在发生连接字符串时，对象引用未设置为c＃中的对象实例 [英] object reference not set to an instance of object in c# while connection string occuring

问题描述

我正在使用Sql server 2008在c＃和VS中使用win应用程序。

当我调试代码时，对象引用未设置为对象的实例

发生错误..我不知道该怎么做以及问题在哪里...请帮帮我

I'm working on win apps in c# and VS using Sql server 2008.
While I'm debugging the code the object reference not set to an instance of object
error occurring..I could not know what to to do and where is issue...please Help me out

public void autoinc()
{
   try
   {

      var connectionString = ConfigurationManager.ConnectionStrings["smallprojects"].ConnectionString;//this is my conncetion strin here I'm getting error and here smallprojects is my ddatabase
      SqlConnection cn = new SqlConnection(connectionString);
      cn.Open();
      SqlCommand command = new SqlCommand("Select isnull(max(S_No),0) from salary", cn);//here salary is my database table and in that table there is column name S_NO
                
      //cn.Open();
      object result = command.ExecuteScalar();

      txtsno.Text = (int.Parse(result.ToString()) + 1).ToString();
      cn.Close();
                
   }
   catch (Exception e1)
   {
      MessageBox.Show(e1.Message);
   }
}

这是我的配置文件。

This my configuration file is followed .

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings>
    <add name="smallprojects"
    connectionString="Data Source=.;Initial Catalog=smallprojects;Integrated Security=True "/> 
  </connectionStrings>
</configuration>

推荐答案

在过去的几天里，有许多未设置对象的问题，这是一个简单的问题一个人找到并解决。学习使用断点。



在VS中按F9，同时在线和断点将被设置。运行Debug配置，你将在那时停止。



第二：有Watch窗口

在该行代码中设置一个并将部分行发送到Watch窗口。您可以在View - >下找到观察窗口。其他窗口或在Debug下 - > Windows（永远不确定哪一个:) :)



在程序执行期间你可以编写表达式（只要你在断点处停止。



如果这有帮助，请花点时间接受解决方案。谢谢。

In the last couple of days there are many "object not set" questions and it is one of the easist one to find and solve. Learn to use break points.

In VS press F9 while on the line and breakpoint will be set. Run Debug configuration and you'll be stopped at that point.

Second: there is Watch window
Set one in that line of code and send parts of the line into Watch window. You can find the watch window under View -> Other windows or under Debug -> Windows (never sure which one :) )

There you can write expressions to observe during execution of your program (as long as you're stopped at the breakpoint.

If this helps, please take time to accept the solution. Thank you.

你好shiv ram ，

在您的情况下..

请首先通过这样的写作检查连接。

Hi shiv ram,
In your case..
Please first check the connection by writing like this.

var connectionString = "Data Source=.;Initial Catalog=smallprojects;Integrated Security=True "

1.检查SQL Server Management Studio中的数据源实例名称。它可能类似于.\SQLExpress 。

2.在该点设置一个断点，然后进行调试。

1. check for the datasource instance name in your SQL Server Management Studio. It may be like ".\SQLExpress".
2. put a break point at the point and then debug.

请看我对该解决方案的评论。你从一个属性得到一个null应该返回一个引用对象（不要使用硬编码顺便说一句，字符串常量，并尝试通过获取它的一些实例成员来取消引用它。你使用过调试器，那又怎样？你需要自己找到这样的东西。



不用担心。这是检测和修复的最简单的案例之一。它只是意味着某些引用类型的某个成员/变量通过使用和它的实例（非静态）成员解除引用，这要求此成员/变量为非null，但实际上它似乎为null。只需在调试器下执行它，它将停止抛出异常的执行。在该行上设置一个断点，重新启动应用程序并再次到达这一点。评估下一行中涉及的所有引用，并查看哪一个为null，而不需要为null。解决这个问题之后，修复代码：要么确保将成员/变量正确初始化为非空引用，要么将其检查为null，如果为null，则执行其他操作。



另请参阅：想要在按钮点击时显示下一条记录。但是如果下一条记录功能的条件对象引用没有设置为对象的实例 [ ^ ]。



有时候，你不能在调试器下，由于一个或另一个原因做到这一点。一个非常讨厌的情况是，只有在调试信息不​​可用时构建软件时才会出现问题。在这种情况下，你必须使用更难的方式。首先，你需要确保你永远不会通过静默处理异常来阻止异常的传播（这是开发人员对自己的犯罪，但很常见）。您需要在每个线程的最顶层堆栈帧上捕获绝对所有异常。如果处理类型 System.Exception 的异常，则可以执行此操作。在处理程序中，您需要记录所有异常信息，尤其是 System.Exception.StackTrace ：

http://msdn.microsoft.com/en-us/library/system.exception.aspx [ ^ ]，

http://msdn.microsoft.com/en-us/library/system.exception。 stacktrace.aspx [ ^ ]。



堆栈跟踪只是一个字符串，显示从throw语句到处理程序的异常传播的完整路径。通过阅读，您总能找到目的。对于日志记录，使用类 System.Diagnostics.EventLog ：

http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx [ ^ ]。

-SA

Please see my comment to the solution. You got a null from a property which is supposed to return a reference object (don't use hard-coded string constants, by the way), and tried to dereference it by taking some instance members of it. You used the debuggers, so what? You need to be able to find such things by yourself.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx[^],
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx[^].

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx[^].

Good luck,

—SA

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