x-nullable for Swashbuckle

ApplySchemaVendorExtensions.cs

    public class ApplySchemaVendorExtensions : ISchemaFilter
    {
        public void Apply(Schema schema, SchemaRegistry schemaRegistry, Type type)
        {
            if (type.IsArray && type.GetElementType() != typeof(byte))
            {
                // Special case
                return;
            }

            var nullableUnderlyingType = Nullable.GetUnderlyingType(type);

            if (nullableUnderlyingType != null && nullableUnderlyingType.IsValueType && !schema.vendorExtensions.ContainsKey("x-nullable"))
            {
                schema.vendorExtensions.Add("x-nullable", true);
            }

            if (schema.properties == null)
            {
                return;
            }

            foreach (var schemaProperty in schema.properties)
            {
                var property = type.GetProperty(schemaProperty.Key, BindingFlags.Public | BindingFlags.Instance);

                if (property == null)
                {
                    continue;
                }

                var propertyType = property.PropertyType;
                Apply(schemaProperty.Value, schemaRegistry, propertyType);
            }
        }
    }

PhotoDto.cs

namespace My
{
    public class PhotoDto
    {
        public string Id { get; set; }
        public string FileName { get; set; }
        public string Src { get; set; }
        public string ThumbnailSrc { get; set; }
        public int TagId { get; set; }
        public PhotoType PictureTypeId { get; set; }

        public int? TestId { get; set; }
    }
}

swagger.json

 "My": {
      "type": "object",
      "properties": {
        "CaptionId": {
          "format": "int32",
          "type": "integer"
        },
        "FileName": {
          "type": "string"
        },
        "Id": {
          "format": "int32",
          "type": "integer"
        },
        "PictureTypeId": {
          "enum": [
            "Property",
            "MainPhoto"
          ],
          "type": "string"
        },
        "Src": {
          "type": "string"
        },
        "ThumbnailSrc": {
          "type": "string"
        },
        "TestId": {
          "format": "int32",
          "type": "integer",
          "x-nullable": true
        }
      }
    }

SwaggerConfig.cs

public class SwaggerConfig
{
    public static void Register()
    {
        Register(GlobalConfiguration.Configuration);
    }

    public static void Register(SwaggerDocsConfig configuration)
    {	
        // ... other settings
        configuration.SchemaFilter();
        // ... other settings
    }
}