为什么在从int转换为泛型类型时，对Ctype的调用在运行时失败？

Question

I know about the existence of System.Nullable(Of T) Structure.

I am trying to write a class to replace it: The Nullable(Of T) Class [VB code below]. But when I tested the Nullable(Of T) Class (See NullableOfTClassTest Class, Method Main) an exception occurs when using the CType operator to convert from System.Int32.

Why does this happen at runtime, why isn't my Ctype operator method being called?

NOTE: code below reordered and reduced in order to hilite problem.

Namespace MyNamespace
Public NotInheritable Class NullableOfTClassTest
      Private Sub New()
      End Sub

      Public Shared Sub Main()
        Dim dic as new Dictionary(Of String, Object) FROM {{"5", 5}}
        Dim Y as MyNamespace.Nullable(Of System.Int32) = 
            CType(dic.Item("5"), MyNamespace.Nullable(Of Integer))
      End Sub
    End Class

Public NotInheritable Class Nullable(Of T As {Structure})
    'I think this operator should be called by the code above
    Public Shared Widening Operator CType(ByVal x As T) As 
                      MyNamespace.Nullable(Of T)
        Return New MyNamespace.Nullable(Of T)(x)
    End Operator

    Public Shared Narrowing Operator CType(ByVal x As 
                             MyNamespace.Nullable(Of T)) As T
         If (x Is Nothing) OrElse (Not x.HasValue) Then Throw New InvalidCastException
             Return x.Value
    End Operator

    Private _Value As T
    Private _HasValue As Boolean = False

    Public Sub New()
    End Sub

    Public Sub New(ByVal value As T)
        Me.Value = value
    End Sub    

    Public Property Value As T
        Get
           If Not Me.HasValue Then Throw New InvalidOperationException(
                  "El objeto que acepta valores Null debe tener un valor.")
               Return Me._Value
        End Get
        Set(ByVal value As T)
            Me._Value = value
            Me._HasValue = True
        End Set
     End Property

     Public ReadOnly Property HasValue As Boolean
          Get
             Return Me._HasValue
          End Get
     End Property 

    Public Shadows Function ToString() As String
       If Me.HasValue Then
          Return Me.Value.ToString
       Else
          Return Nothing
       End If
    End Function

    End Class


End Namespace

Answer 1

maybe you need to look at how, for an example, linq to sql deal with Nothing and dbnull and copy it functionality? use reflector

what your doing right now is... in fact, i don't even really understand it

Answer 2

Does this illustrate the problem to be solved?:

Dim o as Object = Nothing

Dim v as System.Nullable(Of Integer)

v = 123

o = v 
' o is now a boxed Integer, and 
' NOT a boxed System.Nullable(Of Integer)
' where Value is 123 (and HasValue is True)

v = Nothing 
' Hence, v.HasValue = False

o = v 
' o is now Nothing, and 
' NOT a boxed System.Nullable(Of Integer) 
' where HasValue is False

If so, think long and hard about why you need to do a boxed System.Nullable type. There are better ways to skin the DBNull cat.