Generating Swagger example responses with Swashbuckle

Swashbuckle is a tool for generating Swagger, the API description language, from your ASP.NET Web Api solution.
Using Swashbuckle, which provides Swagger-UI, you can create pretty living documentation of your web api, like this:

Documenting the Response

In this post I am going to show you how to document the Response, and a new way to generate some response examples.

You can specify the type of response for Swashbuckle a number of ways. Consider a simple API endpoint which returns a list of Countries:

public class CountriesController : DefaultController
{
    [HttpGet]
    public async Task<HttpResponseMessage> Get()
    {
        var resource = new List<Country>
        {
            new Country {Code = "AR", Name = "Argentina"},
            new Country {Code = "BR", Name = "Brazil"},
            // etc etc omitted for brevity
        };

        return Request.CreateResponse(HttpStatusCode.OK, resource);
    }
}

One way of describing the response code and content for Swashbuckle is using a combination of XML comments, and the ResponseType attribute, like so:

/// <response code="200">Countries returned OK</response>
[HttpGet]
[ResponseType(typeof(IEnumerable<Country>))]
public async Task<HttpResponseMessage> Get()
{

However, this only allows for one type of response.

If your API method can return multiple types, i.e. in the case of an error, then you can use the new SwaggerResponse attribute:

[HttpGet]
[SwaggerResponse(HttpStatusCode.OK, Type=typeof(IEnumerable<Country>))]
[SwaggerResponse(HttpStatusCode.BadRequest, Type = typeof(IEnumerable<ErrorResource>))]
public async Task<HttpResponseMessage> Get(string lang)
{

The Swagger 2.0 spec allows for examples to be added to the Response. However, at time of writing Swashbuckle doesn’t support this. Fortunately Swashbuckle is extendible so here is a way of doing it.

Create a new SwaggerResponseExamplesAttribute.

public class SwaggerResponseExamplesAttribute : Attribute
{
    public SwaggerResponseExamplesAttribute(Type responseType, Type examplesType)
    {
        ResponseType = responseType;
        ExamplesType = examplesType;
    }

    public Type ResponseType { get; set; }
    public Type ExamplesType { get; set; }
}

Decorate your methods with it:

[SwaggerResponse(HttpStatusCode.OK, Type=typeof(IEnumerable<Country>))]
[SwaggerResponseExamples(typeof(IEnumerable<Country>), typeof(CountryExamples))]
[SwaggerResponse(HttpStatusCode.BadRequest, Type = typeof(IEnumerable<ErrorResource>))]
public async Task<HttpResponseMessage> Get(string lang)

Now you’ll need to add an Examples class, which will generate the example data

public interface IProvideExamples
{
    object GetExamples();
}

public class CountryExamples : IProvideExamples
{
    public object GetExamples()
    {
        return new List<Country>
        {
            new Country {Code = "AA", Name = "Test Country"},
            new Country {Code = "BB", Name = "And another"}
        };
    }
}

Now you can add an ExamplesOperationFilter which implements Swashbuckle’s IOperationFilter.

public class ExamplesOperationFilter : IOperationFilter
{
    public void Apply(Operation operation, SchemaRegistry schemaRegistry, ApiDescription apiDescription)
    {
        var responseAttributes = apiDescription.GetControllerAndActionAttributes<SwaggerResponseExamplesAttribute>();

        foreach (var attr in responseAttributes)
        {
            var schema = schemaRegistry.GetOrRegister(attr.ResponseType);

            var response = operation.responses.FirstOrDefault(x => x.Value.schema.type == schema.type &amp;&amp; x.Value.schema.@ref == schema.@ref).Value;

            if (response != null)
            {
                var provider = (IProvideExamples) Activator.CreateInstance(attr.ExamplesType);
                response.examples = FormatAsJson(provider);
            }
        }
    }

    private static object FormatAsJson(IProvideExamples provider)
    {
        var examples = new Dictionary<string, object>()
        {
            {
                "application/json", provider.GetExamples()
            }
        };

        return ConvertToCamelCase(examples);
    }

    private static object ConvertToCamelCase(Dictionary<string, object> examples)
    {
        var jsonString = JsonConvert.SerializeObject(examples, new JsonSerializerSettings() {ContractResolver = new CamelCasePropertyNamesContractResolver()});
        return JsonConvert.DeserializeObject(jsonString);
    }
}

And finally configure it when you configure Swashbuckle’s startup.

configuration
    .EnableSwagger(c =>
    {
        c.OperationFilter<ExamplesOperationFilter>();
    })
    .EnableSwaggerUi();

Now that we’ve done all that, we should see the examples output in our swagger.json file, which you can get to by starting your solution and navigating to /swagger/docs/v1.

And then, when you browse the swagger-ui at /swagger, instead of an autogenerated example like this:

You’ll see your desired example, like this: