Using Swagger In .NET Core 3+

Are there new .NET Core API projects that don’t use Swagger? It’s one of the first things I install on a new project and even if I end up testing an API with something like Postman, the fact that I can quickly show a GUI to run any API endpoints for testers, other devs, hell even project managers is phenomenal.

Now I’ve actually written about using Swagger in .NET Core before, but with the release of .NET Core 3 came some real gotchas that caused some massive headaches. Suddenly my “let’s just spend 30 seconds adding Swagger” turned into a couple of hours trying to work out why things don’t work quite as seamless as before. I’ll write this as a quick walkthrough that points out the issues along the way. It should be enough to get you back up and running before too long!

Installing Swagger

The first thing is installing Swagger via Nuget. Now the annoying thing is that the actual package you want to install is not called Swagger at all. You actually want to install the Swashbuckle.AspNetCore package which then installs various swagger dependencies. Maybe this nuget browser screenshot will explain better than I can

The other frustrating thing (Although now fixed), is that upon release of .NET Core 3, the release version of Swashbuckle did not actually work with .NET Core 3 and you had to use a pre-release version. That’s hopefully fixed now, but just incase, ensure that you are using atleast version 5.0.0 of Swashbuckle. If you use a version before this you will get a really random error :

Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator': Failed to compare two elements in the array

Again, the fix is to ensure you are running version 5.0.0 or above of Swashbuckle.

Adding Swagger Services

The next step is to add the Swagger services to the ServiceCollection in .NET Core. In your ConfigureServices method of startup.cs, you want to add a line that looks like this :

services.AddSwaggerGen(swagger =>
    swagger.SwaggerDoc("v1", new OpenApiInfo { Title = "My API" });

Now pay attention to the fact that I’m creating an object called “OpenApiInfo”, this used to be called just “Swagger.Info”. So if you are copy and pasting from a previous project or an old tutorial you’re gonna run into something like :

The type or namespace name 'Info' does not exist in the namespace 'Swashbuckle.AspNetCore.Swagger'

This goes for almost all configuration with Swagger there has been a rename to prepend “OpenApi” to almost all configuration properties. If you are upgrading a project, 9 times out of 10, you will just have to tack on the “OpenApi” part.

It’s an annoying thing to do but for the most part you can guess your way through it and have everything working.

Adding Swagger Middleware

The final step is to add our Swagger middleware to actually serve up our nice HTML interface. There are two calls to add to our Configure method in our startup.cs. These are :

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

These should come *before* any call to UseMVC or similar that might try and serve the request before Swagger can get to it. So in practice, near the top.

The first call to UseSwagger is the one that adds the “JSON” serving capability to your app. The second call is what adds the actual HTML interface for Swagger. Nice and easy!

NewtonSoft.JSON Support

This one got me good and caused massive headaches. So by now we know that .NET Core 3+ will not use JSON.NET as the default JSON serializer. We can fix that with some easy code (post on that here!). But even once you change your API to use JSON.NET as the serializer, Swagger will still use the default System.Text.Json serializer anyway!

But of course, there is a nuget package for that! Run the following from your package manager console :

Install-Package Swashbuckle.AspNetCore.Newtonsoft

Then in your ConfigureServices method of your startup.cs, add the following :


Now Swagger will also abide by JSON.NET serialization rules and will align with your API.

Never Tested Your API?

After building your API, now’s the time to test it! It’s always difficult with development deadlines to get in enough testing, but it is absolutely critical to have a real testing plan tailored specifically to API building. Check out the guide below for an easy to use overview of testing an API for all things to consider before the next update of your API app.

2 thoughts on “Using Swagger In .NET Core 3+”

  1. > This one got me good and caused massive headaches

    Oh yes, I struggled with this for a couple of days! Swagger UI simply freezing when trying to serialize System.Type. Thanks mate, for giving me the solution to those headaches.

  2. Thanks a lot for posting this help !! Unbelievable they just rip things out and make a mess of the versions 🙁 headaches indeed , massive !! :p


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