使用不可變的類型和屬性

發行項
03/14/2024

不可變的類型是一種可讓您在物件具現化之後，變更物件之任何屬性或欄位值的類型。該類型可能是記錄、沒有公用屬性或欄位、具有唯讀屬性，或具有私人或僅限 init setter 的屬性。 System.String 是不可變類型的範例。 System.Text.Json 提供將 JSON 還原序列化為不可變類型的不同方式。

參數化建構函式

依預設，System.Text.Json 使用預設公用無參數建構函式。不過，您可以使用參數化建構函式，如此可還原序列化不可變的類別或結構。

針對類別，如果唯一的建構函式是參數化建構函式，則會使用該建構函式。

針對結構或具有多個建構函式的類別，請套用 [JsonConstructor] 屬性來指定要使用的項目。未使用屬性時，一律會使用公開的無參數建構函式 (如果存在)。

下列範例會使用 [JsonConstructor] 屬性：

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

namespace ImmutableTypes
{
    public struct Forecast
    {
        public DateTime Date { get; }
        public int TemperatureC { get; }
        public string Summary { get; }

        [JsonConstructor]
        public Forecast(DateTime date, int temperatureC, string summary) =>
            (Date, TemperatureC, Summary) = (date, temperatureC, summary);
    }

    public class Program
    {
        public static void Main()
        {
            string json = """
                {
                    "date":"2020-09-06T11:31:01.923395-07:00",
                    "temperatureC":-1,
                    "summary":"Cold"
                }
                """;
            Console.WriteLine($"Input JSON: {json}");

            var options = JsonSerializerOptions.Web;

            Forecast forecast = JsonSerializer.Deserialize<Forecast>(json, options);

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

            string roundTrippedJson =
                JsonSerializer.Serialize<Forecast>(forecast, options);

            Console.WriteLine($"Output JSON: {roundTrippedJson}");
        }
    }
}

// Produces output like the following example:
//
//Input JSON: { "date":"2020-09-06T11:31:01.923395-07:00","temperatureC":-1,"summary":"Cold"}
//forecast.Date: 9 / 6 / 2020 11:31:01 AM
//forecast.TemperatureC: -1
//forecast.Summary: Cold
//Output JSON: { "date":"2020-09-06T11:31:01.923395-07:00","temperatureC":-1,"summary":"Cold"}

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

Namespace ImmutableTypes

    Public Structure Forecast
        Public ReadOnly Property [Date] As Date
        Public ReadOnly Property TemperatureC As Integer
        Public ReadOnly Property Summary As String

        <JsonConstructor>
        Public Sub New([Date] As Date, TemperatureC As Integer, Summary As String)
            Me.Date = [Date]
            Me.TemperatureC = TemperatureC
            Me.Summary = Summary
        End Sub

    End Structure

    Public NotInheritable Class Program

        Public Shared Sub Main()
            Dim json As String = "{""date"":""2020-09-06T11:31:01.923395-07:00"",""temperatureC"":-1,""summary"":""Cold""}"
            Console.WriteLine($"Input JSON: {json}")

            Dim forecast1 As Forecast = JsonSerializer.Deserialize(Of Forecast)(json, JsonSerializerOptions.Web)

            Console.WriteLine($"forecast.Date: {forecast1.[Date]}")
            Console.WriteLine($"forecast.TemperatureC: {forecast1.TemperatureC}")
            Console.WriteLine($"forecast.Summary: {forecast1.Summary}")

            Dim roundTrippedJson As String = JsonSerializer.Serialize(forecast1, JsonSerializerOptions.Web)

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

    End Class

End Namespace

' Produces output like the following example:
'
'Input JSON: { "date":"2020-09-06T11:31:01.923395-07:00","temperatureC":-1,"summary":"Cold"}
'forecast.Date: 9 / 6 / 2020 11:31:01 AM
'forecast.TemperatureC: -1
'forecast.Summary: Cold
'Output JSON: { "date":"2020-09-06T11:31:01.923395-07:00","temperatureC":-1,"summary":"Cold"}

