Question

我试图找出为什么我的代码抛出空引用异常。

我正在尝试将对象添加到列表中。该对象可以是4种类型中的一种，我在所讨论的代码之后具有所有定义。此代码位于select case语句

中4个图片框的按钮单击处理程序中

以下是有问题的代码

    Dim count As Integer = 0
    Dim a_component As Object = Nothing

    Select Case (p.Name)
        Case TranspositorPictureBox.Name
            Form2.ShowDialog(Me)
            count = TNumericUpDown.Value
            a_component = New Transpositor(tempTranspositorDivert)
        Case ZonePictureBox.Name
            count = ZNumericUpDown.Value
            a_component = New Zone()
        Case InductionPictureBox.Name
            count = IndNumericUpDown.Value
            a_component = New Induction()
        Case InclinePictureBox.Name
            count = IncNumericUpDown.Value
            a_component = New Incline()
    End Select

    For i = 1 To count
        Dim newPic As PictureBox = New PictureBox()
        newPic.Image = p.Image
        newPic.Size = p.Size
        newPic.SizeMode = p.SizeMode

        sys.Add(a_component)

        LayoutFlowLayout.Controls.Add(newPic)
    Next

这是类定义。变量sys的类型为TranSorter

Public Class TranSorter
Public width As Integer
Public components As List(Of Object)

Public Sub New(ByVal the_width As Integer)
    Me.width = the_width
    Me.components = New List(Of Object)
End Sub

Public Sub Add(ByVal next_component As Object)
    Me.components.Add(next_component)
End Sub

End Class

Public Class Transpositor
Public length As Integer
Public divert As Object

Public Sub New(ByVal a_divert As Object)
    Me.divert = a_divert
    Me.length = ComponentLengths.TranspositorLength
    Form1.Transpositors += 1
End Sub
End Class

Public Class Zone
Public length As Integer

Public Sub New()
    Me.length = ComponentLengths.ZoneLength
    Form1.Microzones += 1
End Sub
End Class

Public Class Induction
Public length As Integer

Public Sub New()
    Me.length = ComponentLengths.InductionLength
    Form1.Inductions += 1
End Sub
End Class

Public Class Incline
Public length As Integer

Public Sub New()
    Me.length = ComponentLengths.InclineLength
    Form1.Inclines += 1
End Sub
End Class

sys.add行抛出异常。这是我在

中初始化sys的代码

Dim sys As TranSorter

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs)

sys = New TranSorter(temp_width)

Answer 1

我怀疑由于某种原因，你的p.Name不等于你在If子句中检查的四种情况之一。我建议考虑添加一个毯子Else子句和/或检查a_component是否为空，然后再添加它作为额外的预防措施。

Select Case (p.Name)
    Case TranspositorPictureBox.Name
        Form2.ShowDialog(Me)
        count = TNumericUpDown.Value
        a_component = New Transpositor(tempTranspositorDivert)
    Case ZonePictureBox.Name
        count = ZNumericUpDown.Value
        a_component = New Zone()
    Case InductionPictureBox.Name
        count = IndNumericUpDown.Value
        a_component = New Induction()
    Case InclinePictureBox.Name
        count = IncNumericUpDown.Value
        a_component = New Incline()
    Case Else
        Exit Sub ' Function/etc
End Select

if a_component IsNot Nothing Then
  For i = 1 To count
    Dim newPic As PictureBox = New PictureBox()
    newPic.Image = p.Image
    newPic.Size = p.Size
    newPic.SizeMode = p.SizeMode

    sys.Add(a_component)

    LayoutFlowLayout.Controls.Add(newPic)
  Next
End If

Answer 2

除非sys为Nothing，否则您的代码不应抛出任何异常这可能是因为Form_Load事件，您在哪里初始化sys var，不会执行。只需在行

上设置断点即可轻松检查这种情况

 sys = New TranSorter(temp_width)

然后在

上放置另一个断点

 sys.Add(a_component)

行并检查sys var是否为

NullReferenceException但在调试期间我有一个对象？

2 个答案: