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Swashbuckle and Swagger with ASP.NET Core

December 25, 2017 mgroves 0 Comments
Tags: swagger swashbuckle asp.net asp.net core .net .net core csharp c# csadvent

Swashbuckle is a handy library to easily bring Swagger support to your ASP.NET Core (or ASP.NET) application. It is especially handy when developing an HTTP based API. It creates a form of interactive documentation based on the OpenAPI Specification.

Before diving into Swashbuckle: Merry Christmas! This blog is being posted on December 25th, 2017. It’s the final post of the very first C# Advent Calendar. Please check out the other 24 posts in the series! This event has gone so well, that I’m already planning on doing it again in 2018. Thank you, again, to everyone who participated (whether you are a writer or you’ve just been following along).

The full source code used in this example is available on Github.

ASP.NET Core HTTP API

I’m going to assume some level of familiarity with ASP.NET Core and creating a REST API. Here’s an example of a GET and a POST. These endpoints are reading/writing from a JSON text file (in a way that is probably not thread-safe and definitely not efficient, but it’s fine for this example).

public class ValuesController : Controller
{
    [HttpGet]
    [Route("api/teams")]
    public IActionResult GetTeams()
    {
        var jsonFile = System.IO.File.ReadAllText("jsonFile.json");
        var teams = JsonConvert.DeserializeObject<List<Team>>(jsonFile);
        return Ok(teams);
    }

    [HttpPost]
    [Route("api/team")]
    public IActionResult PostTeam([FromBody]Team team)
    {
        var jsonFile = System.IO.File.ReadAllText("jsonFile.json");
        var teams = JsonConvert.DeserializeObject<List<Team>>(jsonFile);
        teams.Add(team);
        System.IO.File.WriteAllText("jsonFile.json",JsonConvert.SerializeObject(teams));
        return Ok(team);
    }

    // etc...

To try out the GET endpoint, the simplest thing I can do is open a browser and view the results. But to try out the POST endpoint, I need something else. I could install Postman or Fiddler (and you should). Here’s how that would look.

Postman

Postman is great for interacting with endpoints, but Postman alone doesn’t really tell us anything about the endpoint or the system as a whole. This is where Swagger comes in.

Swagger

Swagger is a standard way to provide specifications for endpoints. Usually, that specification is automatically generated and then used to generate an interactive UI.

We could write the Swagger spec out by hand, but fortunately ASP.NET Core provides enough information to generate a spec for us. Look at the PostTeam action above. Just from reading that we know:

  • It expects a POST

  • The URL for it is /api/team

  • There’s a Team class that we can look at to see what kind of body is expected

From that, we could construct a Swagger spec like the following (I used JSON, you can also use YAML).

{
	"swagger": "2.0",
	"info": { "version": "v1", "title": "Sports API" },
	"basePath": "/",
	"paths": {
		"/api/team": {
			"post": {
				"consumes": ["application/json"],
				"parameters": [{
					"name": "team",
					"in": "body",
					"required": false,
					"schema": { "$ref": "#/definitions/Team" }
				}]
			}
		}
	},
	"definitions": {
		"Team": {
			"type": "object",
			"properties": {
				"name": { "type": "string" },
				"stadiumName": { "type": "string" },
				"sport": { "type": "string" }
			}
		}
	}
}

But why on earth would you want to type that out? Let’s bring in a .NET library to do the job. Install Swashbuckle.AspNetCore with NuGet (there’s a different package if you want to do this with ASP.NET).

You’ll need to add a few things to Startup.cs:

In the ConfigureServices method:

services.AddSwaggerGen(c =>
{
    c.SwaggerDoc("v1", new Info { Title = "Sports API", Version = "v1"});
});

In the Configure method:

app.UseSwagger();

Aside: With ASP.NET, NuGet actually does all this setup work for you.

Once you’ve done this, you can open a URL like http://localhost:9119/swagger/v1/swagger.json and see the generated JSON spec.

Swagger spec in JSON

Swagger UI with Swashbuckle

That spec is nice, but it would be even nicer if we could use the spec to generate a UI.

Back in the Configure method, add this:

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

Now, open your site and go to /swagger:

Swagger UI generated by Swashbuckle

Some cool things to notice:

  • Expand/collapse by clicking the URL of an endpoint (note that you must use Route attributes for Swashbuckle to work with ASP.NET Core).

  • "Try it out!" buttons. You can execute GET/POST right from the browser

  • The "parameter" of the POST method. Not only can you paste in some content, but you get an example value that acts like a template (just click it).

Giving some swagger to your Swagger

Swagger and Swashbuckle have done a lot with just a little bit. It can do even more if we add a little more information in the code.

  • Response: The ProducesResponseType attribute will let Swagger know what the response will look like (this is especially useful if you are using IActionResult and/or an endpoint could return different types in different situations).

  • Comments: If you are using XML comments, you can have these included with the Swagger output.

services.AddSwaggerGen(c =>
{
    c.SwaggerDoc("v1", new Info { Title = "Sports API", Version = "v1" });
    var filePath = Path.Combine(PlatformServices.Default.Application.ApplicationBasePath, "swashbuckle-example.xml");
    c.IncludeXmlComments(filePath);
});

(Also make sure you XML Documentation output for your project enabled)

Here’s an example of a GetTeams method with both XML comments and ProducesResponseType:

/// <summary>
/// Gets all the teams stored in the file
/// </summary>
/// <remarks>Baseball is the best sport</remarks>
/// <response code="200">List returned succesfully</response>
/// <response code="500">Something went wrong</response>
[HttpGet]
[Route("api/teams2")]
[ProducesResponseType(typeof(Team), 200)]
public IActionResult GetTeams2()
{
    var jsonFile = System.IO.File.ReadAllText("jsonFile.json");
    var teams = JsonConvert.DeserializeObject<List<Team>>(jsonFile);
    return Ok(teams);
}
  • Customize your info: there’s more to the Info class than just Title and Version. You can specify a license, contact, etc.

services.AddSwaggerGen(c =>
{
    c.SwaggerDoc("v1", new Info
    {
        Title = "Sports API",
        Version = "v1",
        Description = "An API to list and add sports teams",
        TermsOfService = "This is just an example, not for production!",
        Contact = new Contact
        {
            Name = "Matthew Groves",
            Url = "https://crosscuttingconcerns.com"
        },
        License = new License
        {
            Name = "Apache 2.0",
            Url = "http://www.apache.org/licenses/LICENSE-2.0.html"
        }
    });
    var filePath = Path.Combine(PlatformServices.Default.Application.ApplicationBasePath, "swashbuckle-example.xml");
    c.IncludeXmlComments(filePath);
});

Here’s a screenshot of the UI that has all three of the above enhancements: response type, XML comments, and more customized info.

Swagger with more swagger

Summary

Working on HTTP-based APIs? Bring Swashbuckle and Swagger into your life!

More resources:

Thanks again for reading the 2017 C# Advent!

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Matthew D. Groves

About the Author

Matthew D. Groves lives in Central Ohio. He works remotely, loves to code, and is a Microsoft MVP.

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