.Resources>
        <Style TargetType="Image">
            <Setter Property="Opacity" Value="0.5"/>
        </Style>
    </Style.Resources>
    <Style.Triggers>
        <EventTrigger RoutedEvent="MouseEnter">
            <BeginStoryboard>
                <Storyboard>
                    <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="Icon.Opacity">
                        <EasingDoubleKeyFrame KeyTime="0" Value="0.5"/>
                        <EasingDoubleKeyFrame KeyTime="0:0:0.3" Value="1"/>
                    </DoubleAnimationUsingKeyFrames>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
        <EventTrigger RoutedEvent="MouseLeave">
            <BeginStoryboard>
                <Storyboard>
                    <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="Icon.Opacity">
                        <EasingDoubleKeyFrame KeyTime="0" Value="1"/>
                        <EasingDoubleKeyFrame KeyTime="0:0:0.3" Value="0.5"/>
                    </DoubleAnimationUsingKeyFrames>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </Style.Triggers>
</Style>

The only problem with this is that it does not actually set the Icon.Opacity but the opacity of all images that may occur within the MenuItem.

Answer 2

I don't think you can access a Property of a Property like that so the casting itself isn't the problem. Even if Icon was of Type Image that still wouldn't work. You can try with Backgrounds Opacity for a Grid for example. Background is a Dependency Property for Grid and Opacity is a Dependency Property for Brush but the following line won't work

<Grid Background.Opacity="0.8"/>

You'll get an error saying

The attachable property 'Opacity' was not found in type 'Background'.

You would have to set this in the Background itself like this

<Grid>
    <Grid.Background>
        <SolidColorBrush Opacity="0.8"/>
    </Grid.Background>
</Grid>

So what this means as when you do something like this

<Grid TextBlock.Foreground="Red">
    <TextBlock Text="Test"/>
</Grid>

you're actually using the Attached Property Foreground for TextBlock.

Image doesn't have an Attached Property called Opacity so you can't do this either

<MenuItem Image.Opacity="0.8" />

Another workaround besides the one you're already doing is to use something like this (top-most MenuItem or wherever you want to use it).

<MenuItem x:Name="topMenuItem"
          ...>
    <MenuItem.Resources>
        <Style TargetType="Image">
            <Setter Property="Opacity" Value="0.5"/>
        </Style>
    </MenuItem.Resources>
    <!-- ... -->
</MenuItem>