IOException when referencing App.xaml's ResourceDictionary - wpf

Question

I'm trying to reference App.xaml's ResourceDictionary from a separate WPF window. I want to use resources from there in different windows, and this seems like the recommended way to do it. Unfortunately, I don't seem to be able to effectively reference App.xaml from the other window. Here is my App.xaml:
<Application x:Class="View.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
xmlns:ViewModel="clr-namespace:ViewModel;assembly=ViewModel"
xmlns:local="clr-namespace:View"
StartupUri="ClockView.xaml">
<Application.Resources>
<ResourceDictionary>
<local:PriorityToIconConverter x:Key="PriorityToIconConverter" />
</ResourceDictionary>
</Application.Resources>
Notes: I'm not using MainWindow, so I've replaced the startup URI with a form that always comes up. I noted that in some other answers, the location of MainWindow is sometimes the issue. In my case, I haven't seen any difference between using ClockView or MainWindow. Both ClockView and MainWindow exist in the root namespace, MainWindow is just never loaded. I also have more resources, but I've removed them for the sake of conciseness.
Here's a simplified example of the code where I'm trying to reference the ResrouceDictionary from App.xaml:
<local:AssistantWindow
x:Class="View.AutomatorView"
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:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
xmlns:properties="clr-namespace:View.Properties"
xmlns:local="clr-namespace:View"
mc:Ignorable="d"
Title="Tool"
x:Name="Tool"
Background="Transparent"
Height="600"
Width="450"
Topmost="{Binding Source={x:Static properties:Settings.Default}, Path=ToolAlwaysOnTop}"
MinHeight="515"
MinWidth="150">
<Window.Resources>
<ResourceDictionary Source="App.xaml" />
</Window.Resources>
Again, this is simplified to be concise. When I try to load this form, I get the exception:
System.Windows.Markup.XamlParseException: ''Set property 'System.Windows.ResourceDictionary.Source' threw an exception.' Line number '21' and line position '10'.'
Inner Exception
IOException: Cannot locate resource 'tool/app.xaml'.
The view for "Tool" is located in a folder that is also named "Tool." However, the xaml and code behind don't reference this namespace, I'm just using the folder to organize my classes. It looks like it's looking for App.xaml in the folder the view resides in. App.xaml resides in the root namespace (View). I've tried modifying the source in the xaml for Tool to:
- View.App.xaml
- View:App.xaml
- View/App.xaml
How can I get this reference to work, so I can share resources throughout my application? Thank you.

Answer 1

You can't load App.xaml like you're trying to do because it's not actually a ResourceDictionary. You can only specify ResourceDictionary files as the target of Source.
However, if you declare a resource in App.xaml, you can reference it anywhere without needing to load the file it's in. That's done for you automatically. Therefore, you can reference your converter at any time with {StaticResource PriorityToIconConverter}.
Note that if you moved it from the default starting location (the base project folder) you may have to update its location. Right click your project, then Properties. Navigate to the "Application" tab (should be the uppermost element on the left-hand sidebar) and look for the "Startup object" field. Set that to [ProjectName].[Namespace?].[Namespace?].App. When I tested it, mine worked without needing to manually change the location, but your setup may be different.