我的代码有问题吗？

Question

我的代码有问题吗？我遇到“从类型 'DBNull' 到类型'String' 的转换无效”错误。

函数调用 (GetNoofHoursOfTImeIn)：

Public Shared Function GetNoofHoursofTImeIn(ByVal search As String, ByVal dfrom As   DateTime, ByVal dto As DateTime) As DataTable
Dim dtoffenseinfo As New DataTable

If Not DBConnection.State = ConnectionState.Open Then       
    DBConnection.Open()
End If

Using cmd As New OleDbCommand
    cmd.Connection = DBConnection
    cmd.CommandText = _
            "SELECT SUM(No_of_Hour) AS THour FROM EmployeeAttendance " & _
            "WHERE EmployeeID = ? " & _
                "AND WorkingDate BETWEEN ? AND ?"
    cmd.Parameters.AddWithValue("?", search)
    cmd.Parameters.AddWithValue("?", dfrom.Date)
    cmd.Parameters.AddWithValue("?", dto.Date)
    Using adapter As New OleDbDataAdapter(cmd)
        adapter.Fill(dtoffenseinfo)
    End Using
End Using

DBConnection.Close()
Return dtoffenseinfo
End Function

后面的代码：

 txtPresentDays.Text = ClsAttendance.GetNoofHoursofTImeIn(EmployeeID.Text, MyDateString, MyDateString2).Rows(0).Item("THour")

我知道这个错误很常见，但直到现在我还不太清楚我是如何或为什么会出现这个错误的。

感谢您的启发！

Answer 1

当有零个符合条件的记录时，您会得到一个空值。您可以使用iif 函数将该值变为零：

SELECT IIF(SUM(No_of_Hour) Is Null, 0, SUM(No_of_Hour)) ...