如何在C#反序列化时将非扁平JSON映射到扁平类？

先拿你提到的嵌套JSON示例来说：

{
"id": "123",
"data": {
"username": "jason",
"email": "jason@example.com",
"age": 30
}
}

我们要把它直接转成这样的扁平User类：

public class User
{
    public string Id { get; set; }
    public string Username { get; set; }
    public string Email { get; set; }
    public int Age { get; set; }
}

一、用.NET内置的System.Text.Json实现 #

方法1：自定义JsonConverter #

写一个转换器手动映射嵌套属性，适合需要复用的场景：

public class UserConverter : JsonConverter<User>
{
    public override User Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        using var jsonDoc = JsonDocument.ParseValue(ref reader);
        var root = jsonDoc.RootElement;
        
        return new User
        {
            Id = root.GetProperty("id").GetString(),
            Username = root.GetProperty("data").GetProperty("username").GetString(),
            Email = root.GetProperty("data").GetProperty("email").GetString(),
            Age = root.GetProperty("data").GetProperty("age").GetInt32()
        };
    }

    // 如果需要把User序列化回原JSON结构，实现下面的Write方法即可
    public override void Write(Utf8JsonWriter writer, User value, JsonSerializerOptions options)
    {
        writer.WriteStartObject();
        writer.WriteString("id", value.Id);
        writer.WriteStartObject("data");
        writer.WriteString("username", value.Username);
        writer.WriteString("email", value.Email);
        writer.WriteNumber("age", value.Age);
        writer.WriteEndObject();
        writer.WriteEndObject();
    }
}

使用转换器反序列化：

var json = "{\"id\":\"123\",\"data\":{\"username\":\"jason\",\"email\":\"jason@example.com\",\"age\":30}}";
var options = new JsonSerializerOptions();
options.Converters.Add(new UserConverter());
var user = JsonSerializer.Deserialize<User>(json, options);

方法2：LINQ to JSON直接映射 #

如果只是单次使用，不用写转换器，直接解析后手动赋值更省事：

var jsonDoc = JsonDocument.Parse(json);
var user = new User
{
    Id = jsonDoc.RootElement.GetProperty("id").GetString(),
    Username = jsonDoc.RootElement.GetProperty("data").GetProperty("username").GetString(),
    Email = jsonDoc.RootElement.GetProperty("data").GetProperty("email").GetString(),
    Age = jsonDoc.RootElement.GetProperty("data").GetProperty("age").GetInt32()
};

二、用Newtonsoft.Json（Json.NET）实现 #

Newtonsoft的方案更简洁，直接给属性标注路径就行：

方法1：JsonProperty指定嵌套路径 #

不用写转换器，直接在User类的属性上标注JsonProperty，指定JSON里的嵌套路径：

public class User
{
    [JsonProperty("id")]
    public string Id { get; set; }

    [JsonProperty("data.username")]
    public string Username { get; set; }

    [JsonProperty("data.email")]
    public string Email { get; set; }

    [JsonProperty("data.age")]
    public int Age { get; set; }
}

然后直接反序列化：

var json = "{\"id\":\"123\",\"data\":{\"username\":\"jason\",\"email\":\"jason@example.com\",\"age\":30}}";
var user = JsonConvert.DeserializeObject<User>(json);

方法2：自定义JsonConverter（按需使用） #

如果需要更复杂的逻辑，也可以写转换器：

public class UserConverter : JsonConverter<User>
{
    public override User ReadJson(JsonReader reader, Type objectType, User existingValue, bool hasExistingValue, JsonSerializer serializer)
    {
        var jo = JObject.Load(reader);
        return new User
        {
            Id = jo["id"].ToString(),
            Username = jo["data"]["username"].ToString(),
            Email = jo["data"]["email"].ToString(),
            Age = (int)jo["data"]["age"]
        };
    }

    // 序列化回原结构的逻辑
    public override void WriteJson(JsonWriter writer, User value, JsonSerializer serializer)
    {
        var jo = new JObject();
        jo["id"] = value.Id;
        var dataObj = new JObject();
        dataObj["username"] = value.Username;
        dataObj["email"] = value.Email;
        dataObj["age"] = value.Age;
        jo["data"] = dataObj;
        jo.WriteTo(writer);
    }
}

使用转换器的代码：

var settings = new JsonSerializerSettings();
settings.Converters.Add(new UserConverter());
var user = JsonConvert.DeserializeObject<User>(json, settings);

内容的提问来源于stack exchange，提问作者Jason Steinshouer