使用下面的代码得到空引用错误 [英] Am getting a null reference error with the code below

问题描述

好的大师我收到此错误IBCARIP.exe中发生了'System.NullReferenceException'类型未处理的异常



附加信息：对象引用未设置为一个对象的实例。



当我点击保存按钮时会发生这种情况。我想做的是在工资计算中保存来自labells和textboxes的信息表单到访问数据库。代码必须检查表名，如果存在则将值插入新记录中，否则它会创建表，记录和更新。我突出显示代码中的错误点，

gooday gurus i am getting this error "An unhandled exception of type 'System.NullReferenceException' occurred in IBCARIP.exe

Additional information: Object reference not set to an instance of an object."

this happens when i click the save button.what i am trying to do is to save information from labells and textboxes on a salary calculation form to an access database. the code has to check for the table name, if it exists it inserts the values into a new record, if not then it creates both table and record and updates. I HAVE HIGHLIGHTED WHERE THE ERROR POINTS TO IN THE CODE,

Private Sub lblPrint_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lblPrint.Click

        Dim Cn As New OleDb.OleDbConnection
        Cn.ConnectionString = ("Provider=Microsoft.ace.OLEDB.12.0;Data Source=C:\Users\Blessing\Documents\IBCARIP.accdb")

        'Open the connection
        Cn.Open()

        'Get database schema
        Dim dbSchema As DataTable = Cn.GetOleDbSchemaTable(OleDb.OleDbSchemaGuid.Tables, New Object() {Nothing, Nothing, "Jongilanga_High", "TABLE"})

        Cn.Close()

        ' If the table exists, the count = 1
        If dbSchema.Rows.Count > 0 Then
            ' do whatever you want to do if the table exists

            Cn.Open()
            Dim sql2 As String = _
                    "insert into Jongilanga_High(EmpCode, Surname&Initials, Days, NRate, Total,Hours,O_T Rate,T_Overtime,Gross,UIF,Deductions,T_Deductions,Net_Salary) " _
                    & "values('" & txtID.Text & "', '" _
                 & lblSAI.Text & "', " & lblSAI.Text & "', " & txtDays.Text & "','" & lblRate.Text & "','" & lblTSal.Text & "','" _
                 & txtHours.Text & "','" & lblOT.Text & "','" & lblTOvertym.Text & "','" & lblGross.Text & "','" & lblUIF.Text & "','" _
                 & txtDed.Text & "','" & lblTDed.Text & "','" & lblNet.Text & ")"


            Dim da2 As New OleDb.OleDbDataAdapter(sql2, Cn)
            Dim dsC As New DataSet

            da2.InsertCommand.CommandText = sql2
            da2.InsertCommand.ExecuteNonQuery()

            Cn.Close()

            Cn.Open()
            Dim sql3 As String = "update Jongilanga_High set = '" & txtID.Text & "', '" _
                 & lblSAI.Text & "', " & lblSAI.Text & "', " & txtDays.Text & "','" & lblRate.Text & "','" & lblTSal.Text & "','" _
                 & txtHours.Text & "','" & lblOT.Text & "','" & lblTOvertym.Text & "','" & lblGross.Text & "','" & lblUIF.Text & "','" _
                 & txtDed.Text & "','" & lblTDed.Text & "','" & lblNet.Text & ")"

            da2.UpdateCommand.CommandText = sql3 <<<<<<ERROR POINTS THIS LINE<<<<<<            da2.UpdateCommand.ExecuteNonQuery()

            Cn.Close()

            Exit Sub

            MsgBox("New Record added to the Database")
        Else
            'Try
            'do whatever you want to do if the table does not exist
            ' e.g. create a table
            Dim cmd As New OleDb.OleDbCommand("CREATE TABLE [" + lblCCS.Text + "] ([EmpCode] TEXT(30), [Surname&Initials] TEXT(50), [Days] text(5), [NRate] text(10), [Total] text(20), [Hours] text(5), [O_T Rate] text(20), [T_Overtime] text(20), [Gross] text(20), [UIF] text(20), [Deductions] text(20), [T_Deductions] text(20), [Net_Salary] text (20))", Cn)
            Cn.Open()
            cmd.ExecuteNonQuery()

            Cn.Close()
            
            Try
                Cn.Open()
                Dim sql2 As String = _
                    "insert into Jongilanga_High(EmpCode, Surname&Initials, Days, NRate,Total,Hours,O_T Rate,T_Overtime,Gross,UIF,Deductions,T_Deductions,Net_Salary) " _
                    & "values('" & txtID.Text & "', '" _
                 & lblSAI.Text & "', " & lblSAI.Text & "', " & txtDays.Text & "','" & lblRate.Text & "','" & lblTSal.Text & "','" _
                 & txtHours.Text & "','" & lblOT.Text & "','" & lblTOvertym.Text & "','" & lblGross.Text & "','" & lblUIF.Text & "','" _
                 & txtDed.Text & "','" & lblTDed.Text & "','" & lblNet.Text & ")"
                Dim da2 As New OleDb.OleDbDataAdapter(sql2, Cn)
                Dim dsC As New DataSet

                da2.InsertCommand.CommandText = sql2
                da2.InsertCommand.ExecuteNonQuery()

            Catch exceptionObject As Exception
                MsgBox(exceptionObject.Message)
            Finally
                Cn.Close()
            End Try

            Try
                Cn.Open()

                Dim sql3 As String = "update Jongilanga_High set = '" & txtID.Text & "', '" _
                 & lblSAI.Text & "', " & lblSAI.Text & "', " & txtDays.Text & "','" & lblRate.Text & "','" & lblTSal.Text & "','" _
                 & txtHours.Text & "','" & lblOT.Text & "','" & lblTOvertym.Text & "','" & lblGross.Text & "','" & lblUIF.Text & "','" _
                 & txtDed.Text & "','" & lblTDed.Text & "','" & lblNet.Text & ")"

                Dim da2 As New OleDb.OleDbDataAdapter(sql3, Cn)
                Dim dsC As New DataSet
                da2.UpdateCommand.CommandText = sql3
                da2.UpdateCommand.ExecuteNonQuery()

            Catch ex As Exception
                '
                MsgBox(ex.Message)
            Finally
                Cn.Close()
            End Try

            MsgBox("New Record added to the Database")
            
        End If

推荐答案

因此，在该行上放置一个断点并查看变量。当您发现哪一个为null时，将断点置高并再次运行。这一次，单步查看变为null的值，并找出它从not-null切换到null的时间。

So, put a breakpoint on the line and look at the variables. The when you find out which one is null, put a breakpoint higher up and run it again. This time, single step through look at the value that became null, and find out when it switched from "not-null" to "null".

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