gRPC JSON transcoding / Support custom JsonConverter

[colotiline]

Summary

Allow to use custom JsonConverter in gRPC JSON transcoding.

Motivation and goals

Motivation

• There is no decimal (for example) in protobuf well-known types;
• You implement your own (https://learn.microsoft.com/en-us/dotnet/architecture/grpc-for-wcf-developers/protobuf-data-types#decimals);
• It work flawless via gRPC;
• But in REST API you now have units and nanos instead of decimal;
• You fixed Swagger documentation via MapType;
• You write custom JsonConverter for REST API;
• You can't use it.

Because of the last point all REST API consumers must write wrappers for decimal or you must use string for decimal.

Goals

Add JsonConverter support for gRPC JSON transcoding.

Valuabiity

• 28k downloads of https://www.nuget.org/packages/Microsoft.AspNetCore.Grpc.JsonTranscoding;
• decimal implementation is a common issue which is solved via custom type Decimal Type? protocolbuffers/protobuf#4406;
• Guid also Provide support for UUID type (a.k.a. GUID) protocolbuffers/protobuf#2224;
• Potentially there are more situations when you need a gRPC-BCL type implicitly mapping;
• Developers use different JSON serialization on purpose https://thomaslevesque.com/2020/06/27/exposing-custom-type-as-json-string-in-asp-net-core-api/;
• grpc-gateway supports custom marshalers Response body as a single field grpc-ecosystem/grpc-gateway#217.

In scope

• As a developer I want to implement custom JsonConverter for gRPC JSON transcoding.
• As a developer I want that gRPC and JSON types could have a different representation for the same .NET type.

Out of scope

–

Risks / unknowns

Risks

–

Unknowns

@JamesNK explicitly wrote in gRPC JSON transcoding documentation:

Transcoding doesn't support advanced JSON customization. Apps requiring precise JSON structure control should consider using ASP.NET Core Web API.
https://learn.microsoft.com/en-us/aspnet/core/grpc/json-transcoding-binding?view=aspnetcore-7.0#customize-json

I haven't found decision clarification for this behaviour.

Examples

• When you AddJsonTranscoding there could be a property Converters where you can add your own JsonConverter like in JsonSerializerOptions;
• gRPC JSON transcoding also could look for JsonConverter attribute on a not well-known types.

[LukaszRacon]

In case someone is interested in a workaround: #50401 (comment)

[vpetryaev]

In case someone is interested in a workaround: #50401 (comment)

This workaround not working with custom converters. Exception appear, after registering custom converter using reflection:
System.InvalidOperationException: Invalid JsonTypeInfo operation for JsonTypeInfoKind 'None'.

[aspmaker]

In case someone is interested in a workaround: #50401 (comment)

This workaround not working with custom converters. Exception appear, after registering custom converter using reflection:
System.InvalidOperationException: Invalid JsonTypeInfo operation for JsonTypeInfoKind 'None'.

hello, did you find a solution for that? i can not cast Google.Protobuf.WellKnownTypes.

i get this error :

Exception was thrown by handler. InvalidCastException: Unable to cast object of type 'Google.Protobuf.WellKnownTypes.StringValue' to type 'System.String'.

thank you.

[vpetryaev]

i get this error :

Exception was thrown by handler. InvalidCastException: Unable to cast object of type 'Google.Protobuf.WellKnownTypes.StringValue' to type 'System.String'.

Are you sure, that you specify 'string?' (nullable)?

[aspmaker]

i get this error :

Exception was thrown by handler. InvalidCastException: Unable to cast object of type 'Google.Protobuf.WellKnownTypes.StringValue' to type 'System.String'.

Are you sure, that you specify 'string?' (nullable)?

i created new issue :
#55254