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WinUI XAML: Using a ResourceDictionary from another project

In my WinUI 3 application, I am trying to use a ResourceDictionary that is located in another project.
Lets say the referenced project is ResourceTestLib and this library project has a folder "Styles" which has a file "_Thickness.xaml".
In the app.xaml file of the main application, I tried these two approaches below the "Other merged dictionaries here" comment, but none of them seem to work, i.e. that app crashes on startup with that message "Cannot locate resource ...." message.
This is my app.xaml:
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
<!-- Other merged dictionaries here -->
<ResourceDictionary Source="/ResourceTestLib;component/Styles/_Thickness.xaml"/>
<ResourceDictionary Source="pack://application:,,,/ResourceTestLib;component/Styles/_Thickness.xaml"></ResourceDictionary>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
When I hover over the respective line, Visual Studio shows a tooltip with "Path X not found" for the first referenced ResourceDictionary and a "Invalid characters in path" (probably because of "application:,,,") message for the second.
I suppose that that WinUI XAML might be different than WPF XAML in that respect? Or even that this is not supported yet?

Pack URIs are only used in WPF.
UWP and Win UI use the ms-appx URI scheme to refer to a file that resides in another assembly so try this:
<ResourceDictionary Source="ms-appx:///ResourceTestLib/Styles/_Thickness.xaml" />
More on UWP style URI schemes:
https://learn.microsoft.com/en-us/windows/uwp/app-resources/uri-schemes

Styles from resource dictionary not found in user control

I have a resource dictionary with a bunch of styles that I am linking too in my user controls like so:
<UserControl.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="../Theme/ThemedResources.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</UserControl.Resources>
This all works fine during runtime.
However I get a recurring problem in that one of the styles in the resource dictionary 'cannot be found' in whichever user control is the most recent, until the solution is rebuilt. This error will then appear again after I stop the debugging process, and again will disappear with a rebuild.
I don't think this is linked to a specific style, as when I add new styles the style that can't be found seems to change.
Any ideas how I can stop this from happening?

You have used a Relative URI, so it will looks for your resource file in a somewhere that you used your UserControl. It can't find you resource file because your resource file is not in the AbsolutePath.
AbsolutePath = CurrentPath (r.g Where you used your UserControl) + RelativePath
so Use an AbsolutePath:
<ResourceDictionary Source="pack://application:,,,/{YourAssemblyName};component/Theme/ThemedResources.xaml" />

How can I make all Resources in one place

I work on C# WPF
I have more than one DataGridView all of them has the same style (Background, row width, column height...etc) but each one has different numbers of columns
So my question is Can I make one style for these grids and share between them?
like css in Web

You can create a resource file in wpf where you can store all your style and template. Don't confuse it with the resource file in the Properties directory. You can add a Resource file by right clicking your project in visual studio and adding an existing xaml file or creating a new one.
After that, define all your styles and templates. Then you only need to include it in your available resources
<Application
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Class="UsingResourceDictionaries.App">
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="myDirectory/myDictionary.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
Don't forget to give a key to each one of your style and template, so that you can use them in your User controls

What kind of "Key" do I need to supply when loading Resource Dictionaries?

In trying to set a default ResourceDictionary I receive the following warning:
The designer does not support loading dictionaries that mix
'ResourceDictionary' items without a key and other items in the same
collection. Please ensure that the 'Resources' property does not
contain 'ResourceDictionary' items without a key, or that the
'ResourceDictionary' item is the only element in the collection.
This is the code that I am using in my App.xaml file, that received the above warning:
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Lang.en-US.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
This is the exact same code that I've used to set a ResourceDictionary in Visual Studio 2008. I am now using VS 2010. What Key do I need to provide this ResourceDictionary for it to work correctly?
This is the line in my MainWindow.xaml that I am currently testing along with this code:
<MenuItem Header="{DynamicResource new_test}" />

Since you haven't posted your complete XAML file, i suspect there are other resources apart from merged dictionary in your resources section.
As per MSDN -
It is legal to define resources within a ResourceDictionary that is
specified as a merged dictionary, either as an alternative to
specifying Source, or in addition to whatever resources are included
from the specified source. However, this is not a common scenario; the
main scenario for merged dictionaries is to merge resources from
external file locations. If you want to specify resources within the
markup for a page, you should typically define these in the main
ResourceDictionary and not in the merged dictionaries.
Try moving other resources in separate resource dictionary and make sure all other resources have x:Key set on them -
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Lang.en-US.xaml" />
</ResourceDictionary.MergedDictionaries>
<ResourceDictionary>
<BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter"/>
<ContextMenu x:Key="MyContextMenu"/>
</ResourceDictionary>
</ResourceDictionary>
</Application.Resources>

Use resource file for translations. Its better than resource dictionary.
Here is an example:
Set prefix like this for usage in xaml.
xmlns:const="clr-namespace:FileExplorer.Properties"
Resources are located in properties.
To use them in XAML you will need following:
<TextBox Text="{x:Static const:Resources.Window_Title_String}"/>
If you have different languages then create for each language own resource file following naming convention.
For example:
Resources.resx (this will be default)
Resources.de-DE.resx (this is for german)
Now you just have to set current culture to german for your app to be on german and the proper resource file will be used automatically.
Like this in Main method:
Thread.CurrentThread.CurrentUICulture = new CultureInfo("de-DE");

Xaml: C# How to keep style consistent

I'm working on a semi-large windows application using wpf and C# in VS 2010. While working on the xaml, I added a tag so that all buttons and datagrids are styled in the same way. I've copied and pasted this block into several of my .xaml files and that works fine. Of course the problem I'm running into now is that I've added to and changed the style several times.
What is the best way to keep style consistent between my different Windows? Is it subclassing, using Resources.resx, or another way enirely?

If you define the style in the Application level ResourceDictionary (App.xaml), then it will automatically be inherited by your other XAML Windows/Controls.

yeah, if you were to create a new file called Resources.xaml and then add this to your Application.xaml file:
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Resources.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
then you should be able to reference the styles in the Resources.xaml from all the windows in your application.