ASP.NET Core API - Get 404 on Azure App Service, but works ok on Localhost

Question

I have an ASP.NET Core 2.1 app that I've hosted in an Azure app service. When executed locally I'm able to access the controller. But when I host in an Azure app I receive a 404. Here are the minimal steps to reproduce.

In Visual Studio 2017 add a new project. Select ASP.NET Core Web Application. Select ASP.NET Core 2.1, API project template, no authentication, configure for HTTPS. Run the new app as a self hosted (not using IIS). Browse to https://localhost:5001/api/values. I get the expected response (although there is an exception on the command line about failure to authenticate HTTPS connection).

Right click the project and select publish. Select to create a new app service. I selected my existing subscription, hosting plan, and resource group. I left the default app name. Create the app.

Browse to the url https://app_name.azurewebsites.net and I see the default page. Browse to https://appname.azurewebsites.net/api/values and I get a 404.

I'm sure I'm missing something quite stupid, but I just can't figure it out.

It's hard to tell without any more data. Do you have diagnostics or Application Insights enabled so you can share more information? — kamil-mrzyglod
– kamil-mrzyglod, Commented Apr 12, 2019 at 20:27
Not reproduce with your steps ,could you please provide more details ? — Nan Yu
– Nan Yu, Commented Apr 15, 2019 at 6:57
I've followed the steps listed above and I've reproduced it again. I made sure to create a new hosting plan and resource group. I don't know what else to do. — Brian Heilig
– Brian Heilig, Commented Apr 15, 2019 at 17:08

StuartLC · Accepted Answer · 2021-10-22 10:27:21Z

13

One possible cause of differing behaviour is the standard if (env.IsDevelopment()) branch in Startup.cs -> Configure(), which will change depending on the value of the ASPNETCORE_ENVIRONMENT or DOTNET_ENVIRONMENT env vars. These will default to production

In my case, I had removed the Home controller entirely, but inadvertently left the app.UseExceptionHandler middleware in Startup.Configure pointed at the default Home/Error.

So when my app was deployed to Azure, an exception happened which I didn't receive during local testing (e.g. SQL Firewall IP blocking issue), and this meant that any redirection to the error page resulted in a 404.

if (env.IsDevelopment())
   ... << Local environment (ASPNETCORE_ENVIRONMENT = 'Development')
else
{
    app.UseExceptionHandler("/Home/Error"); << Watch for this
    ...
}

edited Oct 22, 2021 at 10:27

answered Jun 23, 2020 at 14:19

StuartLC

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2 Comments

mathkid91 Over a year ago

This got me on the right track. Turns out my entire Swagger configuration was inside the "if (env.IsDevelopment())" brackets. I dont know if I was unfocused while setting up the application, or if I used a weird template, but I am so glad this was the problem. With just a 404 and the lack of any error messages, I started to lose faith. But as always, it was on me....

mathkid91 Over a year ago

Continuing from my last comment. The "ASP.NET Core Web Api" template in Visual Studio (at least the one I use) adds this as default inside the if(env.IsDevelopment()) condition. However, now that my head is more clear, I can easily see the benefits of exposing our API documentation only to our developers, and not the entire world :-D The template is not weird, you just have to place app.UseSwagger() and app.UseSwaggerUI() correctly, based on what you want to achieve.

Dharman · Accepted Answer · 2020-02-14 00:32:37Z

9

I was able to reproduce the error and below solution worked for me. Try this if you haven't tried earlier.

Ensure the configuration is set to Release Mode before publishing the app to Azure.

Add [Route("/")] attribute on top of your GET method in ValuesController like below.

[Route("/")]
[HttpGet("{id}")]
public ActionResult<string> Get(int id)
{
    return "value";
}

Basically, Any controller methods that do not have a route attribute use convention-based routing.

When you use [Route] attribute, you define attribute routing and so conventional routing is not used for that action/controller.

As an option, you can use the fact, that attribute routes can be combined with inheritance. Set a Route attribute on the entire controller and this will work as route prefix (the same behavior as [RoutePrefix] attribute in WebApi):

[Route("api/[controller]")]
public class ValuesController: ControllerBase
{

}

edited Feb 14, 2020 at 0:32

Dharman♦

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answered Apr 15, 2019 at 18:52

Monika Reddy

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4 Comments

Brian Heilig Over a year ago

Thank you for the help. This suddenly started working. I followed the exact same steps multiple times, and now it is working. It's very strange.

Brian Heilig Over a year ago

Oops, I forgot to mention I upgraded Visual Studio 2017 from 15.9.10 to 15.9.11.

EduardoUstarez Over a year ago

I put [Route("/")] in the Get action, but is the only action that works

aj go Over a year ago

its weird..its not working for some app service but when deployed to other app service, it works..is there other things need to consider with this error? :(

tomRedox · Accepted Answer · 2021-11-08 18:19:42Z

9

Similarly to the @StuartLC's answer, there's another development-time-only option to be wary of with Visual Studio's .NET Core Web API template:

I spent a very frustrating hour foolishly expecting a newly deployed Azure WebApp to show me the Swagger page for my API when I navigated to it in the browser. Instead I was just getting a 404. What I should have done was used Postman or whatever to check.

Only when I checked the code did I realise that the Swagger UI is only wired in at development time by default in the template:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
        app.UseSwagger();
        app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "WebApplication6 v1"));
    }

answered Nov 8, 2021 at 18:19

tomRedox

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1 Comment

Adam Cox Over a year ago

Thanks! I recall swagger being scaffolded in to my new Web API project and this made me check on that. Sure enough, it's under the dev config. This makes sense because it exposes good info to anyone who may have less than productive intentions. Cheers!

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