跨项目序列化派生类遇XmlSerializer报错，如何解决类型未预期问题？

问题原因 #

当使用基类Car的泛型序列化器序列化派生类SpecialCar实例时，XmlSerializer在编译期仅知晓基类类型，无法识别运行时传入的派生类，因此抛出类型未预期的错误。由于基类项目无法引用派生类项目，直接在Car类上添加XmlInclude特性不可行。

解决方案 #

修改基类项目中的泛型序列化器，增加支持传入额外类型的重载方法，在创建XmlSerializer时显式告知需要处理的派生类型。

1. 修改基类项目的Seriallizer类 #

添加重载的序列化方法，接受额外类型数组参数：

Public Class Seriallizer(Of T)
    ' 原无参方法，调用重载方法
    Function SerializeObjToString(obj As T) As String
        Return SerializeObjToString(obj, Nothing)
    End Function

    ' 新增重载方法，支持传入额外类型
    Function SerializeObjToString(obj As T, additionalTypes As Type()) As String
        Dim serializer As XmlSerializer
        Dim memStream As New MemoryStream
        Dim xmlWriter As New XmlTextWriter(memStream, Encoding.UTF8)
        Try
            ' 根据是否有额外类型选择XmlSerializer构造函数
            If additionalTypes IsNot Nothing AndAlso additionalTypes.Length > 0 Then
                serializer = New XmlSerializer(GetType(T), additionalTypes)
            Else
                serializer = New XmlSerializer(GetType(T))
            End If
            serializer.Serialize(xmlWriter, obj)
            Dim xml As String
            xml = Encoding.UTF8.GetString(memStream.GetBuffer())
            xml = xml.Substring(xml.IndexOf(Convert.ToChar(60)))
            xml = xml.Substring(0, xml.LastIndexOf(Convert.ToChar(62)) + 1)
            Return xml
        Catch e As Exception
            MsgBox($"Error writing data to string: {e.Message}")
        Finally
            xmlWriter.Close()
            memStream.Close()
            xmlWriter = Nothing
            serializer = Nothing
            memStream = Nothing
        End Try
        Return ""
    End Function
End Class

2. 在派生类项目中调用序列化方法 #

调用时传入SpecialCar的类型，告知XmlSerializer需要处理该派生类型：

Dim spezicalCar As New SpecialCar
spezicalCar.Name = "Demo Car"
spezicalCar.VentorName = "Demo Vendor"
Dim serializer As New Seriallizer(Of Car)
' 传入包含SpecialCar的类型数组
Dim str As String = serializer.SerializeObjToString(spezicalCar, {GetType(SpecialCar)})

原理说明 #

XmlSerializer提供了带Type[]参数的构造函数，用于指定编译期未知但运行时需要序列化的派生类型。这种方式无需修改基类的特性，避免了基类项目对派生类项目的依赖，完美解决跨项目派生类的序列化问题。

内容的提问来源于stack exchange，提问作者gegy