如何处理无效的强制转换操作 [英] How to handle invalid cast operation

问题描述

我有bankname combobox，accountno文本框和Accounttype combobox。如果我从组合框中选择bankname。它在datagridview中显示数据库表中的相关数据。如果选择

从bankcombobox选择文本。它没有在datagridview中显示任何数据。我想在我点击addbutton时添加datagridview的新行顶部。新行正在成功添加。在datagridview中我有像slno，startno，noofcheques和endingno以及status.after进入的文本框startno和noofcheques都将添加填充数据结束没有文本我正在使用单元格离开事件。当我加载表单第一次添加新行和值计算正常。当我第一次选择bankname然后选择选择文本单击addnew行按钮行正在添加，但是当我尝试在startno文本框中输入值时。它会抛出错误。在mscorlib.dll.Object中发生的'System.InvalidCastException'无法从DBNull强制转换为其他类型。

我很新鲜，请帮助我。



我尝试了什么：

I have bankname combobox,accountno textbox and Accounttype combobox.If i select bankname from the combobox.It shows the related data in datagridview from database table.If select
"Select " text from the bankcombobox.It doesn't show any data in datagridview.I want to add new row top of the datagridview when i click addbutton.new row is adding succesfully.In datagridview i have textboxes like slno,startno,noofcheques and endingno and status.after entering startno and noofcheques both will add populate data in ending no text for that i am using cell leave event.when i load the form firsttime new row is adding and values are calculating fine.when i first select bankname and then select "select" text click addnew row button row is adding but when i try to enter values into the startno textbox.It throws error.'System.InvalidCastException' occurred in mscorlib.dll.Object cannot be cast from DBNull to other types.
I'm fresher please help me on this.

What I have tried:

       private void dgvwChqs_CellLeave(object sender, DataGridViewCellEventArgs e)
       {

          if (dgvwChqs.Rows[e.RowIndex].Cells["startno"].Value != null)
           {
               dgvwChqs.Rows[e.RowIndex].Cells["endingno"].Value =
           Convert.ToInt32(dgvwChqs.Rows[e.RowIndex].Cells["startno"].Value) + Convert.ToInt32(dgvwChqs.Rows[e.RowIndex].Cells["noofcheques"].Value);
           }
       }


private void btnadd_Click(object sender, EventArgs e)
       {

           dts.Rows.InsertAt(dts.NewRow(), 0);



           }

推荐答案

错误消息：< mscorlib.dll.Object中发生了code> System.InvalidCastException'，无法从DBNull强制转换为其他类型。非常清楚。这意味着 startno 和 noofcheques 字段之一为空！

您可以查看是否使用 IsDbNull 时该值不为空[ ^ ]方法或者你可以用默认值替换 null ：

An error message: System.InvalidCastException' occurred in mscorlib.dll.Object cannot be cast from DBNull to other types. is quite clear. It means that one of startno and noofcheques field is null!
You can check if the value is not null by using IsDbNull[^] method or you can replace null with default value:

//first method
int startno = 0;
int noofcheques = 0;
if(!Convert.IsDbNull(dgvwChqs.Rows[e.RowIndex].Cells["startno"].Value))
    startno = dgvwChqs.Rows[e.RowIndex].Cells["startno"].Value; //assuming that startno field stores integer values
if(!Convert.IsDbNull(dgvwChqs.Rows[e.RowIndex].Cells["noofcheques"].Value))
    noofcheques = dgvwChqs.Rows[e.RowIndex].Cells["noofcheques"].Value;

//second method
int startno = Convert.IsDbNull(dgvwChqs.Rows[e.RowIndex].Cells["startno"].Value)==true ? dgvwChqs.Rows[e.RowIndex].Cells["startno"].Value : 0;
int noofcheques = Convert.IsDbNull(dgvwChqs.Rows[e.RowIndex].Cells["noofcheques"].Value)==true ? dgvwChqs.Rows[e.RowIndex].Cells["noofcheques"].Value : 0;

//finally:
dgvwChqs.Rows[e.RowIndex].Cells["endingno"].Value = startno + noofcheques;

另一种方法是使用可空数据类型。有关详细信息，请参阅：使用可空类型（C＃编程指南）| Microsoft Docs [ ^ ]

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