c# - 有趣的属性行为

Question

任何人都知道为什么这不起作用(C# 或 VB.NET 或其他 .NET 语言无关紧要)。这是我的问题的一个非常简化的示例(对于 VB.NET 很抱歉):

    Private itsCustomTextFormatter As String
    Public Property CustomTextFormatter As String
        Get
            If itsCustomTextFormatter Is Nothing Then CustomTextFormatter = Nothing  'thinking this should go into the setter - strangely it does not'
            Return itsCustomTextFormatter
        End Get
        Set(ByVal value As String)
            If value Is Nothing Then
                value = "Something"
            End If
            itsCustomTextFormatter = value
        End Set
    End Property

如果你这样做:

Dim myObj as new MyClass
Console.WriteLine(myObj.CustomTextFormatter)

您会对结果感到惊讶。它将打印“无”。任何人都知道为什么它不打印“Something”

这是每个建议的单元测试:

Imports NUnit.Framework

<TestFixture()> _
Public Class Test
   Private itsCustomTextFormatter As String
    Public Property CustomTextFormatter As String
        Get
            If itsCustomTextFormatter Is Nothing Then CustomTextFormatter = Nothing 'thinking this should go into the setter - strangely it does not' 
            Return itsCustomTextFormatter
        End Get
        Set(ByVal value As String)
            If value Is Nothing Then
                value = "Something"
            End If
            itsCustomTextFormatter = value
        End Set
    End Property

    <Test()>
    Public Sub Test2()
        Assert.AreEqual("Something", CustomTextFormatter)
    End Sub
End Class

返回:

Test2 : Failed  
  Expected: "Something"
  But was:  null

at NUnit.Framework.Assert.That(Object actual, IResolveConstraint expression, String message, Object[] args)
at NUnit.Framework.Assert.AreEqual(Object expected, Object actual)

Answer 1

您的评论:

'thinking this should go into the setter - strangely it does not'    

指出你的错误是什么。在 Visual Basic 中，有两种方法可以从函数返回某些内容:

Function GetSomeValue() As String
    Return "Hello"
End Function

或

Function GetSomeValue() As String
    GetSomeValue = "Hello"
End Function

混合使用这两种样式是完全合法的，但却是一种令人困惑的错误做法:

Function GetSomeValue() As String
    GetSomeValue = "Hello" ' I should return Hello... '
    Return "GoodBye"       ' ...or perhaps not. '
End Function

如您所见，您在 getter 中混合了两种样式。 设置该变量不会调用 setter ；它通知运行时，当 getter 返回时，这是它应该返回的值。 然后，您可以使用 Return 语句覆盖该值。

如果您这样做时感到疼痛，那么不要那样做。