Expose string as enum with NSwag

Question

I have this controller

public class Person
{
   public int Id { get; set; }
   public string Position { get; set; }
}

[ApiController]
[Route("[controller]")]
public class MyCtrl : ControllerBase
{
   [HttpGet("{id}")]
   public ActionResult<Person> Get(int id)
   {
      // load person with ID == id from db
      return new ActionResult<Person>(person);
   }
}

Now, the Position is just a string, but based on configuration in the database, it can only be a set of finite values. I would really like if I could mimic that Position was an enum in the Swagger interface (I use NSwag).

I would like if the API looked like it had been generated from this (where PositionType is generated from the configuration in the DB):

public enum PositionType
{
   Student,
   Teacher
}

public class Person
{
   public int Id { get; set; }
   public PositionType Position { get; set; }
}

Answer 1

I ended up doing this.

[AttributeUsage(AttributeTargets.Property)]
public class PositionEnum : Attribute
{
}

public class Person
{
   public int Id { get; set; }
   [PositionEnum]
   public string Position { get; set; }
}

public class PositionEnumProcessor : ISchemaProcessor
{
    public void Process(SchemaProcessorContext context)
    {
        var propertyInfos =
            context.ContextualType.Properties
                .Where(x => Attribute.IsDefined(x.PropertyInfo, typeof(PositionEnum)))
                .ToList();
        if (!propertyInfos.Any())
        {
            return;
        }
        var jsonSchema = context.Generator.Generate(typeof(PositionType), context.Resolver);
        var containerSchema = JsonSchema.FromType<object>();
        containerSchema.Type = JsonObjectType.None;
        containerSchema.Reference = jsonSchema;
        foreach (var propertyInfo in propertyInfos)
        {
            var camelCasePropertyName = System.Text.Json.JsonNamingPolicy.CamelCase.ConvertName(propertyInfo.Name);
            if (context.Schema.Properties.TryGetValue(camelCasePropertyName, out var schemaProperty))
            {
                schemaProperty.Type = JsonObjectType.None;
                schemaProperty.OneOf.Add(containerSchema);
            }

        }
    }
}

Then in builder.Services.AddOpenApiDocument() I have added this:

...
openApiConfiguration.SchemaSettings.SchemaProcessors.Add(new PositionEnumProcessor());
...

If anyone has a more elegant solution, please let me know 😊