ASP.NET Core 3.1系列（29）——System.Text.Json实现JSON的序列化和反序列化

1、前言

在Web开发中，JSON数据可以说是无处不在。由于具有轻量、易读等优点，JSON已经成为当前主流的数据传输格式。在ASP.NET Core 3.0之前，大多数项目都会使用Newtonsoft.Json组件来实现JSON的序列化和反序列化操作，而从ASP.NET Core 3.1开始，微软提供的System.Text.Json已经相当出色，其效率相比前者可以说是有过之而无不及，下面就来介绍一下它的使用方法。

2、引入System.Text.Json

新建一个Web API项目，使用NuGet引入如下组件：

System.Text.Json

新建一个实体类Person，代码如下：

using System;

namespace App
{
    public class Person
    {
        public int Id { get; set; }
        public string PersonName { get; set; }
        public string PersonGender { get; set; }
        public DateTime? BirthOfDate { get; set; }
    }
}

新建一个控制器PersonController，添加一个Get方法，代码如下：

using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public JsonResult Get()
        {
            List<Person> list = new List<Person>
            {
                new Person { Id = 1, PersonName = "张三", PersonGender = "男", BirthOfDate = new DateTime(2000, 1, 1) },
                new Person { Id = 2, PersonName = "李四", PersonGender = "女", BirthOfDate = new DateTime(2000, 2, 2) },
                new Person { Id = 3, PersonName = "王五", PersonGender = "男", BirthOfDate = new DateTime(2000, 3, 3) }
            };
            return new JsonResult(list);
        }
    }
}

运行结果如下所示：

[{"id":1,"personName":"\u5F20\u4E09","personGender":"\u7537","birthOfDate":"2000-01-01T00:00:00"},{"id":1,"personName":"\u674E\u56DB","personGender":"\u5973","birthOfDate":"2000-02-02T00:00:00"},{"id":1,"personName":"\u738B\u4E94","personGender":"\u7537","birthOfDate":"2000-03-03T00:00:00"}]

上面的代码实现了一个简单的JSON序列化操作，但里面的问题也有很多，比如中文乱码、时间格式等等，下面就来说一说如何在System.Text.Json中去设置它们。

3、序列化操作

3.1、JSON数据编码

上面的运行结果显示中文乱码，我们可以使用JsonSerializerOptions中的Encoder来指定编码格式，代码如下：

using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Text.Encodings.Web;
using System.Text.Json;
using System.Text.Unicode;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public JsonResult Get()
        {
            List<Person> list = new List<Person>
            {
                new Person { Id = 1, PersonName = "张三", PersonGender = "男", BirthOfDate = new DateTime(2000, 1, 1) },
                new Person { Id = 2, PersonName = "李四", PersonGender = "女", BirthOfDate = new DateTime(2000, 2, 2) },
                new Person { Id = 3, PersonName = "王五", PersonGender = "男", BirthOfDate = new DateTime(2000, 3, 3) }
            };
            return new JsonResult(list, new JsonSerializerOptions
            {
                Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping
            });
        }
    }
}

运行结果如下所示，可以发现中文编码正确。

[{"Id":1,"PersonName":"张三","PersonGender":"男","BirthOfDate":"2000-01-01T00:00:00"},{"Id":2,"PersonName":"李四","PersonGender":"女","BirthOfDate":"2000-02-02T00:00:00"},{"Id":3,"PersonName":"王五","PersonGender":"男","BirthOfDate":"2000-03-03T00:00:00"}]

3.2、JSON文本格式化

默认输出的JSON文本是未经格式化的，如果你希望JSON看起来清楚一些，可以设置WriteIndented属性值，代码如下：

using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Text.Encodings.Web;
using System.Text.Json;
using System.Text.Unicode;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public JsonResult Get()
        {
            List<Person> list = new List<Person>
            {
                new Person { Id = 1, PersonName = "张三", PersonGender = "男", BirthOfDate = new DateTime(2000, 1, 1) },
                new Person { Id = 2, PersonName = "李四", PersonGender = "女", BirthOfDate = new DateTime(2000, 2, 2) },
                new Person { Id = 3, PersonName = "王五", PersonGender = "男", BirthOfDate = new DateTime(2000, 3, 3) }
            };
            return new JsonResult(list, new JsonSerializerOptions
            {
                Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
                WriteIndented = true
            });
        }
    }
}

运行结果如下所示，可以发现JSON文本已经被格式化了。

[
  {
    "Id": 1,
    "PersonName": "张三",
    "PersonGender": "男",
    "BirthOfDate": "2000-01-01T00:00:00"
  },
  {
    "Id": 2,
    "PersonName": "李四",
    "PersonGender": "女",
    "BirthOfDate": "2000-02-02T00:00:00"
  },
  {
    "Id": 3,
    "PersonName": "王五",
    "PersonGender": "男",
    "BirthOfDate": "2000-03-03T00:00:00"
  }
]

3.3、JSON字段命名格式

上面的输出结果遵循的是首字母大写的帕斯卡命名格式，如果希望输出结果采用驼峰式进行命名，则可以对PropertyNamingPolicy进行设置，代码如下：

using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Text.Encodings.Web;
using System.Text.Json;
using System.Text.Unicode;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public JsonResult Get()
        {
            List<Person> list = new List<Person>
            {
                new Person { Id = 1, PersonName = "张三", PersonGender = "男", BirthOfDate = new DateTime(2000, 1, 1) },
                new Person { Id = 2, PersonName = "李四", PersonGender = "女", BirthOfDate = new DateTime(2000, 2, 2) },
                new Person { Id = 3, PersonName = "王五", PersonGender = "男", BirthOfDate = new DateTime(2000, 3, 3) }
            };
            return new JsonResult(list, new JsonSerializerOptions
            {
                Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
                WriteIndented = true,
                PropertyNamingPolicy = JsonNamingPolicy.CamelCase
            });
        }
    }
}

运行结果如下所示，可以发现JSON文本采用的是驼峰式命名。

[
  {
    "id": 1,
    "personName": "张三",
    "personGender": "男",
    "birthOfDate": "2000-01-01T00:00:00"
  },
  {
    "id": 2,
    "personName": "李四",
    "personGender": "女",
    "birthOfDate": "2000-02-02T00:00:00"
  },
  {
    "id": 3,
    "personName": "王五",
    "personGender": "男",
    "birthOfDate": "2000-03-03T00:00:00"
  }
]

3.4、忽略JSON中的null值

在序列化时，如果对象的属性值为null，则结果中也会显示为null。如果希望忽略null值，则可以对DefaultIgnoreCondition进行设置，代码如下：

using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;
using System.Text.Encodings.Web;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Text.Unicode;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public JsonResult Get()
        {
            List<Person> list = new List<Person>
            {
                new Person { Id = 1, PersonName = "张三", PersonGender = "男", BirthOfDate = null },
                new Person { Id = 2, PersonName = "李四", PersonGender = "女", BirthOfDate = null },
                new Person { Id = 3, PersonName = "王五", PersonGender = "男", BirthOfDate = null }
            };
            return new JsonResult(list, new JsonSerializerOptions
            {
                Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
                WriteIndented = true,
                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
            });
        }
    }
}

运行结果如下所示，可以发现BirthOfDate字段的值被忽略了。

[
  {
    "id": 1,
    "personName": "张三",
    "personGender": "男"
  },
  {
    "id": 2,
    "personName": "李四",
    "personGender": "女"
  },
  {
    "id": 3,
    "personName": "王五",
    "personGender": "男"
  }
]

3.5.、JSON忽略只读字段

一般来说，由于只读字段无法进行反序列化操作，因此在序列化时可以考虑忽略。现在对Person代码进行修改，添加一个只读字段Info，代码如下：

using System;

namespace App
{
    public class Person
    {
        public int Id { get; set; }
        public string PersonName { get; set; }
        public string PersonGender { get; set; }
        public DateTime? BirthOfDate { get; set; }
        public string Info
        {
            get { return $"姓名：{PersonName}，性别：{PersonGender}，出生日期：{BirthOfDate}"; }
        }
    }
}

