Blazor Movie Database app tutorial - Part 4 concurrency sample is missing a `return` after `NavigationManager.NotFound()`

[shaunplumb-sue]

Description

The UpdateMovie sample assumes that NavigationManager.NotFound() ends the
method. It doesn't always. The framework halts execution by throwing a
NavigationException, and .NET 10 lets an app opt out of that with
<BlazorDisableThrowNavigationException>true</BlazorDisableThrowNavigationException>.
When the throw is disabled, execution leaves the catch block and reaches
NavigationManager.NavigateTo("/movies") on a response whose status code is
already 404.

Following the four test steps in that section, step 4 says the browser is
navigated to the Not Found page with a 404 status code. I got no Not Found page
and an unhandled exception instead. Stepping through it, MovieExists returns
false and the debugger reaches NotFound(), so the throw; branch is not
involved.

Suggested fix, which adds the return and moves the success redirect onto the
success path:

    try
    {
        await context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException)
    {
        if (MovieExists(Movie!.Id))
        {
            throw;
        }

        NavigationManager.NotFound();
        return;
    }

    NavigationManager.NavigateTo("/movies");

The return makes the sample behave the same way whether or not the
NavigationException is thrown. It also stops the sample from teaching that a
thrown exception is the method's control flow, which is a surprising thing for a
beginner tutorial to model.

One other suggestion. The test steps could say that the Not Found content needs
somewhere to go. UseStatusCodePagesWithReExecute covers static SSR, and
Router.NotFoundPage is what applies once the components are interactive after
Part 8. With neither configured, NotFound() sets the status code and nothing
renders, which is easy to mistake for the sample being broken.

Page URL

https://learn.microsoft.com/en-gb/aspnet/core/blazor/tutorials/movie-database-app/part-4?view=aspnetcore-10.0&pivots=vsc

Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/blazor/tutorials/movie-database-app/part-4.md

Document ID

ce694ba5-636a-fe1b-dd40-694ec3489d06

Platform Id

bd9facb3-c39a-53cb-f6bb-eeb8d81f7cbf

Article author

@guardrex

Metadata

• ID: ce694ba5-636a-fe1b-dd40-694ec3489d06
• PlatformId: bd9facb3-c39a-53cb-f6bb-eeb8d81f7cbf
• Service: aspnet-core
• Sub-service: blazor

Related Issues

[github-actions]

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[guardrex]

Hello @shaunplumb-sue! Thanks for your issue.

I got no Not Found page and an unhandled exception instead.

Interesting! I can't get that to reproduce here. In a test here, I opened the Edit page for The Road Warrior in Edge. I went to the app in a window of Firefox and deleted The Road Warrior. I went back to the Edit page in Edge and tried to update the title of the movie to "The Road Warrior UPDATED." I received the described result ...

... and the 404 ...

We'll need to get the product unit involved to see if they'd like an issue opened on one of the engineering repos. If there's an potential underlying bug in Not Found code/behavior, they'd want the issue on the ASP.NET Core framework repo (dotnet/aspnetcore). If there's an underlying issue in the scaffolded code, they'd want the issue on the scaffolder repo (dotnet/Scaffolding).

BTW, you can see the code for the Edit component here ...

https://github.com/dotnet/Scaffolding/blob/main/src/dotnet-scaffolding/dotnet-scaffold/AspNet/Templates/net11.0/BlazorCrud/Edit.tt#L100-L122

... and BlazorDisableThrowNavigationException is set to true by the project template here ...

https://github.com/dotnet/aspnetcore/blob/main/src/ProjectTemplates/Web.ProjectTemplates/BlazorWebCSharp.csproj.in#L11

The tutorial, which is maintained by me 🦖 here on the docs repo, relies on the scaffolder code and the underlying Not Found feature behavior being correct. You may have found a bug in that logic that the product unit should take a look at. It's also possible that the tutorial isn't calling out something correctly about Not Found behavior, but I'm not sure what that would be at the moment given that I can't repro your report here (yet anyway 😄).

Pinging @danroth27, the PM for ASP.NET Core and Blazor, to triage your issue. If he doesn't see the ping and respond within 24 hours, I'll message him offline. Stand-by for his feedback and direction ......................

[guardrex]

Actually ... I think I may see the problem here ... maybe.

@shaunplumb-sue ... Can you do me a favor with your app exhibiting that behavior? Make the Movies Index page interactive and try it again.

Open the movie Index component file (Components/Pages/MoviePages/Index.razor), and add the following @rendermode directive immediately after the @page directive to make the component interactive:

@rendermode InteractiveServer

Try it again and see if you get the correct behavior.

What I think is happening here (and the problem with the tutorial) is that when the component is rendered statically only a 404 is thrown. It's with interactive rendering that I think you'll get the Not Found behavior. I looked it up to confirm what's probably going on here ...

https://learn.microsoft.com/en-us/aspnet/core/blazor/fundamentals/navigation?view=aspnetcore-10.0#not-found-responses

... and if you get that behavior, then I'll make an update to the tutorial to address this problem with the steps. It isn't until Step 8 at ...

https://learn.microsoft.com/en-us/aspnet/core/blazor/tutorials/movie-database-app/part-8?view=aspnetcore-10.0&pivots=vsc#adopt-interactivity

... that the app gets interactivity.

[guardrex]

@shaunplumb-sue ... I checked my static-rendering hypothesis on that, and that didn't seem to be the cause of what you're seeing, probably due to the presence of status code pages with re-execute middleware in the request processing pipeline ...

app.UseStatusCodePagesWithReExecute("/not-found", createScopeForStatusCodePages: true);

I can't reproduce the behavior that you report for either static or interactive rendering. Let's see what Dan thinks about your report.

[shaunplumb-sue]

Thanks for checking.

I can confirm that if I revert to the code as in the tutorial, the problem still happens - hitting the 'Save' button doesn't change the page at all, but we do see a 404 for a 'fetch' in the network tab. I also reproduced it with the sample github code (after converting to SQLite).

I was testing by simply duplicating the tab in Chrome before deleting, but just to be sure I copied the URL and opened it in Safari - same result.

Some more info ... this is running in VS Code 1.136.0 on a MacBook Air M3 and I'm using Chrome 152.0.7977.65

The full 'about' from VS Code is:

Version: 1.136.0
Commit: 520fb30b2d3d324b4cb2342f6e88e2cd93751de1
Date: 2026-09-02T09:13:42Z (1 day ago)
Electron: 42.10.0
ElectronBuildId: 15109253
Chromium: 148.0.7778.280
Node.js: 24.18.1
V8: 14.8.178.38-electron.0
@github/copilot: 1.0.81-0
@github/copilot-sdk: 1.0.11
OS: Darwin arm64 25.6.0

[guardrex]

Thanks for that extra info. Also, take all of the output from dotnet --info and paste it into an issue comment here. As long as you don't include the CLI prompt that may include a path with your machine/user name, there won't be any PII in the output, so nothing private will be revealed from your system.

Next, we'll see how Dan wants to triage this. If we don't hear from him by tomorrow morning, I'll message him offline.