VB.Net 中的隐式转换警告

Question

第一个，我的代码如下：

    Dim eventType As Object = Nothing
    eventType = GetEventType()
    If Not (eventType Is Nothing) Then
        If TypeOf eventType Is ClassSelectEvents Then
            m_selectEvents = eventType  ' Warning BC 42016: Implicit type conversion from 'Object' to 'ClassSelectEvents'.
        End If
        If TypeOf eventType Is ClassMouseEvents Then
            m_mouseEvents = eventType   ' Warning BC 42016: Implicit type conversion from 'Object' to 'ClassMouseEvents'.
        End If
        If TypeOf eventType Is ClassTriadEvents Then
            m_triadEvents = eventType   ' Warning BC 42016: Implicit type conversion from 'Object' to 'ClassTriadEvents'.
        End If
    End If

由于编译器后显示警告，我按如下所示对其进行了修改，但仍显示警告。

在第二个 If 语句中，我认为eventType类型是Object .这有什么不同吗？我的代码哪里有问题请告诉我如何隐藏警告？

提前谢谢。

    Dim eventType As Object = Nothing
    eventType = GetEventType()
    If Not (eventType Is Nothing) Then
        If TypeOf eventType Is ClassSelectEvents Then
            'm_selectEvents = eventType
            'm_selectEvents = TryCast(eventType, ClassSelectEvents)
            m_selectEvents = DirectCast(eventType, ClassSelectEvents)
        End If
        If TypeOf eventType Is ClassMouseEvents Then
            'm_mouseEvents = eventType
            'm_selectEvents = TryCast(eventType, ClassMouseEvents)  ' Warning BC42016: Implicit type conversion from 'ClassMouseEvents' to 'ClassSelectEvents'.
            m_selectEvents = DirectCast(eventType, ClassMouseEvents)    ' Warning BC42016: Implicit type conversion from 'ClassMouseEvents' to 'ClassSelectEvents'.
        End If
        If TypeOf eventType Is ClassTriadEvents Then
            'm_triadEvents = eventType
            'm_selectEvents = TryCast(eventType, ClassTriadEvents)  ' Warning BC42016: Implicit type conversion from 'ClassTriadEvents' to 'ClassSelectEvents'.
            m_selectEvents = DirectCast(eventType, ClassTriadEvents)    ' Warning BC42016: Implicit type conversion from 'ClassTriadEvents' to 'ClassSelectEvents'.
        End If
    End If

Answer 1

当最后两个应该m_mouseEvents和m_triadEvents时，您将分配给所有三个If块中的m_selectEvents。

顺便说一句，在那里使用TryCast是没有意义的，因为您的If语句已经保证了演员表将起作用。 你应该只使用DirectCast. 如果你想使用TryCast那么你可以这样做：

m_selectEvents = TryCast(eventType, ClassSelectEvents)
If m_selectEvents Is Nothing Then
    m_mouseEvents = DirectCast(eventType, ClassMouseEvents)
    If m_mouseEvents Is Nothing Then
        m_triadEvents = DirectCast(eventType, ClassTriadEvents)
    End If
End If

如果转换失败，TryCast将返回Nothing，因此您在尝试转换后测试Nothing。 如果您在使用TryCast后没有测试Nothing那么几乎肯定您一开始就不应该使用TryCast。 编辑：嗯...我看到您在我发布答案后/期间将TryCast更改为DirectCast。希望我的解释对一些有所帮助。