Adding an additional assembly to App.razor router in Blazor causes _Host.cshtml to throw 'Object reference not set to an instance of an object'

Question

I have a Razor Class Library (RCL) that contains both Razor components and pages. This RCL has a folder named "Pages" which contains an Index.razor page and one named Details.razor. I have added the RCL to a Blazor Server project, referenced it, and am trying to let the router in the host project know where to find the Details.razor page (the host project already contains an Index page and will not use the RCL version). Here is my router code from App.razor:

<Router AppAssembly="@typeof(App).Assembly"
    AdditionalAssemblies="new[] { 
        typeof(MyRcl.Pages.Details).Assembly
    }">
    <Found Context="routeData">
        <RouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />
        <FocusOnNavigate RouteData="@routeData" Selector="h1" />
    </Found>
    <NotFound>
        <PageTitle>Not found</PageTitle>
        <LayoutView Layout="@typeof(MainLayout)">
            <p role="alert">Sorry, there's nothing at this address.</p>
        </LayoutView>
    </NotFound>
</Router>

Building the project throws the following exception from the last line of the _Host.cshtml file:

@page "/"
@namespace MyProject.Pages
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
@{
    Layout = "_Layout";
}

<component type="typeof(App)" render-mode="ServerPrerendered" />

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Call stack:

