从System.Text.Json诞生，就在努力增加功能和提升性能，在.NET7中，又带来了多态的适配。下面是一个父类Customer，两个子类，WechatCustomer和LineCustomer。

public class Customer
{
    public string Name { get; set; }
    public string Address { get; set; }
    public string City { get; set; }
    public string Region { get; set; }
    public string PostalCode { get; set; }
    public string EMail { get; set; }
    public string Tel { get; set; }
}

public class WechatCustomer : Customer
{
    public string? WechatNo { get; set; }
}
public class LineCustomer : Customer
{
    public string? LineNo { get; set; }
}

分别用一个打印方法PrintCustomer输出两个子类对象。

var wechatCustomer = new WechatCustomer
{
    Name = "张三",
    City = "东京",
    Region = "中央区",
    Address = "1-56-326",
    PostalCode = "3000235",
    EMail = "abcde@gmail.com",
    Tel = "08-9563-2356",
    WechatNo = "wechat_gsw" 
};
var lineCustomer = new LineCustomer
{
    Name = "张三",
    City = "东京",
    Region = "中央区",
    Address = "1-56-326",
    PostalCode = "3000235",
    EMail = "abcde@gmail.com",
    Tel = "08-9563-2356",
    LineNo = "line_gsw"
};
//共用打印方法
void PrintCustomer(Customer customer)
{
    var options = new JsonSerializerOptions
    {
        Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
        WriteIndented = true
    };
    var json = JsonSerializer.Serialize<Customer>(customer, options);
    Console.WriteLine(json);
}

PrintCustomer(wechatCustomer);
PrintCustomer(lineCustomer);

输出结果是：

{
  "Name": "张三",
  "Address": "1-56-326",
  "City": "东京",
  "Region": "中央区",
  "PostalCode": "3000235",
  "EMail": "abcde@gmail.com",
  "Tel": "08-9563-2356"
}
{
  "Name": "张三",
  "Address": "1-56-326",
  "City": "东京",
  "Region": "中央区",
  "PostalCode": "3000235",
  "EMail": "abcde@gmail.com",
  "Tel": "08-9563-2356"
}

现在丢失了子类的属性WechatNo和LineNo，在.NET7中，可以通过两种方式来保证不丢失这种多态产生的新的属性：特性和自定义类型信息解析器。
特性：

[JsonPolymorphic(TypeDiscriminatorPropertyName = "CustomerType")]
[JsonDerivedType(typeof(WechatCustomer), typeDiscriminator: "wechatCustomer")]
[JsonDerivedType(typeof(LineCustomer), typeDiscriminator: "lineCustomer")]
public class Customer
{
……
}

通过给父类Customer添加特性，来达到json化的效果。再次输出如下：

{
  "CustomerType": "wechatCustomer",
  "WechatNo": "wechat_gsw",
  "Name": "张三",
  "Address": "1-56-326",
  "City": "东京",
  "Region": "中央区",
  "PostalCode": "3000235",
  "EMail": "abcde@gmail.com",
  "Tel": "08-9563-2356"
}
{
  "CustomerType": "lineCustomer",
  "LineNo": "line_gsw",
  "Name": "张三",
  "Address": "1-56-326",
  "City": "东京",
  "Region": "中央区",
  "PostalCode": "3000235",
  "EMail": "abcde@gmail.com",
  "Tel": "08-9563-2356"
}

可以看到，JsonPolymorphic特性是定义一个标注子类名称的属性customerType，而JsonDerivedType是定义两个子类的特性。

自定义类型信息解析器：
首先定义自定义类型信息解析器class，可以看出，他的作用与上面的特性是一样的。

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

        Type basePointType = typeof(Customer);
        if (jsonTypeInfo.Type == basePointType)
        {
            jsonTypeInfo.PolymorphismOptions = new JsonPolymorphismOptions
            {
                TypeDiscriminatorPropertyName = "CustomerType",
                DerivedTypes =
                {
                    new JsonDerivedType(typeof(WechatCustomer), "wechatCustomer"),
                    new JsonDerivedType(typeof(LineCustomer), "lineCustomer")
                }
            };
        }
        return jsonTypeInfo;
    }
}

这个时候，Customer上的特性就可以没有了，只要在PrintCustomer方法中给options加个属性即可。

void PrintCustomer(Customer customer)
{
    var options = new JsonSerializerOptions
    {
        TypeInfoResolver = new PolymorphicTypeResolver(),
        Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
        WriteIndented = true
    };
    var json = JsonSerializer.Serialize<Customer>(customer, options);
    Console.WriteLine(json);
}

输出结果与特性方式是一样的。这里不再赘述。上面两种方式，也可以把对应的json字符串反序列化回来。