VB.Net 中的隐式转换警告

Question

首先，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

由于编译后显示警告，我修改如下图，但还是显示警告。

在第二个 If 语句中，我认为 eventType 的类型是 Object。那有什么不同吗？我的代码哪里错了请告诉我如何隐藏警告？

提前致谢。

    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

当最后两个应该是 m_mouseEvents 和 m_triadEvents 时，您将在所有三个 If 块中分配给 m_selectEvents。

顺便说一句，在那里使用TryCast 是没有意义的，因为您的If 声明已经保证强制转换会起作用。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

TryCast 将在转换失败时返回 Nothing，因此您在尝试转换后测试 Nothing。如果您在使用TryCast 之后没有测试Nothing，那么您几乎肯定不应该从一开始就使用TryCast。 编辑：嗯...我看到您在我发布答案之后/同时将TryCast 更改为DirectCast。希望我的解释对一些人有所帮助。