我该如何解决System.NullReferenceException错误 [英] how do i solve the System.NullReferenceException error

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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