System.Text.Json 和多态代码：不适用于 WebApi 控制器

随着 .NET 7.0 的发布，System.Text.Json应该支持多态代码。不幸的是，当您需要从控制器的方法返回派生类型的实例时，它似乎无法开箱即用。例如，假设以下模型：

public class Base {}

public class Derived1: Base  { }

public class Derived2: Base  { }

还假设我们有以下动态类型信息解析器：

public class JsonHierarchyTypeInfoResolver : DefaultJsonTypeInfoResolver {    
    public override JsonTypeInfo GetTypeInfo(Type type, JsonSerializerOptions options) {
        var jsonTypeInfo = base.GetTypeInfo(type, options);

        if( typeof(Base) == jsonTypeInfo.Type ) {
            jsonTypeInfo.PolymorphismOptions = new( ) {
                                                          TypeDiscriminatorPropertyName = "$type", 
                                                          IgnoreUnrecognizedTypeDiscriminators = true,
                                                          UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FailSerialization,
                                                          DerivedTypes = {
                                                                             new JsonDerivedType(typeof(Derived1), typeof(Derived1).AssemblyQualifiedName!),
                                                                             new JsonDerivedType(typeof(Derived2), typeof(Derived2).AssemblyQualifiedName!)
                                                                         }
                                                      };
            
        }
        return jsonTypeInfo;
    }
}

然后，通过执行以下操作在应用程序中使用它：

builder.Services.AddControllers( )
       .AddJsonOptions(options => {
                           options.JsonSerializerOptions.PropertyNameCaseInsensitive = true;
                           options.JsonSerializerOptions.TypeInfoResolver = new JsonHierarchyTypeInfoResolver( );
                       });

我们还假设我们有一个简单的控制器，它有一个简单的方法，如下所示：

[ApiController]
[Route("[controller]")]
public class DEMOController : ControllerBase {
    [HttpGet]
    public ActionResult<Base> GetAsync() {
        var derived = new Derived1(  );
        return Ok(derived);
    }
}

每当调用该方法时，它都不会像我预期的那样生成带有类型鉴别器的 json。看来问题出在SystemTextJsonOutputFormatter当它尝试使用以下代码序列化对象时：

await JsonSerializer.SerializeAsync(responseStream, context.Object, objectType, SerializerOptions, httpContext.RequestAborted);

方法如下objectType已初始化：

// context.ObjectType reflects the declared model type when specified.
// For polymorphic scenarios where the user declares a return type, but returns a derived type,
// we want to serialize all the properties on the derived type. This keeps parity with
// the behavior you get when the user does not declare the return type and with Json.Net at least at the top level.
var objectType = context.Object?.GetType() ?? context.ObjectType ?? typeof(object);

由于该方法使用派生类型，因此自定义信息类型解析器将无法发挥其魔力。我错过了什么吗？这是一个已知的问题？这是否意味着我应该继续使用 json.net 而不是尝试迁移到 System.Text.Json？

ASP.NET Core 解析的类型GetAsync将会Derived1它没有任何多态信息，因此它不会添加任何多态序列化数据。一种解决方案是为层次结构中的每种类型添加多态性信息：

public class JsonHierarchyTypeInfoResolver : DefaultJsonTypeInfoResolver
{
    public override JsonTypeInfo GetTypeInfo(Type type, JsonSerializerOptions options)
    {
        var jsonTypeInfo = base.GetTypeInfo(type, options);
        // if type inherits or is Base
        if (jsonTypeInfo.Type.IsAssignableTo(typeof(Base)))
        {
            jsonTypeInfo.PolymorphismOptions = new()
            {
                TypeDiscriminatorPropertyName = "$type",
                IgnoreUnrecognizedTypeDiscriminators = true,
                UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FailSerialization,
            };

            var derivedTypes = new[]
                {
                    typeof(WeatherForecastController.Derived1),
                    typeof(WeatherForecastController.Derived2)
                }
                .Where(t => jsonTypeInfo.Type.IsAssignableTo(t)) // add only appropriate types
                .Select(t => new JsonDerivedType(t, t.AssemblyQualifiedName!));
            foreach (var derivedType in derivedTypes)
            {
                jsonTypeInfo.PolymorphismOptions.DerivedTypes.Add(derivedType);
            }
        }

        return jsonTypeInfo;
    }
}