Question

我的代码出现问题。它在错误中说我应该在代码中加New但我不知道在哪里放它。这是该函数的完整代码：

Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged
    Try
        Dim TA As New POSCanteenTableAdapters.ItemsTableAdapter
        Dim TB = TA.GetDataByBarcode(TextBox1.Text)
        If TB.Rows.Count = 0 Then
            TextBox2.Text = ""
            TextBox3.Text = ""
            Button1.Enabled = False
            Exit Sub
        End If
        Button1.Enabled = True
        Dim IR As POSCanteen.ItemsRow = TB.Rows(0)
        TextBox2.Text = IR.ItemName
        TextBox3.Text = IR.SellPrice
        Button2.Tag = IR
    Catch ex As Exception
        MsgBox(ex.Message, MsgBoxStyle.Critical Or MsgBoxStyle.OkOnly)
    End Try
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim IR As POSCanteen.ItemsRow = Button1.Tag
    Dim RI As New Integer
    Dim ItemLoc As Integer = -1
    For RI = 0 To CDGV3.Rows.Count - 1
        If IR.Barcode = CDGV3.Rows(RI).Cells(0).Value Then
            ItemLoc = RI
            Exit For
        End If
    Next
    If ItemLoc = -1 Then
        CDGV3.Rows.Add(IR.Barcode, IR.ItemName, IR.BuyPrice, IR.SellPrice, 1, IR.SellPrice)
    Else
        Dim ItemCount As Long = CDGV3.Rows(ItemLoc).Cells(4).Value
        ItemCount += 1
        Dim NewPrice As Decimal = IR.SellPrice * ItemCount
        CDGV3.Rows(ItemLoc).Cells(4).Value = ItemCount
        CDGV3.Rows(ItemLoc).Cells(5).Value = NewPrice
    End If
    TextBox1.Text = ""
    TextBox1.Focus()
End Sub

应该在数据网格中添加一个项目，但由于该错误，它不会。我不知道在哪里放它，因为编译器没告诉我在哪里。

这是异常消息：Object reference not set to an instance of an object.

Answer 1

你需要提前考虑一个对象不能转换的可能性（尤其是当你让编译器猜测时）。不确定这是否是导致问题的对象，但您可能知道。了解如何使用调试器逐步完成代码。

Dim IR = TryCast(Button1.Tag, {type})
If Not IR Is Nothing Then
  'rest of your code
End If

Answer 2

我无法得到你的线，所以我建议你这样做。

尝试以下任何一项

Dim IR As New POSCanteen.ItemsRow = Button1.Tag

Dim ItemCount As New Long = CDGV3.Rows(ItemLoc).Cells(4).Value

Dim NewPrice As New Decimal = IR.SellPrice * ItemCount

VB.Net数据网格视图

2 个答案: