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

解决方案

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

解决方案

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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