.NET Core 3.0提供了一個名為System.Text.Json的全新命名空間，它支持reader/writer，文檔對象模型（DOM）和序列化程序。在此博客文章中，我將介紹它如何工作以及如何使用。

獲取JSON庫

• 如果以.NET Core為目標,請安裝.NET Core 3.0及以上版本，該版本提供了新的JSON庫和ASP.NET Core集成。

• 如果以.NET Standard或.NET Framework為目標。安裝System.Text.Json NuGet軟件包（確保安裝.NET Framework4.6.0或更高版本）。為了與ASP.NET Core集成，必須以.NET Core 3.0為目標。

NET Core 3.0中JSON特性

新的JSON API通過使用Span進行性能優化，并且可以直接處理UTF-8，而無需轉碼為UTF-16 string 實例。這兩個方面對于ASP.NET Core都是至關重要的，在ASP.NET Core中，吞吐量是關鍵要求。使用System.Text.Json，可以將速度提高大約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

• 學習.Net Core最好的方式是查看源碼，下面是JsonSerializer Serialize的部分源碼：

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);}} }

可以看到序列化最終調用的是ToStringInternal這個方法。

• 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用于在序列化時設置配置， 例如處理注釋，null處理，尾隨逗號和命名策略。

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); }

自定義屬性來控制序列化行為

可以使用自定義屬性來控制序列化行為，例如，忽略屬性或者指定屬性的名稱：

class Sample {//忽略[JsonIgnore]public DateTimeOffset Date { get; set; }//指定名稱[JsonPropertyName("test")]public string Summary { get; set; } }

使用DOM

有時，不想反序列化JSON但仍然希望對其內容進行結構化訪問,這時可以使用JsonDocument

var options = new JsonDocumentOptions{AllowTrailingCommas = true}; using (JsonDocument document = JsonDocument.Parse(json, options)) {//todoforeach (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.

具體請參考官方文檔。

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);}}

總結

• 在.NET Core 3.0中，System.Text.Json API提供對JSON的內置支持，包括reader/writer，只讀DOM和序列化器/反序列化器。

• 主要目標是實現高性能和低內存分配。

• ASP.NET Core 3.0包括對System.Text.Json的支持，默認情況下啟用。

補充

有博友提問了下圖中的幾個問題：

當前API版本反序列化不會將JSON字符串中的數字強制轉換；但是JsonConverter轉換器功能可以逐個屬性有效地選擇轉換：

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);

當前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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