Swashbuckle.aspnetcore: Schema cannot be generated

[Zenuka]

After upgrading to 5.3.0 or 5.3.1 we're getting the following exception:

System.NotSupportedException: Schema cannot be generated for type System.Collections.Generic.Dictionary`2[TestWebApplication.Controllers.Enum1,System.Collections.Generic.List`1[TestWebApplication.Controllers.Class2]] as it's not supported by the System.Text.Json serializer
   at Swashbuckle.AspNetCore.SwaggerGen.JsonSerializerDataContractResolver.GetDataContractForType(Type type)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchemaForType(Type type, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GeneratePropertySchema(DataProperty serializerMember, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateObjectSchema(DataContract dataContract, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateInlineSchema(DataContract dataContract, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.<>c__DisplayClass7_0.<GenerateReferencedSchema>b__0()
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaRepository.GetOrAdd(Type type, String schemaId, Func`1 factoryMethod)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateReferencedSchema(DataContract dataContract, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchemaForType(Type type, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchema(Type type, SchemaRepository schemaRepository, MemberInfo memberInfo, ParameterInfo parameterInfo)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateArraySchema(DataContract dataContract, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateInlineSchema(DataContract dataContract, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchemaForType(Type type, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchema(Type type, SchemaRepository schemaRepository, MemberInfo memberInfo, ParameterInfo parameterInfo)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.CreateResponseMediaType(ModelMetadata modelMetadata, SchemaRepository schemaRespository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.<>c__DisplayClass18_0.<GenerateResponse>b__2(String contentType)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateResponse(ApiDescription apiDescription, SchemaRepository schemaRepository, String statusCode, ApiResponseType apiResponseType)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateResponses(ApiDescription apiDescription, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperation(ApiDescription apiDescription, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperations(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GeneratePaths(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwagger(String documentName, String host, String basePath)
   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)`

I tried to find a reason in the release notes but I didn't see any changes to the serialization or I missed something? Has this become unsupported or is this a bug? When I downgrade to 5.2.1 everything works as intended.

I've created a simple solution to reproduce the problem: TestWebApplication.zip
But the general idea is this:

public enum Enum1
{
    Value1,
    Value2
}

public class Class1
{
    public Dictionary<Enum1, List<Class2>> Params { get; set; } = new Dictionary<Enum1, List<Class2>>();
}

public class Class2
{
    public string Property { get; set; } = "Test";
}

[ApiController]
[Route("[controller]")]
public class TestController : ControllerBase
{
    [HttpGet]
    public IEnumerable<Class1> Get()
    {
        return new []{ new Class1() };
    }
}

[adrian-lopez-softtek]

System.Text.Json serializer doesn't support non string dictionary keys. You need to add Swashbuckle.AspNetCore.Newtonsoft nuget package and call AddSwaggerGenNewtonsoftSupport() on your IServiceCollection to register it as default serializer.

[adrian-lopez-softtek]

Just a question for mantainers... shouldn't changing the default serializer be a breaking change? Considering all the limitations that System.Text.Json currently has, I would say yes, but not an expert on semantic versioning 😄

[domaindrivendev]

@adrian-lopez-softtek honoring STJ by default _is_ a breaking change _but_ that change was introduced in v5.0.0 - a major version increment. It's well documented in the 5.0.0 release notes.

Throwing the exception on a type that's unsupported by that serializer was introduced in 5.3.1 because I consider that to be a "bug fix".

[adrian-lopez-softtek]

Yeah, makes sense! My bad for not reading the release notes of version first 5 release. Thought it was changed now because didn't have the errors before. Thanks for the response!

[Zenuka]

Thanks! All makes sense now!

[radwan92]

Just because Dictionary serialization is not supported out-of-the-box doesn't mean we can't add it ourselves.

This won't work ATM because JsonSerializerDataContractResolver has a hardcoded exception for dictionaries with enums as keys, regardless of the converters provided by the JsonSerializerOptions. This should not be the case.