Swashbuckle/Swagger on .NET Core 2.1 has stopped working since upgrade

Question

I have a .NET Core 2.0 application, using Swashbuckle/Swagger to generate API documentation. When we were on 2.1.0-preview, Swagger was working fine. Then we did the big upgrade to 2.1.0 release and SDK 2.1.300. We didn't notice exactly when things broke, but now our Swagger docs won't load. Here's what we see:

Project has a reference to Swashbuckle.AspNetCore version 2.5.0. The relevant code in Startup.cs is below. In ConfigureServices():

services.AddSwaggerGen(swaggerOptions =>
{
    // Register a swagger doc
    swaggerOptions.SwaggerDoc("v1", new Info
    {
        // Optional descriptive info that will be included in the Swagger output
        Contact = new Contact
        {
            Name = "LightSail",
            Url = "https://myurl.com/"
        },
        Description = "A description of the API can go here",
        Title = "My API",
        Version = "v1"
    });

    // Xml file to get comment information from
    swaggerOptions.IncludeXmlComments("App_Data/Api.xml");
});

And in Configure():

app.UseSwagger();

app.UseSwaggerUI(swaggerUiOptions => swaggerUiOptions.SwaggerEndpoint("/swagger/v1/swagger.json", "My API v1"));

I found lots of other similar questions, one of which suggested that there might be duplicate endpoints; I tried adding a call to .ResolveConflictingEndpoints() but that made no difference. I have searched through my project folders and there is no file called swagger.json, so I'm guessing that's the problem.

Any ideas why this is not working, or how to fix?

Answer 1

This is usually indicative of controllers/actions that Swashbuckle doesn't support for one reason or another.

It's expected that you don't have a swagger.json file in your project. Swashbuckle creates and serves that dynamically using ASP.NET Core's ApiExplorer APIs. What's probably happening here is that Swashbuckle is unable to generate Swagger.json and, therefore, the UI is failing to display.

As HelderSepu said, it's hard to know exactly what caused the failure, so the best way to debug is probably just to remove half your controllers (just move the files to a temporary location) and check whether the issues persists. Then you'll know which half of your controllers contains the troublesome action. You can 'binary search' removing controllers (and then actions) until you figure out which action method is causing Swashbuckle to not be able to generate Swagger.json. Once you know that, it should be obvious whether this is some issue in your code or an issue that should be filed in the Swashbuckle repo.

For example, Swashbuckle appears to not support open generics, so having a response type attribute like [ResponseType(typeof(IEnumerable<>))] could cause this sort of behavior. It could also be an issue with ambiguous routes or something like that tripping Swashbuckle up. Once you've narrowed down the cause of failure to something more specific like that, it can either be fixed or filed, as appropriate.