WPF Class library throws System.Windows.Markup.XamlParseException when referencing assembly WPFLocalizeExtension

Check if all package dependencies are pulled in:

Also try to explicitly add all these dependencies to the application assembly (the one that generates the executable file, for example, *.exe).

After a bit of research and some trial and error, I've come up with the answer.

The main application, revit.exe, loads each addin/plugin from an .addin/.xml file in a folder. Here's an example of a typical .adding xml file it will load:

<?xml version="1.0" encoding="utf-8"?>
<RevitAddIns>
  <AddIn Type="Application">
    <Name>BDS</Name>
                <Assembly>D:\dev2\BerkshireDesignServices\bin\Debug\BerkshireDesignServices.dll</Assembly>
    <AddInId>98f0af1f-a857-44e0-8b7a-cb52f24f7543</AddInId>
    <FullClassName>BDS.BDS</FullClassName>
    <VendorId>AJPH</VendorId>
    <VendorDescription>AJPH</VendorDescription>
  </AddIn>
</RevitAddIns>

When revit.exe runs, it loads each .dll from the path specified in the "Assembly" xml tag.

This is typically fine for any application that doesn't use an external library not in .NET framework (.NET core for Revit 2025+). When any plug-in/adding has an additional class library to include, the main application will only search for these additional libraries where the execution has started. So, to resolve this problem in a class library you add a handler at the beginning of the library initialization code, like so:

private void AddLibraryHandler(
{
    AppDomain.CurrentDomain.AssemblyResolve += (sender, args)=>{
    foreach(var e in libariesToLoad)
        {
            if (args.Name.Contains(e)
            {
                return Assembly.LoadFrom(Path.Combine(libPath, $"{e}.dll"));
            }
        }
        return null;
        };
}