在 .NET 7 和舊版中，[JsonConstructor] 屬性只能與公用建構函式搭配使用。

參數化建構函式的參數名稱必須符合屬性名稱與型別。比對不區分大小寫，而且即使使用 [JsonPropertyName] 來為屬性重新命名，建構函式參數也必須符合實際的屬性名稱。在下列範例中，TemperatureC 屬性的名稱在 JSON 中會變更為 celsius，但建構函式參數仍名為 temperatureC：

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

namespace ImmutableTypesCtorParms
{
    public readonly struct Forecast
    {
        public DateTime Date { get; }
        [JsonPropertyName("celsius")]
        public int TemperatureC { get; }
        public string Summary { get; }

        [JsonConstructor]
        public Forecast(DateTime date, int temperatureC, string summary) =>
            (Date, TemperatureC, Summary) = (date, temperatureC, summary);
    }

    public class Program
    {
        public static void Main()
        {
            string json = """
                {
                    "date":"2020-09-06T11:31:01.923395-07:00",
                    "celsius":-1,
                    "summary":"Cold"
                }
                """;
            Console.WriteLine($"Input JSON: {json}");

            var options = JsonSerializerOptions.Web;

            Forecast forecast = JsonSerializer.Deserialize<Forecast>(json, options);

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

            string roundTrippedJson =
                JsonSerializer.Serialize<Forecast>(forecast, options);

            Console.WriteLine($"Output JSON: {roundTrippedJson}");

        }
    }
}

// Produces output like the following example:
//
//Input JSON: { "date":"2020-09-06T11:31:01.923395-07:00","celsius":-1,"summary":"Cold"}
//forecast.Date: 9 / 6 / 2020 11:31:01 AM
//forecast.TemperatureC: -1
//forecast.Summary: Cold
//Output JSON: { "date":"2020-09-06T11:31:01.923395-07:00","celsius":-1,"summary":"Cold"}

除了 [JsonPropertyName] 之外，下列屬性也支援使用參數化建構函式還原序列化：

記錄

序列化和還原串行化也支援記錄，如下列範例所示：

using System.Text.Json;

namespace Records
{
    public record Forecast(DateTime Date, int TemperatureC)
    {
        public string? Summary { get; init; }
    };

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

            string forecastJson = JsonSerializer.Serialize<Forecast>(forecast);
            Console.WriteLine(forecastJson);
            Forecast? forecastObj = JsonSerializer.Deserialize<Forecast>(forecastJson);
            Console.WriteLine(forecastObj);
        }
    }
}

// Produces output like the following example:
//
//{ "Date":"2020-10-21T15:26:10.5044594-07:00","TemperatureC":40,"Summary":"Hot!"}
//Forecast { Date = 10 / 21 / 2020 3:26:10 PM, TemperatureC = 40, Summary = Hot! }

您可以使用屬性 (attribute) 上的 property: 目標，將任何屬性 (attribute) 套用至屬性 (property) 名稱。如需有關位置記錄的詳細資訊，請參閱 C# 語言參考中有關記錄的文章。

非公用成員和屬性存取子

您可以使用 [JsonInclude] 屬性，在屬性上啟用非公用屬性存取子的使用，如下列範例所示：

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

namespace NonPublicAccessors
{
    public class Forecast
    {
        public DateTime Date { get; init; }

        [JsonInclude]
        public int TemperatureC { get; private set; }

        [JsonInclude]
        public string? Summary { private get; set; }
    };

    public class Program
    {
        public static void Main()
        {
            string json = """
                {
                    "Date":"2020-10-23T09:51:03.8702889-07:00",
                    "TemperatureC":40,
                    "Summary":"Hot"
                }
                """;
            Console.WriteLine($"Input JSON: {json}");

