Visual Basic 2012，在编写 DBNull 时遇到问题。 “从类型 'DBNull' 到类型 'String' 的转换无效。”

Question

我遇到了上面列出的错误，我在其他地方读到我必须做的是a DBNull-check...，但我不知道该怎么做。我想知道是否有人可以帮助我？我是 VB 的超级新手。

我也尝试将 LedgerId 和 Net 更改为小数，但它告诉我这一行：

JVNet += Net

错了。 “+ 对 DBNull 和小数无效”之类的内容。

这是相关代码（我认为）。

    Dim SQLStmt, LedgerId, NetDisp As String
    Dim Net, JVNet As Decimal
    Dim NetCounter As Integer
    Cnxn.Open()
    SQLStmt = "SELECT LedgerId, sum(Qty * Each) as Net FROM Orders, Details  where CONVERT(nvarchar(10), AcctgDate, 101) = '" & _
        cbJVDate.Text & _
        "' and Orders.Id = Details.OrderId group by LedgerId order by LedgerId"
    Dim JVCommand As New SqlCeCommand(SQLStmt, Cnxn)
    Dim JVReader As SqlCeDataReader
    lblAdvice.Text = ""
    Debug.Print(vbCrLf & "Got Here with '" & SQLStmt & "'" & vbCrLf)
    Try
        JVReader = JVCommand.ExecuteReader
    Catch ex As Exception
        MsgBox("Execute JVReader with '" & SQLStmt & "' got " & ex.Message)
        Exit Sub
    End Try
    JVNet = 0
    NetCounter = 0
    lbJournal.Items.Clear()
    Do While JVReader.Read()
        LedgerId = JVReader.Item("LedgerId")
        Net = JVReader.Item("Net")
        JVNet += Net
        NetCounter += 1
        NetDisp = Format(Net, "0.00;(0.00)")
        NetDisp = NetDisp.PadLeft(10, " ")
        If Net < 0 Then NetDisp = " " & NetDisp
        lbJournal.Items.Add(LedgerId & NetDisp)
    Loop
    If NetCounter = 0 Then
        lblAdvice.ForeColor = Color.DarkRed
        lblAdvice.Text = "There are no details posted for " & cbJVDate.Text & "..."
    ElseIf JVNet = 0 Then
        lblAdvice.ForeColor = Color.Black
        lblAdvice.Text = "The journal for this date nets zero."
    Else
        lblAdvice.ForeColor = Color.DarkRed
        lblAdvice.Text = "The journal for this date does not net zero." & vbCrLf & _
           vbCrLf & "It nets " & Format(JVNet, "0.00")
    End If
    Cnxn.Close()
End Sub

感谢您的帮助！

Answer 1

在 VB 代码中，数据库 NULL 由DBNull.Value 表示。如果您从数据库中获取数据，那么您可能会执行以下操作：

Dim str As String = Nothing
Dim int As Integer? = Nothing

If Not myDataRow.IsNull("Text") Then
    str = CStr(myDataRow("Text"))
End If

If Not myDataRow.IsNull("Number") Then
    int = CInt(myDataRow("Number"))
End If

反过来，你可能会这样做：

Dim command As New SqlCommand("UPDATE MyTable SET Text = @Text, Number = @Number WHERE ID = @ID", connection)

command.Parameters.Add("@Text", SqlDbType.VarChar, 50).Value = If(str Is Nothing, CObj(DBNull.Value), str)
command.Parameters.Add("@Number", SqlDbType.Int).Value = If(int.HasValue, int.Value, CObj(DBNull.Value))

Answer 2

SQL CE 不支持 IFNULL 或 ISNULL 函数，要复制该函数，一种方法是使用 CASE..END 构造：

SQLStmt = "SELECT LedgerId, 
CASE SUM(Qty * Each) IS NULL THEN 0 END AS Net
FROM Orders, Details
WHERE CONVERT(nvarchar(10), AcctgDate, 101) = '" & cbJVDate.Text & "' AND Orders.Id = Details.OrderId
GROUP BY LedgerId ORDER BY LedgerId"

另一种方法是使用 COALESCE（我无法测试，因为我没有 SQL CE）

SQLStmt = "SELECT LedgerId, 
CAOLESCE(SUM(Qty * Each),0) AS Net
FROM Orders, Details
WHERE CONVERT(nvarchar(10), AcctgDate, 101) = '" & cbJVDate.Text & "' AND Orders.Id = Details.OrderId
GROUP BY LedgerId ORDER BY LedgerId"