如何使用System.Text.Json反序列化含无参公共构造函数的类

在.NET 7.0的C#应用中，使用System.Text.Json进行序列化/反序列化时出现异常：

• 类Works（私有无参构造+公共有参构造）反序列化后，address能正确获取JSON中的值"abc"
• 类Doesnt（标记[JsonConstructor]的公共无参构造+公共有参构造）反序列化后，address始终是无参构造里的"default"（移除初始化则为null）

示例代码 #

正常工作的Works类

public class Works
{
    public string address { get; private set; }

    Works()
    {
        address = "default";
    }
    public Works(string address)
    {
        this.address = address;
    }
}

反序列化异常的Doesnt类

public class Doesnt
{
    public string address { get; private set; }

    [JsonConstructor] 
    public Doesnt()
    {
        address = "default";
    }
    public Doesnt(string address)
    {
        this.address = address;
    }
}

序列化与反序列化逻辑

public byte[] SerializeToBytes<T>(T src)
{
    return JsonSerializer.SerializeToUtf8Bytes(src, SerializerOptions);
}
public object Deserialize(byte[] data, Type type)
{
    object res;
    var str = System.Text.Encoding.UTF8.GetString(data);
    res = JsonSerializer.Deserialize(str, type, SerializerOptions);
    return res;
}

var objs = new object[]{new Works("abc"),new Doesnt("abc")};
var bytes = SerializeToBytes(objs[0]);
Deserialize(bytes, objs[0].GetType());// 返回"abc"

bytes = SerializeToBytes(objs[1]);
Deserialize(bytes, objs[1].GetType()); // 返回"default"

方法1：将[JsonConstructor]标记移至有参构造函数 #

把[JsonConstructor]从无参构造转移到接收address参数的有参构造上，让反序列化时优先调用该构造，直接用JSON中的值初始化属性：

public class Doesnt
{
    public string address { get; private set; }

    public Doesnt()
    {
        address = "default";
    }

    [JsonConstructor] // 标记有参构造
    public Doesnt(string address)
    {
        this.address = address;
    }
}

说明：此方法会跳过无参构造的执行逻辑，直接通过JSON值初始化对象，适合不需要保留无参构造初始化逻辑的场景。

方法2：给私有setter添加[JsonInclude]特性 #

通过[JsonInclude]允许System.Text.Json访问私有setter，这样反序列化时会先执行无参构造初始化，再用JSON值覆盖属性：

public class Doesnt
{
    [JsonInclude] // 允许序列化/反序列化访问私有setter
    public string address { get; private set; }

    [JsonConstructor]
    public Doesnt()
    {
        address = "default";
    }

    public Doesnt(string address)
    {
        this.address = address;
    }
}

说明：此方法会保留无参构造的初始化逻辑，再用JSON值覆盖属性，适合需要保留默认初始化逻辑的场景。

内容的提问来源于stack exchange，提问作者Maxim Kitsenko