System.Text.Json序列化：如何查看转换类型及触发未识别类型异常？

1. 确定当前正在尝试转换的类型 #

当使用PolymorphismOptions配置多态映射时，默认JsonTypeInfoResolver返回基类型的JsonTypeInfo是正常行为，但你可以通过两种方式获取实际要转换的派生类型：

• 利用PolymorphismOptions的鉴别器回调：配置多态选项时，通过OnDiscriminatorResolved回调可直接拿到解析出的目标派生类型。示例：

  var options = new JsonSerializerOptions
  {
      TypeInfoResolver = new DefaultJsonTypeInfoResolver
      {
          Modifiers =
          {
              typeInfo =>
              {
                  if (typeInfo.Type == typeof(BaseType))
                  {
                      typeInfo.PolymorphismOptions = new JsonPolymorphismOptions
                      {
                          TypeDiscriminatorPropertyName = "$type",
                          IgnoreUnrecognizedTypeDiscriminators = false,
                          OnDiscriminatorResolved = (discriminatorValue, resolvedType) =>
                          {
                              // resolvedType即为当前尝试转换的实际派生类型
                              Console.WriteLine($"当前转换类型：{resolvedType.FullName}");
                          },
                          DerivedTypes =
                          {
                              new JsonDerivedType(typeof(DerivedType1), "Derived1"),
                              new JsonDerivedType(typeof(DerivedType2), "Derived2")
                          }
                      };
                  }
              }
          }
      }
  };
  

• 自定义JsonConverter捕获实际类型：如果需要更精细的控制，可编写自定义转换器，读取鉴别器值后匹配对应的派生类型：

  public class BaseTypeConverter : JsonConverter<BaseType>
  {
      public override BaseType Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
      {
          using (var doc = JsonDocument.ParseValue(ref reader))
          {
              var root = doc.RootElement;
              if (root.TryGetProperty("$type", out var discriminatorElem))
              {
                  var discriminator = discriminatorElem.GetString();
                  Type targetType = discriminator switch
                  {
                      "Derived1" => typeof(DerivedType1),
                      "Derived2" => typeof(DerivedType2),
                      _ => throw new JsonException($"未知鉴别器：{discriminator}")
                  };
                  // targetType就是当前尝试转换的实际类型
                  return (BaseType)JsonSerializer.Deserialize(root.GetRawText(), targetType, options);
              }
              throw new JsonException("缺少类型鉴别器");
          }
      }
  
      public override void Write(Utf8JsonWriter writer, BaseType value, JsonSerializerOptions options)
      {
          JsonSerializer.Serialize(writer, value, value.GetType(), options);
      }
  }
  

2. 发现问题时抛出异常 #

System.Text.Json提供多种方式在转换出错时抛出异常：

• 配置PolymorphismOptions处理鉴别器不匹配：设置IgnoreUnrecognizedTypeDiscriminators = false，并通过OnDiscriminatorMismatch回调主动抛出异常：

  typeInfo.PolymorphismOptions = new JsonPolymorphismOptions
  {
      TypeDiscriminatorPropertyName = "$type",
      IgnoreUnrecognizedTypeDiscriminators = false,
      OnDiscriminatorMismatch = (discriminatorValue, baseType) =>
      {
          throw new JsonException($"无法将鉴别器'{discriminatorValue}'映射到{baseType.FullName}的任何派生类型");
      },
      DerivedTypes = { /* 派生类型配置 */ }
  };
  

• 设置JsonSerializerOptions全局异常规则：

  • UnknownTypeHandling = JsonUnknownTypeHandling.ThrowException：遇到无法识别的类型时抛出异常；
  • UnknownPropertyHandling = JsonUnknownPropertyHandling.Throw：JSON中存在目标类型无对应属性时抛出异常；
  • AllowTrailingCommas = false：禁止JSON末尾逗号，否则抛出异常。

  示例：

  var options = new JsonSerializerOptions
  {
      UnknownTypeHandling = JsonUnknownTypeHandling.ThrowException,
      UnknownPropertyHandling = JsonUnknownPropertyHandling.Throw,
      AllowTrailingCommas = false
  };
  

• 自定义转换器中主动抛出异常：在自定义转换器的Read方法中，检测到数据不符合预期（如缺少必要属性、类型不匹配）时，直接抛出JsonException，如前面的BaseTypeConverter示例所示。

内容的提问来源于stack exchange，提问作者Locke