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WinUI 3 "XAML parsing failed." when adding MenuBar

I'm experimenting with WinUI 3 and trying to create a basic user interface.
As soon as I add a MenuBar control, I get a Microsoft.UI.Xaml.Markup.XamlParseException: 'XAML parsing failed.' at the InitializeComponent() line of that view's code-behind when trying to start the app.
I can add other controls without issues, but MenuBar always seems to break the app. What am I missing?

Check your App.xaml to include the <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" /> resource dictionary like this:
<Application
x:Class="Your.Project.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<!-- the following line is important -->
<XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>

Using a Loading Indicator for WPF application

I'm relatively new to WPF and I would like to add a loading indicator. I found https://github.com/100GPing100/LoadingIndicators.WPF which has some nice pre-made loading indicators and I installed the NuGet Package for it but I'm having a hard time implementing them to my window.
So I added
<Window ...
xmlns:loadin="clr-namespace:LoadingIndicators.WPF;assembly=LoadingIndicators.WPF"
...
>
Window.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/LoadingIndicators.WPF;component/Styles/LoadingDoubleBounce.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Window.Resources>
But when I try to add one of the loading indicators to my grid I get an error on the dynamic resource
<Grid x:Name="TagLoadingIndicator" Panel.ZIndex="1">
<loadin:LoadingIndicator SpeedRatio="{Binding SpeedRatio}" IsActive="{Binding IsDoubleBounceActive}" Style="{DynamicResource LoadingIndicatorDoubleBounceStyle}"/>
</Grid>
The error I get is
The resource "LoadingIndicatorDoubleBounceStyle" could not be
resolved.
From what I saw in other websites, I am adding it to the ResourceDictionary correctly... And I'm assuming that by installing the NuGet package, I have the resource already defined. I can still run the application but the indicator is missing. What am I missing?
Thank you

That key exists in the Styles.xaml file. So add a reference to it in your resources.
<Window.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/LoadingIndicators.WPF;component/Styles/LoadingDoubleBounce.xaml"/>
<ResourceDictionary Source="pack://application:,,,/LoadingIndicators.WPF;component/Styles.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Window.Resources>

Using (merged) Resource Dictionary in C# Class Library Project

How can I use a (merged) WPF Resource Dictionary in a C# class library project?
Here is what I did:
In my C# class library project I have a file Dictionary1.xaml like that:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
<Style x:Key="PluginFrameBorderStyle">
...
</Style>
</ResourceDictionary>
Then, I have a UserControl file UserControl1.xaml where I try to use the Dictionary like that:
<UserControl x:Class="EditorPackageA.BackboneMemberB1Editor.BackboneMemberB1Editor"
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"
xmlns:local="clr-namespace:EditorPackageA.EditorBase"
xmlns:prism="http://www.codeplex.com/prism" d:DesignWidth="690.4" d:DesignHeight="460.12">
<UserControl.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/Dictionary1.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</UserControl.Resources>
...
</UserControl>
The project compiles but at runtime I get the error:
with the exception detail:
The same approach works when applied within a WPF project rather than a Class Library project.
What might be the solution here?
Important Addendum:
During design-time I see the effect of the used style that is embedded via the ResourceDictionary, hence the URI of the style and the dictionary must be correct!?

Try to use so called pack URI. I think that you have to explicitly specify where the resource dictionary is located.
<ResourceDictionary Source="pack://application:,,,/TheNameOfClassLibrary;component/Dictionary1.xaml"/>
In the case of a WPF project your approach works because WPF engine by default looks for resource in the assembly being executed (in exe).

You should refer/link the ResourceDictionary xml file to the Source attribute with assembly and component name.
Use relative path as follows:
<ResourceDictionary Source="../Dictionary1.xaml" />
If it is not working, then try PACK URL
<ResourceDictionary Source="pack://application:,,,/Your.Base.AssemblyName;component/DictionaryFolder/Dictionary1.xaml" />
Hope it helps you

You probably need to merge your library's resource dictionary in your application resources. You need to edit your App.xaml file and add something like:
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/YourAssembly;component/Dictionary1.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>

How to place XAML-only resources in a DLL (Reference) file to be used by WPF applications?

My WPF applications are reaching the level of complexity at which it becomes desirable to place some of the code and other resources inside a Reference file (*.dll).
I am sure I can figure out how to do this, by following the myriad applications out there.
In this particular case, however, the files are two, XAML:
Can somebody please provide an example on how to expose or publish the XAML resources? Are *.cs files required?
TIA
The XAML file outlined by #HighCore should look like this:
<Application x:Class="Application"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/ReferencedAssembly;component/Subfolder/ResourceFile.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>

XAML ResourceDictionaries do not generate classes. I'm not sure why you would expect such thing.
Simply move the needed ResourceDictionary definitions to a separate assembly then use the Pack URI Syntax to merge these resources to your application's Resources in app.xaml:
<Application ....>
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/ReferencedAssembly;component/Subfolder/ResourceFile.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>

ResourceReferenceKeyNotFoundException declared on Dictionary.xaml

I have an ObjectDataProvider declared on a dictionary:
<ObjectDataProvider x:Key="Resources"
ObjectType="{x:Type const:CultureResources}"
MethodName="GetResourceInstance"/>
And I have a class that wants to find it like this:
m_provider = (ObjectDataProvider) App.Current.FindResource("Resources");
But when it tries to find the Resource, it launches the error
ResourceReferenceKeyNotFoundException
And can't find my resource... Here you have an image on how my project is divided:
The Default.xaml is my default dictionary.
So, why I can't find my Resources element defined on my dictionary?

I need to answer my question, because it was such a weird thing...
At first, I need to say that I had to add my Dictionary.xaml to my App.xaml:
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="../themes/Default.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
For some reason, it wasn't enough to call it on my MainWindow.xaml. So I erased it from my MainWindow.xaml and added it to App.xaml, but I couldn't have them inserted in both sides. That was the part that I couldn't see clearly...