如何使用 System.Text.Json 实例化 JsonSerializerOptions 实例

项目
02/16/2024

本文介绍如何避免在使用 JsonSerializerOptions 时出现性能问题。还介绍了如何使用可用的参数化构造函数。

重用 JsonSerializerOptions 实例

如果你通过相同的选项重复使用 JsonSerializerOptions，则请勿在每次使用时都创建新的 JsonSerializerOptions 实例。对每个调用重复使用同一实例。本指南适用于你为自定义转换器编写的代码，以及调用 JsonSerializer.Serialize 或 JsonSerializer.Deserialize时。在多个线程中使用相同的实例是安全的。选项实例上的元数据缓存是线程安全的，并且实例在第一次序列化或反序列化之后不可变。

以下代码演示使用新的选项实例所带来的性能损失。

using System.Diagnostics;
using System.Text.Json;

namespace OptionsPerfDemo
{
    public record Forecast(DateTime Date, int TemperatureC, string Summary);

    public class Program
    {
        public static void Main()
        {
            Forecast forecast = new(DateTime.Now, 40, "Hot");
            JsonSerializerOptions options = new() { WriteIndented = true };
            int iterations = 100000;

            var watch = Stopwatch.StartNew();
            for (int i = 0; i < iterations; i++)
            {
                Serialize(forecast, options);
            }
            watch.Stop();
            Console.WriteLine($"Elapsed time using one options instance: {watch.ElapsedMilliseconds}");

            watch = Stopwatch.StartNew();
            for (int i = 0; i < iterations; i++)
            {
                Serialize(forecast);
            }
            watch.Stop();
            Console.WriteLine($"Elapsed time creating new options instances: {watch.ElapsedMilliseconds}");
        }

        private static void Serialize(Forecast forecast, JsonSerializerOptions? options = null)
        {
            _ = JsonSerializer.Serialize<Forecast>(
                forecast,
                options ?? new JsonSerializerOptions() { WriteIndented = true });
        }
    }
}

// Produces output like the following example:
//
//Elapsed time using one options instance: 190
//Elapsed time creating new options instances: 40140

前面的代码使用相同的选项实例对一个小型对象进行了 100,000 次序列化。然后，它对相同的对象进行了相同次数的序列化，且每次都创建一个新的选项实例。典型运行时的差值为 190 毫秒，而不是 40,140 毫秒。如果增加迭代的次数，差值甚至越大。

当向序列化程序传递新的选项实例时，在对象图中每种类型的第一次序列化期间，序列化程序将执行一个预热阶段。此预热包括创建序列化所需的元数据的缓存。元数据包括对属性 Getter 的委托、Setter、构造函数参数、指定特性等。此元数据缓存存储在选项实例中。相同的预热过程和缓存适用于反序列化。

JsonSerializerOptions 实例中元数据缓存的大小取决于要序列化的类型的数量。如果向序列化程序传递多种类型（例如动态生成的类型），缓存大小将继续增长，最终可导致 OutOfMemoryException。

`JsonSerializerOptions.Default` 属性

如果需要使用的 JsonSerializerOptions 的实例为默认实例（具有所有默认设置和默认转换器），请使用 JsonSerializerOptions.Default 属性，而不是创建一个实例选项。有关详细信息，请参阅使用默认系统转换器。

复制 JsonSerializerOptions

存在 JsonSerializerOptions constructor，可让你使用与现有实例相同的选项来创建新的实例，如以下示例中所示：

using System.Text.Json;

namespace CopyOptions
{
    public class Forecast
    {
        public DateTime Date { get; init; }
        public int TemperatureC { get; set; }
        public string? Summary { get; set; }
    };

    public class Program
    {
        public static void Main()
        {
            Forecast forecast = new()
            {
                Date = DateTime.Now,
                TemperatureC = 40,
                Summary = "Hot"
            };

            JsonSerializerOptions options = new()
            {
                WriteIndented = true
            };

            JsonSerializerOptions optionsCopy = new(options);
            string forecastJson =
                JsonSerializer.Serialize<Forecast>(forecast, optionsCopy);

            Console.WriteLine($"Output JSON:\n{forecastJson}");
        }
    }
}

// Produces output like the following example:
//
//Output JSON:
//{
//  "Date": "2020-10-21T15:40:06.8998502-07:00",
//  "TemperatureC": 40,
//  "Summary": "Hot"
//}

Imports System.Text.Json
Imports System.Text.Json.Serialization

Namespace CopyOptions

    Public Class Forecast
        Public Property [Date] As Date
        Public Property TemperatureC As Integer
        Public Property Summary As String
    End Class

    Public NotInheritable Class Program

        Public Shared Sub Main()
            Dim forecast1 As New Forecast() With {
                .[Date] = Date.Now,
                .Summary = Nothing,
                .TemperatureC = CType(Nothing, Integer)
            }

            Dim options As New JsonSerializerOptions() With {
                .DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault
            }

            Dim optionsCopy As New JsonSerializerOptions
            Dim forecastJson As String = JsonSerializer.Serialize(forecast1, optionsCopy)

            Console.WriteLine($"Output JSON:{forecastJson}")
        End Sub

    End Class

End Namespace

' Produces output like the following example:
'
'Output JSON:
'{
'  "Date": "2020-10-21T15:40:06.8998502-07:00",
'  "TemperatureC": 40,
'  "Summary": "Hot"
'}

现有 JsonSerializerOptions 实例的元数据缓存不会复制到新实例。因此，使用此构造函数与重用现有实例 JsonSerializerOptions 不同。

JsonSerializerOptions 的 Web 默认值

对于 Web 应用，以下选项具有不同的默认值：

在 .NET 9 及更高版本中，可使用 JsonSerializerOptions.Web 单一实例通过 ASP.NET Core 用于 Web 应用的默认选项进行序列化。在更低的版本中，请调用 JsonSerializerOptions 构造函数来使用 Web 默认值创建新实例，如以下示例所示：

using System.Text.Json;

namespace OptionsDefaults
{
    public class Forecast
    {
        public DateTime? Date { get; init; }
        public int TemperatureC { get; set; }
        public string? Summary { get; set; }
    };

    public class Program
    {
        public static void Main()
        {
            Forecast forecast = new()
            {
                Date = DateTime.Now,
                TemperatureC = 40,
                Summary = "Hot"
            };

            JsonSerializerOptions options = new(JsonSerializerDefaults.Web)
            {
                WriteIndented = true
            };

            Console.WriteLine(
                $"PropertyNameCaseInsensitive: {options.PropertyNameCaseInsensitive}");
            Console.WriteLine(
                $"JsonNamingPolicy: {options.PropertyNamingPolicy}");
            Console.WriteLine(
                $"NumberHandling: {options.NumberHandling}");

            string forecastJson = JsonSerializer.Serialize<Forecast>(forecast, options);
            Console.WriteLine($"Output JSON:\n{forecastJson}");

            Forecast? forecastDeserialized =
                JsonSerializer.Deserialize<Forecast>(forecastJson, options);

            Console.WriteLine($"Date: {forecastDeserialized?.Date}");
            Console.WriteLine($"TemperatureC: {forecastDeserialized?.TemperatureC}");
            Console.WriteLine($"Summary: {forecastDeserialized?.Summary}");
        }
    }
}

// Produces output like the following example:
//
//PropertyNameCaseInsensitive: True
//JsonNamingPolicy: System.Text.Json.JsonCamelCaseNamingPolicy
//NumberHandling: AllowReadingFromString
//Output JSON:
//{
//  "date": "2020-10-21T15:40:06.9040831-07:00",
//  "temperatureC": 40,
//  "summary": "Hot"
//}
//Date: 10 / 21 / 2020 3:40:06 PM
//TemperatureC: 40
//Summary: Hot

Imports System.Text.Json

Namespace OptionsDefaults

    Public Class Forecast
        Public Property [Date] As Date
        Public Property TemperatureC As Integer
        Public Property Summary As String
    End Class

    Public NotInheritable Class Program

        Public Shared Sub Main()
            Dim forecast1 As New Forecast() With {
                .[Date] = Date.Now,
                .TemperatureC = 40,
                .Summary = "Hot"
                }

            Dim options As New JsonSerializerOptions(JsonSerializerDefaults.Web) With {
                .WriteIndented = True
                }

            Console.WriteLine(
                $"PropertyNameCaseInsensitive: {options.PropertyNameCaseInsensitive}")
            Console.WriteLine(
                $"JsonNamingPolicy: {options.PropertyNamingPolicy}")
            Console.WriteLine(
                $"NumberHandling: {options.NumberHandling}")

            Dim forecastJson As String = JsonSerializer.Serialize(forecast1, options)
            Console.WriteLine($"Output JSON:{forecastJson}")

            Dim forecastDeserialized As Forecast = JsonSerializer.Deserialize(Of Forecast)(forecastJson, options)

            Console.WriteLine($"Date: {forecastDeserialized.[Date]}")
            Console.WriteLine($"TemperatureC: {forecastDeserialized.TemperatureC}")
            Console.WriteLine($"Summary: {forecastDeserialized.Summary}")
        End Sub

    End Class

End Namespace

' Produces output like the following example:
'
'PropertyNameCaseInsensitive: True
'JsonNamingPolicy: System.Text.Json.JsonCamelCaseNamingPolicy
'NumberHandling: AllowReadingFromString
'Output JSON:
'{
'  "date": "2020-10-21T15:40:06.9040831-07:00",
'  "temperatureC": 40,
'  "summary": "Hot"
'}
'Date: 10 / 21 / 2020 3:40:06 PM
'TemperatureC: 40
'Summary: Hot

通过

如何使用 System.Text.Json 实例化 JsonSerializerOptions 实例

重用 JsonSerializerOptions 实例

JsonSerializerOptions.Default 属性

复制 JsonSerializerOptions

JsonSerializerOptions 的 Web 默认值

其他资源

`JsonSerializerOptions.Default` 属性