How to add a model to the swagger file?

[Ghostbird]

I have a middleware that catches and packages exceptions that occur on our API in a common ApiException class. That class is mentioned nowhere on any controllers. I want it included in the swagger.json. I read this stackoverflow but the code examples given there are for older versions of Swashbuckle and neither works with Swashbuckle 6+.

This is the code I tried:

  sealed class GenerateSchemaFor<T> : IDocumentFilter where T : class
  {
    public void Apply(OpenApiDocument swaggerDoc, DocumentFilterContext context)
    {
      if (!context.SchemaRepository.Schemas.ContainsKey(typeof(T).Name))
      {
        context.SchemaGenerator.GenerateSchema(typeof(T), context.SchemaRepository);
      }
      if (!context.SchemaRepository.Schemas.ContainsKey(typeof(T).Name))
      {
        throw new SwaggerGeneratorException($"Could not register schema for {typeof(T).Name}!");
      }
    }
  }

I call it in the swagger configuration:

  .AddSwaggerGen(options => options.AddModel<ApiException>())`

However this always throws: Could not register schema for ApiException!

It's quite odd. I've read through the code of SchemaGenerator.cs and it seems it ought to work. Yet it doesn't.

[martincostello]

We'd need to see more information (an inner exception and a stack trace) to determine whether Could not register schema for ApiException! is a bug or not.

However you should be able to use filters to add an additional error response to all of your operations to handle the behaviour your middleware adds.

The code for the filter you've provided looks wrong as you create a schema, but then you don't actually add it to the document.

[Ghostbird]

As you can see from my provided code Could not register schema for ApiException! is my own error I throw when I have concluded that the schema was not generated and/or added correctly.

Why do you think that I create the schema but don't add it?

I went by the examples from Stack Overflow and this test. When I call GenerateSchema the schema is auto-added to the passed SchemaRepository in this line. None of the examples I've seen actually add it to the document in addition to adding it to the schema repository.

The full call stack that adds the schema to the repository would be:

• GenerateSchema
• GenerateSchemaForType
• GenerateConcreteSchema because dataContract.DataType is DataType.Object which sets returnAsReference it then calls
• GenerateReferencedSchema which calls
• schemaRepository.AddDefinition(schemaId, schema)

[martincostello]

Why do you think that I create the schema but don't add it?

I assumed that because you weren't using the result of the GenerateSchema() call it wasn't added to the OpenApiDocument.

[Ghostbird]

Well, I though it was a bit strange too, but the examples that I saw never add the result to the OpenApiDocument. Apparently that's not necessary? It is added to the context.SchemaRepository.

[Ghostbird]

It must've been something related to the specific type I was trying to generate. I called the code above on another type, and it worked out of the box. I have no idea why.