System.NullReferenceException:未将对象引用设置为对象的实例。 [英] System.NullReferenceException: Object reference not set to an instance of an object.
问题描述
执行此代码时为什么会出现上述错误消息:
Public Sub DropDown_SelectedIndexChanged(ByVal sender As System.Object,
ByVal e As System.EventArgs)
Dim list As DropDownList = CType(sender,DropDownList)
Dim cell = CType(list.Parent,TableCell)
Dim item = CType(cell.Parent,DataGridItem)
Dim index = item.ItemIndex
Dim ds As DataSet
ds = DataGrid1.DataSource
Dim row As DataRow
row = ds.Tables(0).Select(" ID = ''" + item.Cells(0).Text +"''")(0)
''< --- ERROR
End Sub
在线上设置断点
row = ds.Tables(0).Select (" ID =''" + item.Cells(0).Text +"''")(0)
并查看那里的null。
可能是:
ds中没有表格;
否单元格中的单元格(o);
没有项目;
选择不返回任何内容
....
调试它。
Eliyahu
" John Smith" <乔******** @ microsoft.com>在消息中写道
新闻:%Z ******************** @ news.siol.net ...
为什么在执行此代码时会出现上述错误消息:
Public Sub DropDown_SelectedIndexChanged(ByVal sender As System.Object,
ByVal e As System.EventArgs)
Dim cell = CType(list.Parent,TableCell)
Dim item = CType(cell.Parent,DataGridItem)
Dim index = item.ItemIndex
Dim ds As DataSet
ds = DataGrid1.DataSource
Dim row As DataRow
row = ds.Tables(0).Select (" ID =''" + item.Cells(0).Text +"''")(0)
''< --- ERROR
End Sub
谢谢。
" ds"是空的。看来我需要以某种方式将DataSource添加到Session对象
中。初学者的错误。似乎DataGrid1.DataSource不是持久性的?
" ds"是空的。看来我需要以某种方式将DataSource添加到Session对象中。初学者的错误。似乎DataGrid1.DataSource不是持久的吗?
我已经将ds声明为全局共享变量,它到目前为止工作正常。
Why do I get above error message while executing this code:
Public Sub DropDown_SelectedIndexChanged(ByVal sender As System.Object,
ByVal e As System.EventArgs)
Dim list As DropDownList = CType(sender, DropDownList)
Dim cell = CType(list.Parent, TableCell)
Dim item = CType(cell.Parent, DataGridItem)
Dim index = item.ItemIndex
Dim ds As DataSet
ds = DataGrid1.DataSource
Dim row As DataRow
row = ds.Tables(0).Select("ID = ''" + item.Cells(0).Text + "''")(0)
''<--- ERROR
End Sub
Set a break point on the line
row = ds.Tables(0).Select("ID = ''" + item.Cells(0).Text + "''")(0)
and see what is null there.
Could be:
no tables in ds;
no Cells(o) in item;
no item;
select doesn''t return anything
....
Debug it.
Eliyahu
"John Smith" <jo********@microsoft.com> wrote in message
news:%Z********************@news.siol.net...
Why do I get above error message while executing this code:
Public Sub DropDown_SelectedIndexChanged(ByVal sender As System.Object,
ByVal e As System.EventArgs)
Dim list As DropDownList = CType(sender, DropDownList)
Dim cell = CType(list.Parent, TableCell)
Dim item = CType(cell.Parent, DataGridItem)
Dim index = item.ItemIndex
Dim ds As DataSet
ds = DataGrid1.DataSource
Dim row As DataRow
row = ds.Tables(0).Select("ID = ''" + item.Cells(0).Text + "''")(0)
''<--- ERROR
End Sub
Thank you.
"ds" was null. It seems I need to add DataSource into Session object
somehow. Beginner mistake. It seems DataGrid1.DataSource is not persistance?
"ds" was null. It seems I need to add DataSource into Session object
somehow. Beginner mistake. It seems DataGrid1.DataSource is not
persistance?
I''ve declared ds as global Shared variable and it works fine so far.
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