vb.net – 为什么Property Setters的调用频率高于预期？

我在VB.net中观察到一种行为,其中属性设置器被调用的次数比看似必要的更多,并且调用了对姐妹setter方法的调用.

Public Class Form1

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Console.WriteLine("Calling WorkReferenceTypeByReference")
        WorkReferenceTypeByReference(ReferenceTypeData)
        Console.WriteLine("Called WorkReferenceTypeByReference")
        Console.WriteLine("Calling WorkReferenceTypeByValue")
        WorkReferenceTypeByValue(ReferenceTypeData)
        Console.WriteLine("Called WorkReferenceTypeByValue")
    End Sub

    Public Sub WorkReferenceTypeByReference(ByRef ref As Point)
        Dim b As Point = New Point(4, 4) + ref
        Console.WriteLine("    adding (4,4) to " & ref.ToString)
    End Sub

    Public Sub WorkReferenceTypeByValue(ByVal ref As Point)
        Dim b As Point = New Point(4, 4) + ref
        Console.WriteLine("    adding (4,4) to " & ref.ToString)
    End Sub

    Private m_ReferenceType As Point = New Point(0, 0)
    Public Property ReferenceTypeData As Point
        Get
            Console.WriteLine("  Calling ReferenceTypeData getter")
            Console.WriteLine("  returning: " & m_ReferenceType.ToString)
            Return m_ReferenceType
        End Get
        Set(ByVal value As Point)
            Console.WriteLine("  Calling ReferenceTypeData setter")
            Console.WriteLine("  value = " & value.ToString)
            m_ReferenceType = value
        End Set
    End Property
End Class

前面的代码返回到控制台以下输出

Calling WorkReferenceTypeByReference
  Calling ReferenceTypeData getter
  returning: {X=0,Y=0}
    adding (4,4) to {X=0,Y=0}
  Calling ReferenceTypeData setter
  value = {X=0,Y=0}
Called WorkReferenceTypeByReference
Calling WorkReferenceTypeByValue
  Calling ReferenceTypeData getter
  returning: {X=0,Y=0}
  adding (4,4) to {X=0,Y=0}
Called WorkReferenceTypeByValue

请注意方法执行后对属性setter的虚假调用.我认为这种行为是作为一种安全措施而产生的,以防止无意中修改底层财产,尽管这可能是意图.

通过选择适当的ByVal关键字,可以轻松解决ByRef与ByVal使用情况下的这种行为,但是最近发现了一种更加隐蔽的行为,这种行为导致重复调用的堆栈溢出,因为setter调用会更新一个调用的值.只有吸气剂.

Public Sub DoSomething()
    Dim a As New CustomObject(anotherObject.AProperty(getterArgument))
End Sub

Public Class AnotherObject

    Public Property AProperty as SomeType
        Get
            ' Get value
        End Get
        Set
            ' Set value, call DoSomething
        End Set
    End Property
End Class

在前面的示例中,调用DoSomething()将触发AProperty getter方法,但在此用法之后,将触发setter方法,该方法由程序逻辑再次调用DoSomething().这是让我困惑的二传手的自动召唤.

事实上,这是VB.Net的一个特性.在您的代码中,您通过引用传递属性而不是变量.严格地说,传递属性ByRef是不可能的,因为ByRef需要对变量的引用.但是,编译器会代表您自动创建临时本地,并将其传递给您的方法.因为该方法可能会更改ByRef参数(现在是编译器生成的临时参数而不是您的属性),然后编译器会插入对setter的调用.基本上,这样的事情发生了：

Dim temp = Me.ReferenceTypeData
Me.WorkReferenceTypeByReference(temp)
Me.ReferenceTypeData = temp

其他语言(如C#)不允许通过引用传递属性(正确地从参数传递的严格定义中传递),而是要求您自己编写上述代码的等效代码.