我该如何解决System.NullReferenceException错误 [英] how do i solve the System.NullReferenceException error
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问题描述
当我尝试在我的数据库上运行查询时,我一直收到此错误,无论是选择还是插入。我该怎么解决这个问题。
I keep getting this error anytime i try to run a query on my database be it a select or insert. how do i solve this problem.
System.NullReferenceException: Object reference not set to an instance of an object
此错误指向我的代码的第39行。代码如下所示:
This error points to line 39 of my code. the code is shown below:
String sql, connection, lname, fileloc;
sql = "select FileLoc, Fname, Lname from apps where Lname = '"+searchString.Text+"'";
connection = Properties.Settings.Default.cvmanagerConnectionString;
SqlDataReader reader = null;
SqlConnection conn = new SqlConnection(connection);
SqlCommand cmd = new SqlCommand(sql, conn);
try
{
conn.Open();
cmd.ExecuteReader();
intsearchLB.Enabled = true;
while (reader.Read())// THIS IS LINE 39
{
lname = reader["Lname"].ToString();
fileloc = reader["FileLoc"].ToString();
intsearchLB.Items.Add(new searchresults(lname, fileloc));
}
}
catch (Exception ex)
{
MessageBox.Show(ex.ToString());
}
finally
{
nsCancel.Enabled = true;
nsView.Enabled = true;
nsEdit.Enabled = true;
nsSave.Enabled = true;
nsDate.Enabled = true;
nsTime.Enabled = true;
}
推荐答案
使用调试器找出它发生的位置。由于我们不知道您的代码是什么样的,这是您将获得的最佳建议。
Use the debugger to find out where it's happening. Since we don't know what your code looks like, that's the best advice you're going to get.
当您看到此异常时而你正在使用VS,90%的问题都解决了。在调试器下运行它 - 它会通过令人讨厌的代码行直接进入你的脸。 (你做到了吗?还有问题吗?在哪里?)
你需要有适当的Debug Exception选项:VS Main menu - >调试 - >例外...(Ctrl + D,E)。默认情况下,选项几乎是最优的。
如果它没有帮助,你需要提出更具体的问题。
As you see this exception and you're using VS, 90% of your problem is solved. Run it under debugger -- it will through the offending line of code right into your face. (Did you do this? Still a problem? Where?)
You need to have proper Debug Exception options: VS Main menu -> Debug -> Exceptions... (Ctrl+D,E). By default, the options are nearly optimal.
If it did not help, you need to ask more concrete question.
好的,更新后这个错误报告很好。
Ok, after update this bug report is good.
SqlDataReader reader = null;
//...
while (reader.Read()) //...
来吧,你自己分配 null
并且从未构建读者
您尝试读取
!
您可能需要
Come on, you assigned null
by yourself and never constructed reader
by the point where you try to Read
!
You probably need
reader = cmd.ExecuteReader();
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