WPF- Compile error on referencing a Resource Dictionary file

Question

Question: When I reference the following Resource Dictionary file in MainWindow.xaml file (shown below), why do I get the following compile error?

CustomWindowStyle resource not found

Remark:

1. I'm following this article where the Creating A Resource Dictionary File section is also showing the exact procedure that I am using on how to reference Resource Dictionary in MainWindow.xaml
2. As shown in the image below, VS2019 does CORRECCTLY recognize CustomWindowStyle when I type Style="{StaticResource ....} line in MainWindow.xaml. But at the compile time (as shown in image 2 below), it throws the above error.
3. I am using .NET Core 3.1. I'm not sure if the issue is related to .NET Core

When typing the line Style="{StaticResource ....} the intellisense correctly shows the option of selecting CustomWindowStyle:

At the compile time, the error shows up:

.

WindowStyle.xaml:

<ResourceDictionary x:Class="WPF_OctDelete.WindowStyle"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:WPF_OctDelete">

    <Style x:Key="CustomWindowStyle" TargetType="{x:Type Window}">
        <Setter Property="WindowChrome.WindowChrome">
            <Setter.Value>
                <WindowChrome CaptionHeight="30"
                              CornerRadius="4"
                              GlassFrameThickness="0"
                              NonClientFrameEdges="None"
                              ResizeBorderThickness="5"
                              UseAeroCaptionButtons="False" />
            </Setter.Value>
        </Setter>
        <Setter Property="BorderBrush" Value="Black" />
        <Setter Property="Background" Value="Gray" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type Window}">
                    <Grid>
                        <Border Background="{TemplateBinding Background}"
                                BorderBrush="{TemplateBinding BorderBrush}"
                                BorderThickness="5,30,5,5">
                            <AdornerDecorator>
                                <ContentPresenter />
                            </AdornerDecorator>
                        </Border>

                        <DockPanel Height="30"
                                   VerticalAlignment="Top"
                                   LastChildFill="False">

                            <TextBlock Margin="5,0,0,0"
                                       VerticalAlignment="Center"
                                       DockPanel.Dock="Left"
                                       FontSize="16"
                                       Foreground="White"
                                       Text="{TemplateBinding Title}" />

                            <Button x:Name="btnClose"
                                    Width="15"
                                    Margin="5"
                                    Click="CloseClick"
                                    Content="X"
                                    DockPanel.Dock="Right"
                                    WindowChrome.IsHitTestVisibleInChrome="True" />


                            <Button x:Name="btnRestore"
                                    Width="15"
                                    Margin="5"
                                    Click="MaximizeRestoreClick"
                                    Content="#"
                                    DockPanel.Dock="Right"
                                    WindowChrome.IsHitTestVisibleInChrome="True" />

                            <Button x:Name="btnMinimize"
                                    Width="15"
                                    Margin="5"
                                    VerticalContentAlignment="Bottom"
                                    Click="MinimizeClick"
                                    Content="_"
                                    DockPanel.Dock="Right"
                                    WindowChrome.IsHitTestVisibleInChrome="True" />
                        </DockPanel>

                    </Grid>

                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

MainWindow.xaml:

<Window x:Class="WPF_OctDelete.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WPF_OctDelete"
        mc:Ignorable="d"
        Style="{StaticResource CustomWindowStyle}"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="WindowStyle.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    <Grid>
        <Button x:Name="btnTest" Content="Test"/>
    </Grid>
</Window>

Answer 1

The XAML processor reads the XAML file from top to bottom so by the time it tries to resolve the CustomWindowStyle resource, it hasn't yet loaded the ResourceDictionary.

You could easily fix this by setting the Resources property before you set the Style property:

<Window x:Class="WPF_OctDelete.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WPF_OctDelete"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="WindowStyle.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    <Window.Style>
        <StaticResource ResourceKey="CustomWindowStyle"/>
    </Window.Style>
    <Grid>
        <Button x:Name="btnTest" Content="Test"/>
    </Grid>
</Window>

The order matters.