我们可以对JsonSerializerOptions中的IgnoreReadOnlyProperties字段进行设置，从而忽略只读字段，代码如下：

using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Text.Encodings.Web;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public JsonResult Get()
        {
            List<Person> list = new List<Person>
            {
                new Person { Id = 1, PersonName = "张三", PersonGender = "男", BirthOfDate = new DateTime(2000, 1, 1) },
                new Person { Id = 2, PersonName = "李四", PersonGender = "女", BirthOfDate = new DateTime(2000, 2, 2) },
                new Person { Id = 3, PersonName = "王五", PersonGender = "男", BirthOfDate = new DateTime(2000, 3, 3) }
            };
            return new JsonResult(list, new JsonSerializerOptions
            {
                Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
                WriteIndented = true,
                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
                IgnoreReadOnlyProperties = true
            });
        }
    }
}

运行结果如下所示，可以发现Info字段并没有被序列化。

[
  {
    "id": 1,
    "personName": "张三",
    "personGender": "男",
    "birthOfDate": "2000-01-01T00:00:00"
  },
  {
    "id": 2,
    "personName": "李四",
    "personGender": "女",
    "birthOfDate": "2000-02-02T00:00:00"
  },
  {
    "id": 3,
    "personName": "王五",
    "personGender": "男",
    "birthOfDate": "2000-03-03T00:00:00"
  }
]

3.6、JSON中的时间格式

在上面的代码中，时间字段BirthOfDate的序列化结果有一些问题，如何把它序列化成我们熟悉的时间格式呢？首先定义一个类DateTimeJsonConverter，该类继承JsonConverter，代码如下：

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

namespace App
{
    public class DateTimeJsonConverter : JsonConverter<DateTime>
    {
        public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            if (reader.TokenType == JsonTokenType.String)
            {
                if (DateTime.TryParse(reader.GetString(), out DateTime dateTime))
                {
                    return dateTime;
                }
            }
            return reader.GetDateTime();
        }

        public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options)
        {
            writer.WriteStringValue(value.ToString("yyyy年MM月dd日 HH时mm分ss秒"));
        }
    }
}

然后在JsonSerializerOptions中的Converters集合中加入它即可，代码如下：

using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Text.Encodings.Web;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Text.Unicode;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public JsonResult Get()
        {
            List<Person> list = new List<Person>
            {
                new Person { Id = 1, PersonName = "张三", PersonGender = "男", BirthOfDate = new DateTime(2000, 1, 1, 0, 0, 0) },
                new Person { Id = 2, PersonName = "李四", PersonGender = "女", BirthOfDate = new DateTime(2000, 2, 2) },
                new Person { Id = 3, PersonName = "王五", PersonGender = "男", BirthOfDate = new DateTime(2000, 3, 3) }
            };

            JsonSerializerOptions options = new JsonSerializerOptions
            {
                Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
                WriteIndented = true,
                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
            };
            options.Converters.Add(new DateTimeJsonConverter());

            return new JsonResult(list, options);
        }
    }
}

运行结果如下所示，可以发现BirthOfDate字段的值已经被格式化。

[
  {
    "id": 1,
    "personName": "张三",
    "personGender": "男",
    "birthOfDate": "2000年01月01日 00时00分00秒"
  },
  {
    "id": 2,
    "personName": "李四",
    "personGender": "女",
    "birthOfDate": "2000年02月02日 00时00分00秒"
  },
  {
    "id": 3,
    "personName": "王五",
    "personGender": "男",
    "birthOfDate": "2000年03月03日 00时00分00秒"
  }
]

4、反序列化操作

4.1、常规操作

JSON的反序列化操作比较简单，只需要调用Deserialize即可。代码如下：

using Microsoft.AspNetCore.Mvc;
using System.Text.Json;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public ActionResult<string> Get()
        {
            string json = "{\"Id\":1,\"PersonName\":\"张三\",\"PersonGender\":\"男\",\"BirthOfDate\":\"2000-01-01T00:00:00\"}";
            Person person = JsonSerializer.Deserialize<Person>(json);
            return $"姓名：{person.PersonName}\n性别：{person.PersonGender}\n出生日期：{person.BirthOfDate}";
        }
    }
}

运行结果如下所示：

姓名：张三
性别：男
出生日期：2000/1/1 0:00:00

4.2、特殊情况的处理

在进行反序列化操作时，有一种情况需要特别注意，那就是属性值末尾存在逗号的情况。在JavaScript中，下面的代码是允许的，即：在BirthOfDate的属性值后面允许添加一个逗号：

