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我很难从一个多次重载的子程序中选择正确的子程序，每个子程序都有两个参数。也许某种形式的双重分派是合适的，但是当有两个参数需要检查两个（可能）不同的类型时，我看不到任何整洁的方法。

从本质上讲，我创建的示例代码可以按需要执行，但在我看来这是一种非常糟糕的方法。由于类型是在编译时确定的，因此我不能通过传入我要测试的两个对象来简单地调用我想要的 sub： 

test(store(0),store(1))

只会调用带有签名的测试子程序

Sub test(cA As Class1, cB As Class1)

因为 store(x) 的静态类型总是 Class1。

某种复杂的双重调度形式可行还是有更好的解决方案？

Option Strict On
Module Module1

    Sub Main()
        Dim store As List(Of Class1) = New List(Of Class1)
        store.Add(New Class2)
        store.Add(New Class3)
        If TypeOf (store(0)) Is Class2 Then
            If TypeOf store(1) Is Class2 Then
                test(CType(store(0), Class2), CType(store(1), Class2))
            ElseIf TypeOf store(1) Is Class3 Then
                test(CType(store(0), Class2), CType(store(1), Class3))
                'and so on...
            End If

        ElseIf TypeOf (store(0)) Is Class3 Then
            'relevant tests of store(1)
        ElseIf TypeOf (store(0)) Is Class4 Then
            'relevant tests of store(1)
        End If

        Console.ReadLine()
    End Sub

    Sub test(cA As Class2, cB As Class2)
        Console.WriteLine(cA.class2SecondUniqueProperty)
        Console.WriteLine("2 and a 2")
    End Sub
    Sub test(cA As Class2, cB As Class3)
        Console.WriteLine("2 and a 3")
    End Sub
    Sub test(cA As Class2, cB As Class4)
        Console.WriteLine(cB.class4UniqueProperty)
        Console.WriteLine("2 and a 4")
    End Sub
    Sub test(cA As Class3, cB As Class2)
        Console.WriteLine("3 and a 2")
    End Sub
    Sub test(cA As Class3, cB As Class3)
        Console.WriteLine("3 and a 3")
    End Sub
    Sub test(cA As Class3, cB As Class4)
        Console.WriteLine("3 and a 4")
    End Sub
    'and so on, for many (but not all) pairs of classes inherited from Class 1.
    Public Class Class1
    End Class
    Public Class Class2
        Inherits Class1
        Property class2UniqueProperty As String = "only present in Class 2"
        Property class2SecondUniqueProperty As String = "only present in Class 2"
        Sub class2UniqueSub()
            'do something that the other classes derrived from Class1 don't
        End Sub
    End Class
    Public Class Class3
        Inherits Class1
        Property class3UniqueProperty As String = "only present in Class 3"
    End Class
    Public Class Class4
        Inherits Class1
        Property class4UniqueProperty As String = "only present in Class 4"
    End Class
    'will be more than 4 classes
End Module
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1 回答 1

1

编译器选择重载方法（重载解析）。您可能正在寻找一种在运行时选择方法的方法。您的类型匹配方法可能是一种解决方案，但最好将类型解析代码放在test方法中。

从面向对象编程的角度来看，您的代码并不好。更好的方法是在类中定义一个方法并调用它。

Public MustInherit Class Class1
  Public Property TypeNumber as  integer
  Public MustOverride Sub Test(Ca As Class1)
  Public MustOverride Sub SomeMethod()
End Class
Public Class Class2
  Inherits Class1
  Public Sub New()
       TypeNumber = 2
  End Sub

  Property class2UniqueProperty As String = "only present in Class 2"
  Public Overrides Sub Test(Ca As Class1)
      Ca.SomeMethod()
      Console.Writeln(Me.TypeNumber & " and a " & Ca.TypeNumber)
  End Sub
  Public MustOverride Sub SomeMethod()
      Console.Writeln(class2UniqueProperty)
  End Sub
End Class

'... 继续其他课程

并调用现在使用

store(0).Test(Store(1))

没有类型匹配逻辑可能无法解决您的问题，但通常最好避免它。面向对象模式通常比使用模块方法更易于管理。

于 2013-10-13T15:36:15.990 回答