>   
MyProject.dll!MyProject.Pages.Pages__Host.ExecuteAsync() Line 8 C#
    Microsoft.AspNetCore.Mvc.RazorPages.dll!Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure.RazorPageAdapter.ExecuteAsync()  Unknown
    Microsoft.AspNetCore.Mvc.Razor.dll!Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(Microsoft.AspNetCore.Mvc.Razor.IRazorPage page, Microsoft.AspNetCore.Mvc.Rendering.ViewContext context) Unknown
    Microsoft.AspNetCore.Mvc.Razor.dll!Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(Microsoft.AspNetCore.Mvc.Razor.IRazorPage page, Microsoft.AspNetCore.Mvc.Rendering.ViewContext context, bool invokeViewStarts)  Unknown
    Microsoft.AspNetCore.Mvc.Razor.dll!Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(Microsoft.AspNetCore.Mvc.Rendering.ViewContext context) Unknown
    Microsoft.AspNetCore.Mvc.ViewFeatures.dll!Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(Microsoft.AspNetCore.Mvc.Rendering.ViewContext viewContext, string contentType, int? statusCode)  Unknown
    Microsoft.AspNetCore.Mvc.RazorPages.dll!Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure.PageResultExecutor.ExecuteAsync(Microsoft.AspNetCore.Mvc.RazorPages.PageContext pageContext, Microsoft.AspNetCore.Mvc.RazorPages.PageResult result)  Unknown
    Microsoft.AspNetCore.Mvc.RazorPages.dll!Microsoft.AspNetCore.Mvc.RazorPages.PageResult.ExecuteResultAsync(Microsoft.AspNetCore.Mvc.ActionContext context)   Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultAsync(Microsoft.AspNetCore.Mvc.IActionResult result)  Unknown
    Microsoft.AspNetCore.Mvc.RazorPages.dll!Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure.PageActionInvoker.InvokeResultAsync(Microsoft.AspNetCore.Mvc.IActionResult result)   Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext<Microsoft.AspNetCore.Mvc.Filters.IResultFilter, Microsoft.AspNetCore.Mvc.Filters.IAsyncResultFilter>(ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.State next, ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Scope scope, ref object state, ref bool isCompleted)  Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeNextResultFilterAsync<Microsoft.AspNetCore.Mvc.Filters.IResultFilter, Microsoft.AspNetCore.Mvc.Filters.IAsyncResultFilter>()    Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext<Microsoft.AspNetCore.Mvc.Filters.IResultFilter, Microsoft.AspNetCore.Mvc.Filters.IAsyncResultFilter>(ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.State next, ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Scope scope, ref object state, ref bool isCompleted)  Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters() Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.State next, ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Scope scope, ref object state, ref bool isCompleted) Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeNextResourceFilter()    Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.State next, ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Scope scope, ref object state, ref bool isCompleted) Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync()   Unknown
    Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeAsync() Unknown
    Microsoft.AspNetCore.Mvc.RazorPages.dll!Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure.PageRequestDelegateFactory.CreateRequestDelegate.AnonymousMethod__0(Microsoft.AspNetCore.Http.HttpContext context)   Unknown
    Microsoft.AspNetCore.Routing.dll!Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext httpContext)  Unknown
    Microsoft.AspNetCore.Routing.dll!Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext httpContext)   Unknown
    Microsoft.AspNetCore.StaticFiles.dll!Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)    Unknown
    Microsoft.AspNetCore.HttpsPolicy.dll!Microsoft.AspNetCore.HttpsPolicy.HttpsRedirectionMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)  Unknown
    Microsoft.AspNetCore.HttpsPolicy.dll!Microsoft.AspNetCore.HttpsPolicy.HstsMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)  Unknown
    Microsoft.AspNetCore.Diagnostics.dll!Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)    Unknown
    Microsoft.AspNetCore.Diagnostics.dll!Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)    Unknown
    Microsoft.AspNetCore.HostFiltering.dll!Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Unknown
    Microsoft.WebTools.BrowserLink.Net.dll!Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.ExecuteWithFilterAsync(Microsoft.WebTools.BrowserLink.Net.IHttpSocketAdapter injectScriptSocket, string requestId, Microsoft.AspNetCore.Http.HttpContext httpContext)   Unknown
    Microsoft.WebTools.BrowserLink.Net.dll!Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.InvokeAsync(Microsoft.AspNetCore.Http.HttpContext context)  Unknown
    Microsoft.AspNetCore.Watch.BrowserRefresh.dll!Microsoft.AspNetCore.Watch.BrowserRefresh.BrowserRefreshMiddleware.InvokeAsync(Microsoft.AspNetCore.Http.HttpContext context) Unknown
    Microsoft.AspNetCore.Http.Abstractions.dll!Microsoft.AspNetCore.Builder.Extensions.MapWhenMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)  Unknown
    Microsoft.AspNetCore.Hosting.dll!Microsoft.AspNetCore.Hosting.HostingApplication.ProcessRequestAsync(Microsoft.AspNetCore.Hosting.HostingApplication.Context context)   Unknown
    Microsoft.AspNetCore.Server.Kestrel.Core.dll!Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests<Microsoft.AspNetCore.Hosting.HostingApplication.Context>(Microsoft.AspNetCore.Hosting.Server.IHttpApplication<Microsoft.AspNetCore.Hosting.HostingApplication.Context> application)    Unknown
    Microsoft.AspNetCore.Server.Kestrel.Core.dll!Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequestsAsync<Microsoft.AspNetCore.Hosting.HostingApplication.Context>(Microsoft.AspNetCore.Hosting.Server.IHttpApplication<Microsoft.AspNetCore.Hosting.HostingApplication.Context> application)   Unknown
    Microsoft.AspNetCore.Server.Kestrel.Core.dll!Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.Http2Stream<Microsoft.AspNetCore.Hosting.HostingApplication.Context>.Execute() Unknown
    System.Private.CoreLib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch()  Unknown
    System.Private.CoreLib.dll!System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart() Unknown
    System.Private.CoreLib.dll!System.Threading.Thread.StartCallback()  Unknown

If I remove the "AdditionalAssemblies" statement from the router configuration the app builds and runs fine, so I am pretty sure that is triggering the error but am unsure on what is causing it (I have double-checked the syntax and compared it with other apps where I have consumed Razor pages from RCLs and the code seems correct). Any help or ideas for further clarifying what the underlying problem might be is appreciated.

Answer 1

Found the issue - will post here in case someone else encounters the same problem as is difficult to identify. It is a variation of a .NET 6 issue described here and here which is slotted for consideration in .NET 7. In my case the issue was the duplicate Index.razor files in the host project and RCL (not the Details.razor file I was trying to consume from the RCL). The duplicate files should not be an issue per MS documentation as the file in the hosting app should take precedence. This was indeed the case until I tried to modify App.razor to consume the Details page from the RCL, at which point the exception occurs (even though I am only referencing the unduplicated Details page and not the Index page in the router code). I can resolve the exception by removing the unused Index page in the RCL (but this is not a permanent solution as it is used in other projects). My workaround for now is to override both of the RCL pages and remove the AdditionalAssemblies statement from App.razor.