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

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.

Today I found out that I could just go to the json url in the browser and get some error information for example

myapiurl/api/vi/swagger.json

I was able to solve this error by explicitly adding the http verb attribute to my asp.net core 2.x controller method. The convention of prefixing the method name with the http verb is not enough for Swashbuckle apparently.

[HttpPost] public async Task<IActionResult> AddNewData([FromBody] MyType myType) { … }