.NET Core 3.0提供了一个名为System.Text.Json的全新命名空间，它支持reader/writer，文档对象模型（DOM）和序列化程序。在此博客文章中，我将介绍它如何工作以及如何使用。
官方文档
获取JSON库
- 如果以.NET Core为目标,请安装.NET Core 3.0及以上版本，该版本提供了新的JSON库和ASP.NET Core集成。
- 如果以.NET Standard或.NET Framework为目标。安装System.Text.Json NuGet软件包（确保安装.NET Framework4.6.0或更高版本）。为了与ASP.NET Core集成，必须以.NET Core 3.0为目标。
NET Core 3.0中JSON特性
新的JSON API通过使用Span进行性能优化，并且可以直接处理UTF-8，而无需转码为UTF-16 string 实例。这两个方面对于ASP.NET Core都是至关重要的，在ASP.NET Core中，吞吐量是关键要求。使用System.Text.Json，可以将速度提高大约1.3倍至5倍。
使用System.Text.Json
using System.Text.Json;
using System.Text.Json.Serialization;
使用序列化器Serializer
- 学习.Net Core最好的方式是查看源码，下面是JsonSerializer Serialize的部分源码：
namespace System.Text.Json
{
public static partial class JsonSerializer
{
/// <summary>
/// Convert the provided value into a <see cref="System.String"/>.
/// </summary>
/// <returns>A <see cref="System.String"/> representation of the value.</returns>
/// <param name="value">The value to convert.</param>
/// <param name="options">Options to control the conversion behavior.</param>
/// <remarks>Using a <see cref="System.String"/> is not as efficient as using UTF-8
/// encoding since the implementation internally uses UTF-8. See also <see cref="SerializeToUtf8Bytes"/>
/// and <see cref="SerializeAsync"/>.
/// </remarks>
public static string Serialize<TValue>(TValue value, JsonSerializerOptions options = null)
{
return ToStringInternal(value, typeof(TValue), options);
}
/// <summary>
/// Convert the provided value into a <see cref="System.String"/>.
/// </summary>
/// <returns>A <see cref="System.String"/> representation of the value.</returns>
/// <param name="value">The value to convert.</param>
/// <param name="inputType">The type of the <paramref name="value"/> to convert.</param>
/// <param name="options">Options to control the conversion behavior.</param>
/// <remarks>Using a <see cref="System.String"/> is not as efficient as using UTF-8
/// encoding since the implementation internally uses UTF-8. See also <see cref="SerializeToUtf8Bytes"/>
/// and <see cref="SerializeAsync"/>.
/// </remarks>
public static string Serialize(object value, Type inputType, JsonSerializerOptions options = null)
{
VerifyValueAndType(value, inputType);
return ToStringInternal(value, inputType, options);
}
private static string ToStringInternal(object value, Type inputType, JsonSerializerOptions options)
{
return WriteCoreString(value, inputType, options);
}
}
}
可以看到序列化最终调用的是ToStringInternal这个方法。
- 示例
class Sample
{
public DateTimeOffset Date { get; set; }
public string Summary { get; set; }
}
string Serialize(Sample value)
{
return JsonSerializer.Serialize<Sample>(value);
}
//obj
string SerializeObj(object value)
{
return JsonSerializer.Serialize(value);
}
JsonSerializerOptions用于在序列化时设置配置， 例如处理注释，null处理，尾随逗号和命名策略。
public static string ToJson(this object obj)
{
var options = new JsonSerializerOptions() { IgnoreNullValues = true, WriteIndented = true, AllowTrailingCommas = true };
return JsonSerializer.Serialize(obj, options);
}
使用反序列化器Deserialize
Sample Deserialize(string json)
{
var options = new JsonSerializerOptions
{
AllowTrailingCommas = true
};
return JsonSerializer.Deserialize<Sample>(json, options);
}
自定义属性来控制序列化行为
可以使用自定义属性来控制序列化行为，例如，忽略属性或者指定属性的名称：
class Sample
{
//忽略
[JsonIgnore]
public DateTimeOffset Date { get; set; }
//指定名称
[JsonPropertyName("test")]
public string Summary { get; set; }
}
使用DOM
有时，不想反序列化JSON但仍然希望对其内容进行结构化访问,这时可以使用JsonDocument
var options = new JsonDocumentOptions
{
AllowTrailingCommas = true
};
using (JsonDocument document = JsonDocument.Parse(json, options))
{
//todo
foreach (JsonElement element in document.RootElement.EnumerateArray())
{
DateTimeOffset date = element.GetProperty("date").GetDateTimeOffset();
//todo
}
}
