Swagger not loading - Failed to load API definition: Fetch error undefined

Simply navigate to https://localhost:{PortNo}/swagger/v1/swagger.json and get much more details about the error message.

I was able to find the error by opening the network tab and looking at the response for swagger.json

I've been working with .Net Core 3.1 and I spent some time to find out and understanding what was going on.

The issue can arise from many different reasons:

1. Swagger configuration errors

2. Classes with the same name but in different namespaces

3. Public methods without the rest attribute (Get, Post, etc.)

First, take a look the link below just to check if your setup is ok:

Add Swagger(OpenAPI) API Documentation in ASP.NET Core 3.1

Then,

A good tip to find out the problem is to run the application without to use IISExpress and check the console log. Any error found to generate the documentation will be displayed there.

In my case, the problems was that I had a public method (that should be private) without any rest attribute:

After change the method from public to private I solve the issue.

So after a lot of troubleshooting it came down to basically two things, but I feel that in general this could be helpful to someone else in the future so I'm posting an answer.

First- if ever your stuck with the aforementioned error the best way to actually see whats going on is by adding the following line to your Configure() method

app.UseDeveloperExceptionPage();

Now if you navigate to the 'swagger/v1/swagger.json' page you should see some more information which will point you in useful direction.

Second- now for me the error was something along the lines of

'Multiple operations with path 'some_path' and method 'GET' '

However these API were located inside of dependency libraries so I was unable to apply a solution at the point of definition. As a workaround I found that adding the following line to your ConfigureServices() method resolved the issue

services.AddSwaggerGen(c =>
{
     c.SwaggerDoc("v1", new Info { Title = "API WSVAP (WebSmartView)", Version = "v1" });
     c.ResolveConflictingActions(apiDescriptions => apiDescriptions.First()); //This line
});

Finally- After all that I was able to generate a JSON file but still I wasn't able to pull up the UI. In order to get this working I had to alter the end point in Configure()

app.UseSwaggerUI(c =>
{
    c.SwaggerEndpoint("./v1/swagger.json", "My API V1"); //originally "./swagger/v1/swagger.json"
});

I'm not sure why this was necessary, although it may be worth noting the web application's virtual directory is hosted on IIS which might be having an effect.

NOTE: Navigating to swagger/v1/swagger.json will give you more details, for me it was causing issue due to undecorated action. This information is mentioned in comment by @MarkD

Hope this helps someone in the future.