.NET 7中F#自定义JsonConverter失效问题求助

项目从.NET 5/6升级到.NET 7后，原本正常工作的F#自定义JsonConverter突然失效，返回DTO时抛出F#判别式联合序列化不支持的异常。

自定义JsonConverter代码 #

type RouteCheckConverter() =
    inherit JsonConverter<RouteCheckResponse>()

    override _.Read(reader, _, _): RouteCheckResponse =
        match reader.GetInt32() with 
        | 200 -> RouteOk
        | 301 -> RouteRedirect
        | 404 -> RouteNotFound
        | _ -> failwithf "Unknown route check response"

    override _.Write (writer, r, _): unit =
        match r with
        | RouteOk -> writer.WriteNumberValue 200
        | RouteRedirect -> writer.WriteNumberValue 301
        | RouteNotFound -> writer.WriteNumberValue 404

判别式联合定义 #

type RouteCheckResponse = | RouteOk | RouteRedirect | RouteNotFound

使用转换器的DTO类 #

type RouteCheckResultDTO = {
    page: ProductPageDTO option
    redirectTo: string option
    [<JsonConverter(typeof<RouteCheckConverter>)>] 
    response: RouteCheckResponse
}

抛出的异常信息 #

Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
An unhandled exception has occurred while executing the request.
System.NotSupportedException: F# discriminated union serialization is not supported. Consider authoring a custom converter for the type.
at System.Text.Json.Serialization.Converters.FSharpTypeConverterFactory.CreateConverter(Type typeToConvert, JsonSerializerOptions options)
at System.Text.Json.Serialization.JsonConverterFactory.GetConverterInternal(Type typeToConvert, JsonSerializerOptions options)
at System.Text.Json.Serialization.Metadata.DefaultJsonTypeInfoResolver.GetConverterForType(Type typeToConvert, JsonSerializerOptions options, Boolean resolveJsonConverterAttribute)
at System.Text.Json.Serialization.Metadata.DefaultJsonTypeInfoResolver.CreateJsonTypeInfo(Type type, JsonSerializerOptions options)
at System.Text.Json.Serialization.Metadata.DefaultJsonTypeInfoResolver.GetTypeInfo(Type type, JsonSerializerOptions options)
at System.Text.Json.JsonSerializerOptions.GetTypeInfoNoCaching(Type type)
at System.Collections.Concurrent.ConcurrentDictionary2.GetOrAdd(TKey key, Func2 valueFactory)
at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Boolean resolveIfMutable)
at System.Text.Json.Serialization.Metadata.JsonPropertyInfo.Configure()
at System.Text.Json.Serialization.Metadata.JsonTypeInfo.InitializePropertyCache()
at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
at System.Text.Json.Serialization.Metadata.JsonTypeInfo.g__ConfigureLocked|143_0()
--- End of stack trace from previous location ---

原因与解决方案 #

这不是.NET 7的Bug，而是.NET 7中System.Text.Json新增了FSharpTypeConverterFactory，它会优先处理F#判别式联合类型，导致DTO属性上的JsonConverter注解被忽略。

有两种可行的解决方法：

• 全局注册自定义转换器：在配置JsonSerializerOptions时，将自定义转换器添加到Converters集合，覆盖默认的F#转换器工厂处理逻辑：

  builder.Services.AddControllers()
      .AddJsonOptions(options =>
      {
          options.JsonSerializerOptions.Converters.Add(RouteCheckConverter())
      })
  

• 为判别式联合类型直接添加注解：将JsonConverter注解移至判别式联合定义上，而非DTO的属性上：

  [<JsonConverter(typeof<RouteCheckConverter>)>]
  type RouteCheckResponse = | RouteOk | RouteRedirect | RouteNotFound
  

两种方法均可让自定义转换器正常生效，推荐第二种，更贴合类型本身的序列化规则。

内容的提问来源于stack exchange，提问作者eastwing