            Forecast forecastDeserialized = JsonSerializer.Deserialize<Forecast>(json)!;
            Console.WriteLine($"Date: {forecastDeserialized.Date}");
            Console.WriteLine($"TemperatureC: {forecastDeserialized.TemperatureC}");

            json = JsonSerializer.Serialize<Forecast>(forecastDeserialized);
            Console.WriteLine($"Output JSON: {json}");
        }
    }
}

// Produces output like the following example:
//
//Input JSON: { "Date":"2020-10-23T09:51:03.8702889-07:00","TemperatureC":40,"Summary":"Hot"}
//Date: 10 / 23 / 2020 9:51:03 AM
//TemperatureC: 40
//Output JSON: { "Date":"2020-10-23T09:51:03.8702889-07:00","TemperatureC":40,"Summary":"Hot"}

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

Namespace NonPublicAccessors

    Public Class Forecast
        Public Property [Date] As Date

        Private _temperatureC As Integer

        <JsonInclude>
        Public Property TemperatureC As Integer
            Get
                Return _temperatureC
            End Get
            Private Set(Value As Integer)
                _temperatureC = Value
            End Set
        End Property

        Private _summary As String

        <JsonInclude>
        Public Property Summary As String
            Private Get
                Return _summary
            End Get
            Set(Value As String)
                _summary = Value
            End Set
        End Property

    End Class

    Public NotInheritable Class Program

        Public Shared Sub Main()
            Dim json As String = "{""Date"":""2020-10-23T09:51:03.8702889-07:00"",""TemperatureC"":40,""Summary"":""Hot""}"
            Console.WriteLine($"Input JSON: {json}")

            Dim forecastDeserialized As Forecast = JsonSerializer.Deserialize(Of Forecast)(json)
            Console.WriteLine($"Date: {forecastDeserialized.[Date]}")
            Console.WriteLine($"TemperatureC: {forecastDeserialized.TemperatureC}")

            json = JsonSerializer.Serialize(forecastDeserialized)
            Console.WriteLine($"Output JSON: {json}")
        End Sub

    End Class

End Namespace

' Produces output like the following example:
'
'Input JSON: { "Date":"2020-10-23T09:51:03.8702889-07:00","TemperatureC":40,"Summary":"Hot"}
'Date: 10 / 23 / 2020 9:51:03 AM
'TemperatureC: 40
'Output JSON: { "Date":"2020-10-23T09:51:03.8702889-07:00","TemperatureC":40,"Summary":"Hot"}

藉由包含具有私人 setter 的屬性，您仍然可以還原序列化該屬性。

在 .NET 8 和更新版本中，您也可以使用 [JsonInclude] 屬性，選擇將非公用成員加入指定類型的序列化合約。

注意

在來源產生模式中，您無法透過使用 [JsonInclude] 屬性對成員進行標註，來序列化 private 成員或使用 private 存取子。而且，如果他們與產生的 JsonSerializerContext 位於相同的組件中，您才能序列化 internal 成員或使用 internal 存取子。

唯讀屬性

在 .NET 8 和更新版本中，唯讀屬性，或那些沒有私人或公用的 setter 屬性也可以還原序列化。雖然您無法變更屬性所參考的執行個體，但如果屬性的類型是可變的，您可以加以修改。例如，您可以將元素新增至清單。若要還原序列化唯讀屬性，您必須將其物件建立處理行為設定為填入，而不是取代。例如，您可以使用 JsonObjectCreationHandlingAttribute 屬性 (attribute) 標註屬性 (property)。

class A
{
    [JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
    public List<int> Numbers1 { get; } = new List<int>() { 1, 2, 3 };
}

如需詳細資訊，請參閱填入初始化的屬性。

共用方式為

使用不可變的類型和屬性

參數化建構函式

記錄

非公用成員和屬性存取子

唯讀屬性

另請參閱

其他資源