{
  "Id": 1,
  "PersonName": "张三",
  "PersonGender": "男",
  "BirthOfDate": "2000-01-01T00:00:00",
}

但这种情况会导致反序列化报错，代码如下：

using Microsoft.AspNetCore.Mvc;
using System.Text.Json;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public ActionResult<string> Get()
        {
            string json = "{\"Id\":1,\"PersonName\":\"张三\",\"PersonGender\":\"男\",\"BirthOfDate\":\"2000-01-01T00:00:00\",}";
            Person person = JsonSerializer.Deserialize<Person>(json);
            return $"姓名：{person.PersonName}\n性别：{person.PersonGender}\n出生日期：{person.BirthOfDate}";
        }
    }
}

运行结果如下图所示：

如果要允许这种末尾添加逗号的情况，需要设置JsonSerializerOptions的AllowTrailingCommas属性，代码如下：

using Microsoft.AspNetCore.Mvc;
using System.Text.Json;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public ActionResult<string> Get()
        {
            string json = "{\"Id\":1,\"PersonName\":\"张三\",\"PersonGender\":\"男\",\"BirthOfDate\":\"2000-01-01T00:00:00\",}";
            Person person = JsonSerializer.Deserialize<Person>(json, new JsonSerializerOptions
            {
                AllowTrailingCommas = true
            });
            return $"姓名：{person.PersonName}\n性别：{person.PersonGender}\n出生日期：{person.BirthOfDate}";
        }
    }
}

运行结果如下所示：

姓名：张三
性别：男
出生日期：2000/1/1 0:00:00

5、全局配置JSON

上面的代码是在单个方法中设置JSON操作属性，如果当前存在很多方法，则必然会导致代码臃肿。我们可以在全局对JSON进行配置，代码如下：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System.Text.Encodings.Web;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace App
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers().AddJsonOptions(options =>
            {
                // 设置编码格式
                options.JsonSerializerOptions.Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping;

                // 是否格式化文本
                options.JsonSerializerOptions.WriteIndented = true;

                // 添加时间格式化转换器
                options.JsonSerializerOptions.Converters.Add(new DateTimeJsonConverter());

                // 字段采用驼峰式命名
                options.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;

                // 忽略null值
                options.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;

                // 忽略只读字段
                options.JsonSerializerOptions.IgnoreReadOnlyProperties = true;

                // 允许属性值末尾存在逗号
                options.JsonSerializerOptions.AllowTrailingCommas = true;

                // 处理循环引用类型
                options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles;
            });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseHttpsRedirection();
            app.UseRouting();
            app.UseAuthorization();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}

在全局进行配置后，Controller中的方法就不需要单独配置了，代码如下：

using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;

namespace App.Controllers
{
    [Route("api/[controller]/[action]")]
    [ApiController]
    public class PersonController : ControllerBase
    {
        [HttpGet]
        public JsonResult Get()
        {
            List<Person> list = new List<Person>
            {
                new Person { Id = 1, PersonName = "张三", PersonGender = "男", BirthOfDate = new DateTime(2000, 1, 1, 0, 0, 0) },
                new Person { Id = 2, PersonName = "李四", PersonGender = "女", BirthOfDate = new DateTime(2000, 2, 2) },
                new Person { Id = 3, PersonName = "王五", PersonGender = "男", BirthOfDate = new DateTime(2000, 3, 3) }
            };
            return new JsonResult(list);
        }
    }
}

运行结果如下所示：

[
  {
    "id": 1,
    "personName": "张三",
    "personGender": "男",
    "birthOfDate": "2000年01月01日 00时00分00秒"
  },
  {
    "id": 2,
    "personName": "李四",
    "personGender": "女",
    "birthOfDate": "2000年02月02日 00时00分00秒"
  },
  {
    "id": 3,
    "personName": "王五",
    "personGender": "男",
    "birthOfDate": "2000年03月03日 00时00分00秒"
  }
]

6、结语

本文简单介绍了ASP.NET Core中关于JSON序列化和反序列化操作，主要通过System.Text.Json来实现。如果你觉得微软提供的JSON序列化工具不好用，那也可以使用Newtonsoft.Json，我也会在下一篇博客中介绍关于Newtonsoft.Json的使用方法。