如何使用 System.Text.Json 忽略屬性

發行項
02/20/2024

將 C# 物件序列化為 JavaScript 物件標記法 (JSON) 時，所有公用屬性預設都會序列化。如果不想讓其中某些屬性出現在產生的 JSON 中，有幾個選項可供選擇。在本文中，您將了解如何根據各種準則忽略屬性：

個別屬性
所有唯讀屬性
所有 Null 值屬性
所有預設值屬性

忽略個別屬性

若要忽略個別屬性，請使用 [JsonIgnore] 屬性。

下例示範要序列化的類型。它也會顯示 JSON 輸出：

public class WeatherForecastWithIgnoreAttribute
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureCelsius { get; set; }
    [JsonIgnore]
    public string? Summary { get; set; }
}

Public Class WeatherForecastWithIgnoreAttribute
    Public Property [Date] As DateTimeOffset
    Public Property TemperatureCelsius As Integer

    <JsonIgnore>
    Public Property Summary As String

End Class

{
  "Date": "2019-08-01T00:00:00-07:00",
  "TemperatureCelsius": 25,
}

您可以設定 [JsonIgnore] 屬性 (Attribute) 的 Condition 屬性 (Property) 來指定條件式排除。 JsonIgnoreCondition 列舉提供下列選項：

Always - 一律忽略屬性。如果未指定任何 Condition，則會假設為此選項。
Never - 無論DefaultIgnoreCondition、IgnoreReadOnlyProperties 和 IgnoreReadOnlyFields 全域設定為何，一律會序列化與還原序列化該屬性。
WhenWritingDefault - 若為參考型別 null、可為 Null 的實值型別 null 或實值型別 default，則會在序列化時忽略該屬性。
WhenWritingNull - 若為參考型別 null 或可為 Null 的實值型別 null，則會在序列化時忽略該屬性。

下例示範如何使用 [JsonIgnore] 屬性 (Attribute) 的 Condition 屬性 (Property)：

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

namespace JsonIgnoreAttributeExample
{
    public class Forecast
    {
        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
        public DateTime Date { get; set; }

        [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
        public int TemperatureC { get; set; }

        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
        public string? Summary { get; set; }
    };

    public class Program
    {
        public static void Main()
        {
            Forecast forecast = new()
            {
                Date = default,
                Summary = null,
                TemperatureC = default
            };

            JsonSerializerOptions options = new()
            {
                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault
            };

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

            Console.WriteLine(forecastJson);
        }
    }
}

// Produces output like the following example:
//
//{"TemperatureC":0}

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

Namespace JsonIgnoreAttributeExample

    Public Class Forecast

        <JsonIgnore(Condition:=JsonIgnoreCondition.WhenWritingDefault)>
        Public Property [Date] As Date

        <JsonIgnore(Condition:=JsonIgnoreCondition.Never)>
        Public Property TemperatureC As Integer

        <JsonIgnore(Condition:=JsonIgnoreCondition.WhenWritingNull)>
        Public Property Summary As String

    End Class

    Public NotInheritable Class Program

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

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

            Dim forecastJson As String = JsonSerializer.Serialize(forecast1, options)

            Console.WriteLine(forecastJson)
        End Sub

    End Class

End Namespace

' Produces output like the following example:
'
'{"TemperatureC":0}

忽略所有唯讀屬性

如果屬性包含公用 getter，卻不包含公用 setter，則屬性即為唯讀。若要在序列化時忽略所有唯讀屬性，請將 JsonSerializerOptions.IgnoreReadOnlyProperties 設定為 true，如下列範例所示：

var options = new JsonSerializerOptions
{
    IgnoreReadOnlyProperties = true,
    WriteIndented = true
};
jsonString = JsonSerializer.Serialize(weatherForecast, options);

Dim options As JsonSerializerOptions = New JsonSerializerOptions With {
    .IgnoreReadOnlyProperties = True,
    .WriteIndented = True
}
jsonString = JsonSerializer.Serialize(weatherForecast, options)

下例示範要序列化的類型。它也會顯示 JSON 輸出：

public class WeatherForecastWithROProperty
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureCelsius { get; set; }
    public string? Summary { get; set; }
    public int WindSpeedReadOnly { get; private set; } = 35;
}

Public Class WeatherForecastWithROProperty
    Public Property [Date] As DateTimeOffset
    Public Property TemperatureCelsius As Integer
    Public Property Summary As String
    Private _windSpeedReadOnly As Integer

    Public Property WindSpeedReadOnly As Integer
        Get
            Return _windSpeedReadOnly
        End Get
        Private Set(Value As Integer)
            _windSpeedReadOnly = Value
        End Set
    End Property

End Class

{
  "Date": "2019-08-01T00:00:00-07:00",
  "TemperatureCelsius": 25,
  "Summary": "Hot",
}

此選項僅適用於屬性。若要在序列化欄位時忽略唯讀欄位，請使用 JsonSerializerOptions.IgnoreReadOnlyFields 全域設定。

注意

即使 JsonSerializerOptions.IgnoreReadOnlyProperties 設定為 true，唯讀集合型別屬性仍會序列化。

忽略所有 Null 值屬性

若要忽略所有 Null 值屬性，請將 DefaultIgnoreCondition 屬性設定為 WhenWritingNull，如下列範例所示：

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

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

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

            JsonSerializerOptions options = new()
            {
                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
            };

            string forecastJson =
                JsonSerializer.Serialize<Forecast>(forecast, options);
            
            Console.WriteLine(forecastJson);
        }
    }
}

// Produces output like the following example:
//
//{"Date":"2020-10-30T10:11:40.2359135-07:00","TemperatureC":0}

Imports System.Text.Json

Namespace IgnoreNullOnSerialize

    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

            Dim forecastJson As String = JsonSerializer.Serialize(forecast1, options)

            Console.WriteLine(forecastJson)
        End Sub

    End Class

End Namespace

' Produces output like the following example:
'
'{"Date":"2020-10-30T10:11:40.2359135-07:00","TemperatureC":0}

忽略所有預設值屬性

若要防止實值型別屬性中的預設值序列化，請將 DefaultIgnoreCondition 屬性設定為 WhenWritingDefault，如下列範例所示：

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

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

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

            JsonSerializerOptions options = new()
            {
                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault
            };

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

            Console.WriteLine(forecastJson);
        }
    }
}

// Produces output like the following example:
//
//{ "Date":"2020-10-21T15:40:06.8920138-07:00"}

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

Namespace IgnoreValueDefaultOnSerialize

    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 forecastJson As String = JsonSerializer.Serialize(forecast1, options)

            Console.WriteLine(forecastJson)
        End Sub

    End Class

End Namespace

' Produces output like the following example:
'
'{ "Date":"2020-10-21T15:40:06.8920138-07:00"}

此 WhenWritingDefault 設定也會防止 Null 值參考型別和可為 Null 實值型別屬性序列化。

如何使用 System.Text.Json 忽略屬性

忽略個別屬性

忽略所有唯讀屬性

忽略所有 Null 值屬性

忽略所有預設值屬性

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