Question

Is it possible to use a PlaceHolder in a WPF Style that could later be assigned a value by the implementing control? Something like:

<Style x:Key="BigButton" TargetType="{x:Type Button}">
    <Setter Property="Foreground" Value="#58290a" />
    <Setter Property="FontFamily" Value="Lucida Console" />
    <Setter Property="FontSize" Value="24" />
    <Setter Property="Content">
        <Setter.Value>
            <StackPanel>
                <Image Source=[SOME PLACEHOLDER]></Image>
                <TextBlock>[ANOTHER PLACEHOLDER]</TextBlock>
            </StackPanel>
        </Setter.Value>
    </Setter>
</Style>

and then a Button could assign an image path to this place-holder like this:

<Button Name="btn" Style="{StaticResource BigButton}">
    <SOME CLEVER WAY OF ASSIGNING A PATH TO Style.Content.StackPanel.Image.Source and Style.Content.StackPanel.TextBlock.Text>
</Button>

Answer 1

This sounds like a job for Attached properties.

Create a utility class with you “placeholder properties”:

public static class BigButtonUtils
{
    public static Uri GetBBImage(DependencyObject obj)
    {
        return (Uri)obj.GetValue(BBImageProperty);
    }
    public static void SetBBImage(DependencyObject obj, Uri value)
    {
        obj.SetValue(BBImageProperty, value);
    }
    // Using a DependencyProperty as the backing store for BBImage.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty BBImageProperty =
        DependencyProperty.RegisterAttached("BBImage", typeof(Uri), typeof(BigButtonUtils), new UIPropertyMetadata(null));


    public static string GetBBCaption(DependencyObject obj)
    {
        return (string)obj.GetValue(BBCaptionProperty);
    }
    public static void SetBBCaption(DependencyObject obj, string value)
    {
        obj.SetValue(BBCaptionProperty, value);
    }
    // Using a DependencyProperty as the backing store for BBCaption.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty BBCaptionProperty =
        DependencyProperty.RegisterAttached("BBCaption", typeof(string), typeof(BigButtonUtils), new UIPropertyMetadata(null));
}

Apply those properties on a Button..

<Button Style="{StaticResource BigButtonStyle}"
        bb:BigButtonUtils.BBImage="http://programming.enthuses.me/1.png" 
        bb:BigButtonUtils.BBCaption="My caption" />

..and use the property values in your Style:

<Style x:Key="BigButtonStyle" TargetType="{x:Type Button}">
    <Setter Property="Foreground" Value="#58290a" />
    <Setter Property="FontFamily" Value="Lucida Console" />
    <Setter Property="FontSize" Value="24" />
    <Setter Property="Content">
        <Setter.Value>
            <StackPanel DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=Button}}">
                <Image Source="{Binding Path=(bb:BigButtonUtils.BBImage)}" Stretch="None" />
                <TextBlock Text="{Binding Path=(bb:BigButtonUtils.BBCaption)}" />
            </StackPanel>
        </Setter.Value>
    </Setter>
</Style>

NOTE: Don’t set any other content on the Button, like <Button Content="..." /> or <Button ...>SomeContent</Button>, as that would override the “placeholder content” from the Style.