VB.Net中使用System.Json反序列化Object类型类属性的问题咨询

一、解决JsonConverter的废弃错误 #

你遇到的方法签名废弃问题，是因为System.Text.Json的API更新：原Read方法中Utf8JsonReader参数的ByRef写法已被淘汰，现在需要改为ByVal。VB.Net完全支持更新后的API，正确的方法签名应为：

Public Overrides Function Read(ByVal reader As Utf8JsonReader, typeToConvert As Type, options As JsonSerializerOptions) As Object

二、自定义转换器处理Object类型反序列化 #

由于OriginalValue和NewValue是Object类型，反序列化时需要根据PropertyName映射到具体类型（如Demand、Location），可通过自定义转换器实现：

1. 实现Change类的自定义转换器

Public Class ChangeConverter
    Inherits JsonConverter(Of Change)

    Public Overrides Function Read(ByVal reader As Utf8JsonReader, typeToConvert As Type, options As JsonSerializerOptions) As Change
        If reader.TokenType <> JsonTokenType.StartObject Then
            Throw New JsonException("JSON格式错误：预期对象起始标记")
        End If

        Dim change As New Change()
        Dim currentPropName As String = Nothing

        While reader.Read()
            If reader.TokenType = JsonTokenType.EndObject Then
                Return change
            End If

            If reader.TokenType = JsonTokenType.PropertyName Then
                currentPropName = reader.GetString()
                reader.Read()
                Select Case currentPropName
                    Case "PropertyName"
                        change.PropertyName = reader.GetString()
                    Case "OriginalValue"
                        change.OriginalValue = DeserializeTargetValue(reader, change.PropertyName, options)
                    Case "NewValue"
                        change.NewValue = DeserializeTargetValue(reader, change.PropertyName, options)
                    Case Else
                        reader.Skip() ' 跳过未知属性
                End Select
            End If
        End While

        Throw New JsonException("JSON格式错误：未找到对象结束标记")
    End Function

    ' 根据PropertyName映射目标类型并反序列化
    Private Function DeserializeTargetValue(ByVal reader As Utf8JsonReader, propName As String, options As JsonSerializerOptions) As Object
        Dim targetType As Type = propName.ToLower() switch {
            "demand" => GetType(Demand),
            "location" => GetType(Location),
            ' 在此添加其他需要处理的类型映射
            Else => GetType(Object)
        }
        Return JsonSerializer.Deserialize(reader, targetType, options)
    End Function

    Public Overrides Sub Write(ByVal writer As Utf8JsonWriter, value As Change, options As JsonSerializerOptions)
        JsonSerializer.Serialize(writer, value, options)
    End Sub
End Class

2. 反序列化时注册转换器

Dim options As New JsonSerializerOptions()
options.Converters.Add(New ChangeConverter())

' 执行反序列化
Dim model = JsonSerializer.Deserialize(Of Model)(jsonContent, options)

三、替代方案：开启类型信息自动识别 #

若不想编写自定义转换器，可在序列化时开启类型信息保留，让System.Text.Json自动识别目标类型：

Dim serializeOptions As New JsonSerializerOptions()
serializeOptions.WriteIndented = True
serializeOptions.TypeInfoResolver = New DefaultJsonTypeInfoResolver() With {
    .Modifiers = {
        Sub(typeInfo)
            If typeInfo.Type = GetType(Object) Then
                typeInfo.PolymorphismOptions = New JsonPolymorphismOptions() With {
                    .TypeDiscriminatorPropertyName = "$type",
                    .IgnoreUnrecognizedTypeDiscriminators = True,
                    .DerivedTypes = {
                        New JsonDerivedType(GetType(Demand), "Demand"),
                        New JsonDerivedType(GetType(Location), "Location")
                        ' 添加其他需要识别的类型
                    }
                }
            End If
        End Sub
    }
}

' 序列化
Dim jsonString = JsonSerializer.Serialize(model, serializeOptions)
' 反序列化使用相同配置
Dim deserializedModel = JsonSerializer.Deserialize(Of Model)(jsonString, serializeOptions)

关于VB.Net支持Json转换的说明 #

网传“VB.Net不再支持Json转换”的说法完全错误，VB.Net完全兼容System.Text.Json的所有功能，包括自定义转换器。你遇到的废弃提示仅为API版本更新导致的参数写法变更，调整签名即可解决。

内容的提问来源于stack exchange，提问作者Chiel