Blazor Server - Cannot find the fallback endpoint specified by route values: { page: /_Host, area: } exception with RC1

Describe the bug

I have a Blazor Server app (here) which has been running fine with all of the .Net 6 previews. It’s currently deployed on Preview-7 and working without issue.

Last night I upgraded to .Net 6 RC1, and am now see this exception at startup:

InvalidOperationException: Cannot find the fallback endpoint specified by route values: { page: /_Host, area: }.
Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure.DynamicPageEndpointMatcherPolicy.ApplyAsync(HttpContexthttpContext,CandidateSetcandidates)

I have made no code changes.

Reverting to Preview-7 by removing the RC1 SDK (per the instructions here) resolved the problem and the app starts as expected.

Is there a breaking change between Preview7 and RC1 I’ve missed that means I need to make code-changes. I had a look at the breaking changes list but couldn’t spot anything that didn’t happen in earlier previews.

Exceptions (if any)

InvalidOperationException: Cannot find the fallback endpoint specified by route values: { page: /_Host, area: },
Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure.DynamicPageEndpointMatcherPolicy.ApplyAsync(HttpContexthttpContext,CandidateSetcandidates)
Microsoft.AspNetCore.Routing.Matching.DfaMatcher.SelectEndpointWithPoliciesAsync(HttpContexthttpContext,IEndpointSelectorPolicy[]policies,CandidateSetcandidateSet)
Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.<Invoke>g_AwaitMatch|8_1(EndpointRoutingMiddlewaremiddleware,HttpContexthttpContext,TaskmatchTask)
Microsoft.AspNetCore.ResponseCompression.ResponseCompressionMiddleware.InvokeCore(HttpContextcontext)
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContextcontext)

Further technical details

ASP.NET Core version: 6.0.100-rc.1.21458.32
IDE: VS for Mac 2022 Preview
OS: MacOS Monterey 12.0 Beta (21A5506j)

Issue Analytics

State:
Created 2 years ago
Reactions:1
Comments:10 (5 by maintainers)

Top GitHub Comments

2reactions

pranavkmcommented, Sep 15, 2021

There’s some performance improvements in using the source generator, but the biggest benefit is in being able to hot reload .razor and .cshtml files. It does not help with VS4Mac right now as we haven’t done the work to enable hot reload, but having it off would prevent hot reload for .razor / .cshtml from working in VS and dotnet-watch.

2reactions

javiercncommented, Sep 15, 2021

@Webreaper thanks for contacting us.

We’ve identified that the underlying issue for this is a bug in the razor source generator that prevents the views from being compiled into the assembly

You can try use

<PropertyGroup>
  <UseRazorSourceGenerator>false</UseRazorSourceGenerator>
</PropertyGroup>

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