对象引用未设置为对象的实例（asp.net，c＃）

问题描述

我用asp.net（c＃）编写了一些应用程序，这是我的问题。

此代码（带有注释字符串行）效果很好：当我打开这个页面

在Internet Explorer中我在DataGrid2中获取了一些数据来自我的数据库...

....

.. ..


OleDbConnection OleDbConn = new

OleDbConnection（ConfigurationSettings.AppSettings [" connstr]）;


string selectString =" SELECT * FROM myTable"

OleDbCommand myOleDbCommand = OleDbConn.CreateCommand（）;

myOleDbCommand.CommandText = selectString;


OleDbDataAdapter myOleDbDataAdapter = new OleDbDataAdapter（）;

myOleDbDataAdapter.SelectCommand = myOleDbCommand;


DataSet myDataSet = new DataSet（）;

myOleDbDataAdapter.Fill（myDataSet）;


//字符串s =（字符串）myDataSet.Tables [" myTable"]。行[0] [ " myColumn"];


DataGrid2.Da taSource = myDataSet;

DataGrid2.DataBind（）;


......

......

问题：如果我取消注释字符串行，然后我收到此

行的错误消息（

对象引用未设置为对象的实例）当我尝试打开

这个页面在Internet Explorer中。


我在我的数据库表myTable中，在这个表中我有列

myColumn，并在此

列我肯定是第一行（索引为0），但它不起作用......为什么？


请帮助...谢谢...

解决方案

我建议你在调试器中或用类似的东西检查句子：
DataTable table = myDataSet.Tables [" myTable"];
DataRow row = table.Rows [0];
对象值=行[" myColumn"];
你会看到问题出在哪里。

-
Miha Markic [MVP C＃] - RightHand .NET咨询&开发
miha at rthand com

但我收到错误消息：指定的演员表无效。


如果我写而不是：


string s =（string）myDataSet.Tables [& myTable]。行[0] [" myColumn"];


这一行：


DataTable table = myDataSet.Tables [" myTable"];

DataRow row = table.Rows [0];

object value = row [" myColumn"];


我收到相同的错误消息：


"对象引用未设置为对象的实例。


和此消息显示此行：


DataRow row = table.Rows [0];


这可能意味着我可以写了myTable在


DataTable table = myDataSet.Tables [" myTable"];


因为此行没有显示错误消息。 />

I write some application with asp.net (c#) and here is my problem.
This code (with "commented" string line) works well: when i open this page
in Internet Explorer i get in DataGrid2 some data frome my database...
....
....

OleDbConnection OleDbConn = new
OleDbConnection(ConfigurationSettings.AppSettings["connstr"]);

string selectString="SELECT * FROM myTable"

OleDbCommand myOleDbCommand=OleDbConn.CreateCommand();
myOleDbCommand.CommandText=selectString;

OleDbDataAdapter myOleDbDataAdapter = new OleDbDataAdapter();
myOleDbDataAdapter.SelectCommand=myOleDbCommand;

DataSet myDataSet=new DataSet();
myOleDbDataAdapter.Fill(myDataSet);

//string s = (string) myDataSet.Tables["myTable"].Rows[0]["myColumn"];

DataGrid2.DataSource = myDataSet;
DataGrid2.DataBind();

......
......
The problem: if I "uncomment" string line, then i get error message for this
line ("
Object reference not set to an instance of an object") when i try to open
this page in Internet Explorer.

I have in my database table myTable and in this table i have column
myColumn, and in this
column i have surely first row (indexed with "0"), but it doesnt work...why?

please help...Thanks...

解决方案

I suggest you to check the sentence either in debugger or with something
like:
DataTable table = myDataSet.Tables["myTable"];
DataRow row = table.Rows[0];
object value = row["myColumn"];
And you''ll see where the problem is.

--
Miha Markic [MVP C#] - RightHand .NET consulting & development
miha at rthand com
www.rthand.com

but i get error message: Specified cast is not valid.

And if I write instead of:

string s = (string) myDataSet.Tables["myTable"].Rows[0]["myColumn"];

this lines:

DataTable table = myDataSet.Tables["myTable"];
DataRow row = table.Rows[0];
object value = row["myColumn"];

I get the same error message:

"Object reference not set to an instance of an object."

and this message appears for this line:

DataRow row = table.Rows[0];

This could mean that I can wrote "myTable" in

DataTable table = myDataSet.Tables["myTable"];

because the error message dont appear for this line.

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