我从以下VB.NET代码中仅在某些月份遇到此异常:
System.InvalidCastException: Conversion from type 'DBNull' to type 'Date' is not valid.
发生在这一行:
If CDate(dRow("CompleteDate")).ToString("d") = arrWeekYear(i, 1).ToString("d") Then
如果我理解正确的话,问题是我试图将一些NULL值与非NULL值的CompleteDate进行比较。
如何解决这个问题?对于某些日期,上面的代码运行得很好。 但是对于一些更近期的记录,它却不能正常运行。 但是,我能够从具有相同日期范围的T-SQL查询中获得输出,并且没有错误;它也运行得很快。 我还检查了这个T-SQL查询中的“Date_Completed”和“Review_Date”的NULL值,但无论如何,“CompleteDate”始终等于非NULL值。 所以我不明白这是怎么回事。
这是我的VB.NET查询:
commandstring = "SELECT Batch_Records.Part_Number, Batch_Records.Lot_Number, Batch_Records.Date_Received, " & _
"IsNull([Date_Completed], [Review_Date]) AS [CompleteDate], Batch_Records.Error, " & _
"Batch_Records.[Group], Batch_Records.MFG, Batch_Records.MFG2, Batch_Records.QC, Batch_Records.QC2, " & _
"QC_CODES.CODE_DESC, DATEADD(DD, 1 - DATEPART(DW, Batch_Records.Date_Received), Batch_Records.Date_Received) AS SundayDate " & _
"FROM EXCEL.Batch_Records LEFT JOIN EXCEL.QC_CODES ON Batch_Records.Part_Number = QC_CODES.CODE_ID " & _
"WHERE (Batch_Records.[Group]" & TheGroup & " AND Batch_Records.Date_Received > '" & FirstWeek & "' AND Batch_Records.Date_Received < dateadd(Day, 1, '" & LastWeek & "'))" & _
"ORDER BY Batch_Records.Date_Received"
当我在引起错误的那行代码上方添加此行时,我的报告在这些月份超时。那么该怎么办呢?
If Not dRow("CompleteDate") Is System.DBNull.Value Then