The property "Style" can only be set once

Question

I am trying to get this answer to work, however I get the error The property "Style" can only be set once.. Why is that?

This is what I have:

<UserControl x:Class="SFVControls.Controls.Basic.BooleanRectangleView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             mc:Ignorable="d" 
             d:DesignHeight="20" d:DesignWidth="40">

    <StackPanel Name="stackTextPanel" Orientation="Horizontal">
        <StackPanel.Style>
            <Setter Property="Margin" Value="0,8,0,0" />
            <Style TargetType="{x:Type StackPanel}">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding QuickDrawBarPinned}" Value="False">
                        <Setter Property="Margin" Value="0,8,0,0" />
                    </DataTrigger>
                    <DataTrigger Binding="{Binding QuickDrawBarPinned}" Value="True">
                        <Setter Property="Margin" Value="0,48,0,0" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </StackPanel.Style>
    </StackPanel>
</UserControl>

Answer 1

You've got two direct children of <StackPanel.Style>: A Setter for Margin that belongs within the Style, and the Style itself. You can only assign one thing to <StackPanel.Style>, and that thing has to be a Style.

So just move the Margin setter inside the Style element:

<StackPanel Name="stackTextPanel" Orientation="Horizontal">
    <StackPanel.Style>
        <Style TargetType="{x:Type StackPanel}">


            <!-- Gotta be inside the Style, not outside. -->
            <Setter Property="Margin" Value="0,8,0,0" />


            <Style.Triggers>
                <DataTrigger Binding="{Binding QuickDrawBarPinned}" Value="False">
                    <Setter Property="Margin" Value="0,8,0,0" />
                </DataTrigger>
                <DataTrigger Binding="{Binding QuickDrawBarPinned}" Value="True">
                    <Setter Property="Margin" Value="0,48,0,0" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </StackPanel.Style>
</StackPanel>

In short, your style has a fashion faux pas.