使用Utf8JsonWriter
var options = new JsonWriterOptions
{
Indented = true
};
using (var stream = new MemoryStream())
{
using (var writer = new Utf8JsonWriter(stream, options))
{
writer.WriteStartObject();
writer.WriteString("date", DateTimeOffset.UtcNow);
writer.WriteEndObject();
}
string json = Encoding.UTF8.GetString(stream.ToArray());
Console.WriteLine(json);
}
使用Utf8JsonReader
byte[] data = Encoding.UTF8.GetBytes(json);
Utf8JsonReader reader = new Utf8JsonReader(data, isFinalBlock: true, state: default);
while (reader.Read())
{
Console.Write(reader.TokenType);
switch (reader.TokenType)
{
case JsonTokenType.PropertyName:
case JsonTokenType.String:
{
string text = reader.GetString();
Console.Write(" ");
Console.Write(text);
break;
}
case JsonTokenType.Number:
{
int value = reader.GetInt32();
Console.Write(" ");
Console.Write(value);
break;
}
}
Console.WriteLine();
}
System.Text.Json中的DateTime和DateTimeOffset支持
The System.Text.Json library parses and writes DateTime and DateTimeOffset values according to the ISO 8601:-2019 extended profile. Converters provide custom support for serializing and deserializing with JsonSerializer. Custom support can also be implemented when using Utf8JsonReader and Utf8JsonWriter.
具体请参考官方文档。
示例：
public class DateTimeConverterUsingDateTimeParse : JsonConverter<DateTime>
{
/// <summary>
/// 日期格式
/// </summary>
public string dateTimeFormat { get; }
/// <summary>
/// ctor
/// </summary>
/// <param name="dateTimeFormat"></param>
public DateTimeConverterUsingDateTimeParse(string dateTimeFormat)
{
this.dateTimeFormat = dateTimeFormat;
}
public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
Debug.Assert(typeToConvert == typeof(DateTime));
return DateTime.Parse(reader.GetString());
}
public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options)
{
writer.WriteStringValue(value.ToString(dateTimeFormat));
}
}
public static class JsonHelper
{
/// <summary>Converts to json.</summary>
/// <param name="obj">The object.</param>
/// <param name="dateTimeFormat">The date time format.</param>
/// <returns>System.String.</returns>
public static string ToJson(this object obj, string dateTimeFormat = "yyyy-MM-dd")
{
var options = new JsonSerializerOptions() { IgnoreNullValues = true, WriteIndented = true };
options.Converters.Add(new DateTimeConverterUsingDateTimeParse(dateTimeFormat));
return JsonSerializer.Serialize(obj, options);
}
}
总结
- 在.NET Core 3.0中，System.Text.Json API提供对JSON的内置支持，包括reader/writer，只读DOM和序列化器/反序列化器。
- 主要目标是实现高性能和低内存分配。
- ASP.NET Core 3.0包括对System.Text.Json的支持，默认情况下启用。
补充
有博友提问了下图中的几个问题：
当前API版本反序列化不会将JSON字符串中的数字强制转换；但是JsonConverter转换器功能可以逐个属性有效地选择转换：
public class NumberToStringConverter : JsonConverter<int>
{
public override int Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
var number = 0;
if (reader.TokenType == JsonTokenType.String)
{
if (Int32.TryParse(reader.GetString(), out number))
return number;
}
return reader.GetInt32();
}
public override void Write(Utf8JsonWriter writer, int value, JsonSerializerOptions options)
{
writer.WriteStringValue(value.ToString());
}
}
class Sample
{
public string Date { get; set; }
public int Summary { get; set; }
}
var json = "{"Date":"2019-10-31","Summary":"1"}";
var options = new JsonSerializerOptions() { IgnoreNullValues = true, WriteIndented = true };
options.Converters.Add(new NumberToStringConverter());
var deserialize = JsonSerializer.Deserialize<Sample>(json, options);
当前API版本支持System.Text.Json 支持Dictionary<key,value>序列化。
Dictionary<string, object> dictionary = new Dictionary<string, object>
{
{"1", 2}
};
var serialize = JsonSerializer.Serialize(dictionary);
var temjson = "{"1":2}";
var deserializeDictionary = JsonSerializer.Deserialize<Dictionary<string, object>>(temjson);