.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。

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。

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);

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