面试题：Visual Basic反序列化时处理版本兼容性问题

Question

当使用Visual Basic进行序列化与反序列化时，若被序列化的类结构发生了变化（例如增加或删除了字段），怎样在反序列化过程中确保兼容性，避免反序列化失败？请阐述思路并给出可能用到的技术手段和代码示例。

Answer 1

思路

1. 版本控制：在类中添加版本号字段，序列化时记录当前版本，反序列化时根据版本号采取不同策略。
2. 可选字段处理：对于新增字段，在反序列化时设置默认值；对于删除字段，反序列化时忽略。

技术手段

1. 添加版本号字段：在类中定义一个字段专门用于记录版本号。
2. 自定义序列化/反序列化方法：通过实现 ISerializable 接口来自定义序列化和反序列化逻辑。

代码示例

Imports System
Imports System.IO
Imports System.Runtime.Serialization
Imports System.Runtime.Serialization.Formatters.Binary

<Serializable()>
Public Class MyClass
    Implements ISerializable

    Private Const VersionNumber As Integer = 1
    Private _field1 As String
    Private _field2 As Integer
    '新增字段示例
    Private _newField As Double

    Public Sub New()
        '默认构造函数
    End Sub

    Public Sub New(ByVal info As SerializationInfo, ByVal context As StreamingContext) Implements ISerializable.GetObjectData
        Dim version As Integer = info.GetInt32("Version")
        _field1 = info.GetString("Field1")
        _field2 = info.GetInt32("Field2")
        If version >= 1 Then
            _newField = info.GetDouble("NewField")
        End If
    End Sub

    Public Sub GetObjectData(ByVal info As SerializationInfo, ByVal context As StreamingContext) Implements ISerializable.GetObjectData
        info.AddValue("Version", VersionNumber)
        info.AddValue("Field1", _field1)
        info.AddValue("Field2", _field2)
        info.AddValue("NewField", _newField)
    End Sub

    '示例方法
    Public Sub SerializeObject(ByVal filePath As String)
        Using stream As New FileStream(filePath, FileMode.Create)
            Dim formatter As New BinaryFormatter()
            formatter.Serialize(stream, Me)
        End Using
    End Sub

    Public Shared Function DeserializeObject(ByVal filePath As String) As MyClass
        Using stream As New FileStream(filePath, FileMode.Open)
            Dim formatter As New BinaryFormatter()
            Return CType(formatter.Deserialize(stream), MyClass)
        End Using
    End Function
End Class

使用示例

Module Module1
    Sub Main()
        '序列化对象
        Dim obj As New MyClass()
        obj._field1 = "Hello"
        obj._field2 = 42
        obj._newField = 3.14
        obj.SerializeObject("test.bin")

        '反序列化对象
        Dim deserializedObj As MyClass = MyClass.DeserializeObject("test.bin")
        Console.WriteLine($"Field1: {deserializedObj._field1}, Field2: {deserializedObj._field2}, NewField: {deserializedObj._newField}")
    End Sub
End Module