1､使用的命名空间

using System.Text.Json;
using System.Text.Json.Serialization;

2､.NET对象序列化JSON字符串

将 JSON 写入字符串，请调用 JsonSerializer.Serialize 方法。 下面的示例使用具有泛型类型参数的重载：

WeatherForecast weatherForecast;
//...
string json = JsonSerializer.Serialize<WeatherForecast>(weatherForecast);

可以省略泛型类型参数并改用泛型类型推理：

WeatherForecast weatherForecast;
//...
string json = JsonSerializer.Serialize(weatherForecast);

示例类型，其中包含集合和嵌套类：

public class WeatherForecast
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureC { get; set; }
    public string Summary { get; set; }
    public IList<DateTimeOffset> DatesAvailable { get; set;}
    public Dictionary<string, HighLowTemperatures> TemperatureRanges { get; set; }
    public string [] SummaryWords { get; set; }
}
public class HighLowTemperatures
{
    public Temperature High { get; set; }
    public Temperature Low { get; set; }
}
public class Temperature
{
    public int DegreesCelsius { get; set; }
}

JSON字符串：

{"Date":"2019-08-01T00:00:00-07:00","TemperatureC":25,"Summary":"Hot","DatesAvailable":["2019-08-01T00:00:00-07:00","2019-08-02T00:00:00-07:00"],"TemperatureRanges":{"Cold":{"High":{"DegreesCelsius":20},"Low":{"DegreesCelsius":-10}},"Hot":{"High":{"DegreesCelsius":60},"Low":{"DegreesCelsius":20}}},"SummaryWords":["Cool","Windy","Humid"]}

序列化为UTF-8字节数据

调用 JsonSerializer.SerializeToUtf8Bytes 方法：

byte[] utf8Json = JsonSerializer.SerializeToUtf8Bytes<WeatherForecast>(weatherForecast);

序列化为 UTF-8 比使用基于字符串的方法更快5-10%。 不同之处在于，不需要将字节（如 UTF-8）转换为字符串（UTF-16)。

3､JSON字符串反序列化成.NET对象

从字符串进行反序列化，请调用 JsonSerializer.Deserialize 方法，如以下示例中所示：

string json = ... ;
var weatherForecast = JsonSerializer.Deserialize<WeatherForecast>(json);

从UTF-8字节数据反序化

byte[] utf8Json;
//...
var readOnlySpan = new ReadOnlySpan<byte>(utf8Json);
weatherForecast = JsonSerializer.Deserialize<WeatherForecastMin>(readOnlySpan);

或

byte[] utf8Json;
//...
var utf8Reader = new Utf8JsonReader(utf8Json);
weatherForecast = JsonSerializer.Deserialize<WeatherForecastMin>(ref utf8Reader);

4､序列化为格式化的JSON字符串

要整齐地打印 JSON 输出，请将 JsonSerializerOptions.WriteIndented 设置为 true：

var options = new JsonSerializerOptions
{
    WriteIndented = true,
};
json = JsonSerializer.Serialize(weatherForecast, options);

下面是要序列化的示例类型和 JSON 输出：

class WeatherForecast
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureC { get; set; }
    public string Summary { get; set; }
}

格式化后的输出的JSON字符串：

{
  "Date": "2019-08-01T00:00:00-07:00",
  "TemperatureC": 25,
  "Summary": "Hot"
}

官方文档：https://docs.microsoft.com/zh-cn/dotnet/standard/serialization/system-text-json-how-to

相关文档：NET Core 3.0 JsonSerializer(System.Text.Json)自定属性名及允许注释和尾随逗号方法