ASP.NET Core 6 Web API混合返回JSON的问题及修复方案咨询

问题描述 #

开发ASP.NET Core 6 Web API控制器时，尝试在接口中同时返回JSON字符串与普通C#对象，遇到两个问题：

1. JObject类型字段值丢失；
2. 字符串类型的JSON数据被自动编码，无法直接作为JSON结构使用。

复现代码 #

[HttpGet("GetMixedJson")]
public IActionResult GetMixedJson()
{
    string[] Summaries = new[]
    {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

    string json = JsonConvert.SerializeObject(Summaries, Formatting.Indented);

    JObject jsonObject = new JObject();
    jsonObject["thisFiledValueWillBeMissing"] = "JObject won't serialize :(";

    var resultObject = new
    {
        PlainCSharpFiled = "plain c# fields showing fine",
        jsonObject = jsonObject,
        TunneledJsonString = json
    };

    return new JsonResult(resultObject);
}

错误返回结果 #

{
  "plainCSharpFiled": "plain c# fields showing fine",
  "jsonObject": {
    "thisFiledValueWillBeMissing": []
  },
  "tunneledJsonString": "[\n  \"Freezing\",\n  \"Bracing\",\n  \"Chilly\",\n  \"Cool\",\n  \"Mild\",\n  \"Warm\",\n  \"Balmy\",\n  \"Hot\",\n  \"Sweltering\",\n  \"Scorching\"\n]"
}

问题原因 #

• JObject是Newtonsoft.Json的类型，而ASP.NET Core默认使用System.Text.Json序列化，两者兼容性不足，导致JObject字段被错误解析为空数组；
• 直接返回预序列化的JSON字符串时，System.Text.Json会将其视为普通字符串进行转义编码，无法保留原生JSON结构。

修复方案 #

方案一：统一使用Newtonsoft.Json序列化（推荐，低内存开销） #

替换ASP.NET Core默认序列化器为Newtonsoft.Json，完美兼容JObject，同时避免重复序列化操作，降低内存占用。

1. 安装NuGet包：

Install-Package Microsoft.AspNetCore.Mvc.NewtonsoftJson

2. 在Program.cs中配置服务：

builder.Services.AddControllers().AddNewtonsoftJson();

3. 修改控制器方法：

[HttpGet("GetMixedJson")]
public IActionResult GetMixedJson()
{
    string[] Summaries = new[]
    {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

    // 直接转换为JArray，避免先序列化字符串再解析，节省内存
    JArray jsonArray = JArray.FromObject(Summaries);

    JObject jsonObject = new JObject();
    jsonObject["thisFiledValueWillBeMissing"] = "JObject will serialize correctly :)";

    var resultObject = new
    {
        PlainCSharpFiled = "plain c# fields showing fine",
        JsonObject = jsonObject,
        TunneledJson = jsonArray
    };

    return new JsonResult(resultObject);
}

如果必须使用已有的JSON字符串，可解析为JToken再加入结果：

string json = JsonConvert.SerializeObject(Summaries, Formatting.Indented);
JToken jsonToken = JToken.Parse(json);

方案二：使用System.Text.Json的JsonElement替代JObject（无需切换序列化库） #

保留默认System.Text.Json序列化器，用JsonElement处理动态JSON结构：

[HttpGet("GetMixedJson")]
public IActionResult GetMixedJson()
{
    string[] Summaries = new[]
    {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

    // 将数组序列化为JsonElement，避免字符串编码
    using var doc = JsonDocument.Parse(JsonSerializer.Serialize(Summaries));
    JsonElement jsonElement = doc.RootElement.Clone();

    // 构建动态JSON对象
    var jsonObject = new Dictionary<string, object>
    {
        ["thisFiledValueWillBeMissing"] = "JsonElement works correctly"
    };

    var resultObject = new
    {
        PlainCSharpFiled = "plain c# fields showing fine",
        JsonObject = jsonObject,
        TunneledJson = jsonElement
    };

    return new JsonResult(resultObject);
}

方案三：直接写入响应流（最低内存开销，适合大数据量） #

跳过中间对象构建，直接将JSON内容写入响应流，避免内存中生成完整JSON对象：

[HttpGet("GetMixedJson")]
public async Task GetMixedJson()
{
    string[] Summaries = new[]
    {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

    Response.ContentType = "application/json";
    await using var writer = new Utf8JsonWriter(Response.Body);

    writer.WriteStartObject();

    // 写入普通C#字段
    writer.WriteString("PlainCSharpFiled", "plain c# fields showing fine");

    // 写入动态JSON对象
    writer.WriteStartObject("JsonObject");
    writer.WriteString("thisFiledValueWillBeMissing", "Direct writing works");
    writer.WriteEndObject();

    // 直接写入数组，避免预序列化
    writer.WriteStartArray("TunneledJson");
    foreach (var summary in Summaries)
    {
        writer.WriteStringValue(summary);
    }
    writer.WriteEndArray();

    writer.WriteEndObject();
    await writer.FlushAsync();
}

内容的提问来源于stack exchange，提问作者Allan Xu