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I created an external control library which holds some resource dictionaries etc. My problem is when I try to apply a style on a Window element. The style's changes are visible only when running, and not in the Visual Studio designer.

An example of the resource dictionary from my control library:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Style x:Key="Window_Style" TargetType="Window">
        <Setter Property="Background" Value="#FF272727"/>
    </Style>

</ResourceDictionary>

This is how I include my external resource dictionary into my app:

<Application x:Class="SigmaLibMaster.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:SigmaLibMaster"
             StartupUri="MainWindow.xaml">

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/SigmaLib;component/Resources/Styles/Window.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

</Application>

And how I apply it to my window element:

<Window x:Class="SigmaLibMaster.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:SigmaLibMaster"
        mc:Ignorable="d"
        Title="MainWindow" Height="480" Width="840"
        Style="{DynamicResource Window_Style}">

    <Grid>
    </Grid>
    
</Window>

Any idea why is this happening?

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2 Answers 2

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3

You can sometimes find that resources from libraries don't work at design time.

It's a bug IMO.

The work round I use is design time resources.

This is a mechanism which was originally intended for blend. But the wpf designer in visual studio is the same designer as blend now.

I have a library called uilib.

In the properties of that I add a resource dictionary called DesignTimeResources.xaml. It must be that name.

In the csproj I have the following:

<ItemGroup>
<Page Include="Properties\DesignTimeResources.xaml" Condition="'$(DesignTime)'=='true' OR ('$(SolutionPath)'!='' AND Exists('$(SolutionPath)') AND '$(BuildingInsideVisualStudio)'!='true' AND '$(BuildingInsideExpressionBlend)'!='true')">
  <Generator>MSBuild:Compile</Generator>
  <SubType>Designer</SubType>
  <ContainsDesignTimeResources>true</ContainsDesignTimeResources>
</Page>

Note particularly that ContainsDesignTimeResources tag.

That merges a bunch of resource dictionaries I have in uilib:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:UILib">
    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="pack://application:,,,/UILib;component/Resources/Geometries.xaml"/>
        <ResourceDictionary Source="pack://application:,,,/UILib;component/Resources/ControlTemplates.xaml"/>
        <ResourceDictionary Source="pack://application:,,,/UILib;component/Resources/FontResources.xaml"/>
        <ResourceDictionary Source="pack://application:,,,/UILib;component/Resources/UILibResources.xaml"/>
        <ResourceDictionary Source="pack://application:,,,/UILib;component/Resources/HypsoColoursRD.xaml"/>
    </ResourceDictionary.MergedDictionaries>
</ResourceDictionary>

It will not merge these resources in an extra time when you build. The conditions in the tags mean it's design time only. You can probably find a bunch more on this by searching now you know it exists.

https://dennymichael.net/2016/07/28/wpf-design-time-resources-dictionary/

Edit

Having tried this with the code supplied, there's a complication here with usage of x:Type Window.

MainWindow is not a window, it's a MainWindow.

The designer doesn't seem to want to work out MainWindow inherits from Window.

If I change that style to:

<Style x:Key="MainWindow_Style" TargetType="{x:Type Control}">
    <Setter Property="Background" Value="Green"/>
</Style>

This then works for me in my designer.

enter image description here

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16 Comments

A couple of questions. [1] The DesignTimeResources.xaml should be placed into properties folder of my Library project? [2] My MergedDictionaries should be added into DesignTimeResources.xaml which we created before? [3] And what should I do from my application project, just what I showed in my example? Because I do exactly as you wrote and as the link says and it doesn't "work"!!!
What exactly do you mean by doesn't "work"? If your window is in the application project and you don't get the styles applied then the design time merging needs to be in that project ( as well ).
By "doesn't work" I mean that my window doesn't get the style in design mode. And yes, my window is into a separate application project!!! In that case, should I create the DesignTimeResources.xaml into properties folder of my application project and do my merges in there and not into my Library project?
You will need it in your entry point. If you don't do any styling directly on controls in your external library then you don't need it there. Windows are a bit tricky as well incidentally since they have a window tag but their type is not actually a window. They're a mainwindow or whatever.
The problem here is because MainWindow is a MainWindow, not a Window. If you change that to x:Type Control from xType WIndow then it'll work. Or it does for me anyhow.
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As Andy commented into his answer, "The problem here is because MainWindow is a MainWindow, not a Window. If you change that to x:Type Control from xType Window then it'll work".

So in my case, code stays as it is in my question and I just need to change <Style x:Key="MainWindow_Style" TargetType="{x:Type Window}"> to <Style x:Key="MainWindow_Style" TargetType="{x:Type Control}">. Even if I don't use this DesignTimeResources.xaml stuff, it works!!!

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