How do I include subclasses in Swagger API documentation/ OpenAPI specification using Swashbuckle?

It seems Swashbuckle doesn’t implement polymorphism correctly and I understand the point of view of the author about subclasses as parameters (if an action expects an Animal class and behaves differently if you call it with a dog object or a cat object, then you should have 2 different actions…) but as return types I believe that it is correct to return Animal and the objects could be Dog or Cat types.

So to describe my API and produce a proper JSON schema in line with correct guidelines (be aware of the way I describe the disciminator, if you have your own discriminator you may need to change that part in particular), I use document and schema filters as follows:

SwaggerDocsConfig configuration;
.....
configuration.DocumentFilter<PolymorphismDocumentFilter<YourBaseClass>>();
configuration.SchemaFilter<PolymorphismSchemaFilter<YourBaseClass>>();
.....

public class PolymorphismSchemaFilter<T> : ISchemaFilter
{
    private readonly Lazy<HashSet<Type>> derivedTypes = new Lazy<HashSet<Type>>(Init);

    private static HashSet<Type> Init()
    {
        var abstractType = typeof(T);
        var dTypes = abstractType.Assembly
                                 .GetTypes()
                                 .Where(x => abstractType != x && abstractType.IsAssignableFrom(x));

        var result = new HashSet<Type>();

        foreach (var item in dTypes)
            result.Add(item);

        return result;
    }

    public void Apply(Schema schema, SchemaRegistry schemaRegistry, Type type)
    {
        if (!derivedTypes.Value.Contains(type)) return;

        var clonedSchema = new Schema
                                {
                                    properties = schema.properties,
                                    type = schema.type,
                                    required = schema.required
                                };

        //schemaRegistry.Definitions[typeof(T).Name]; does not work correctly in SwashBuckle
        var parentSchema = new Schema { @ref = "#/definitions/" + typeof(T).Name };   

        schema.allOf = new List<Schema> { parentSchema, clonedSchema };

        //reset properties for they are included in allOf, should be null but code does not handle it
        schema.properties = new Dictionary<string, Schema>();
    }
}

public class PolymorphismDocumentFilter<T> : IDocumentFilter
{
    public void Apply(SwaggerDocument swaggerDoc, SchemaRegistry schemaRegistry, System.Web.Http.Description.IApiExplorer apiExplorer)
    {
        RegisterSubClasses(schemaRegistry, typeof(T));
    }

    private static void RegisterSubClasses(SchemaRegistry schemaRegistry, Type abstractType)
    {
        const string discriminatorName = "discriminator";

        var parentSchema = schemaRegistry.Definitions[SchemaIdProvider.GetSchemaId(abstractType)];

        //set up a discriminator property (it must be required)
        parentSchema.discriminator = discriminatorName;
        parentSchema.required = new List<string> { discriminatorName };

        if (!parentSchema.properties.ContainsKey(discriminatorName))
            parentSchema.properties.Add(discriminatorName, new Schema { type = "string" });

        //register all subclasses
        var derivedTypes = abstractType.Assembly
                                       .GetTypes()
                                       .Where(x => abstractType != x && abstractType.IsAssignableFrom(x));

        foreach (var item in derivedTypes)
            schemaRegistry.GetOrRegister(item);
    }
}

What the previous code implements is specified here, in the section “Models with Polymorphism Support. It basically produces something like the following:

{
  "definitions": {
    "Pet": {
      "type": "object",
      "discriminator": "petType",
      "properties": {
        "name": {
          "type": "string"
        },
        "petType": {
          "type": "string"
        }
      },
      "required": [
        "name",
        "petType"
      ]
    },
    "Cat": {
      "description": "A representation of a cat",
      "allOf": [
        {
          "$ref": "#/definitions/Pet"
        },
        {
          "type": "object",
          "properties": {
            "huntingSkill": {
              "type": "string",
              "description": "The measured skill for hunting",
              "default": "lazy",
              "enum": [
                "clueless",
                "lazy",
                "adventurous",
                "aggressive"
              ]
            }
          },
          "required": [
            "huntingSkill"
          ]
        }
      ]
    },
    "Dog": {
      "description": "A representation of a dog",
      "allOf": [
        {
          "$ref": "#/definitions/Pet"
        },
        {
          "type": "object",
          "properties": {
            "packSize": {
              "type": "integer",
              "format": "int32",
              "description": "the size of the pack the dog is from",
              "default": 0,
              "minimum": 0
            }
          },
          "required": [
            "packSize"
          ]
        }
      ]
    }
  }
}