如何解决system.nullreferenceexception：对象引用未设置为对象的实例？ [英] How to solve the system.nullreferenceexception: object reference not set to an instance of an object?

问题描述

'/'应用程序中的服务器错误。



对象引用未设置为对象的实例。



描述：执行当前Web请求期间发生了未处理的异常。请查看堆栈跟踪以获取有关错误及其源自代码的位置的更多信息。



异常详细信息：System.NullReferenceException：对象引用未设置为对象的实例。



来源错误：





第32行：

第33行：字符串reportName =会话[ID] .ToString（）;

第34行：字符串reportParams = Session [param]。ToString（）;

第35行：

行36：string [] reportServerURL = reportName.Split（'？'）;



我尝试过：



尝试过（reportParams！= null

{

string reportParams = Session [param]。ToString（）;

}

解决方案

错误很明显。您的Session对象为null，它们没有存储在具有名称ID或param的Session中的值。



在尝试使用Session变量之前，你应该总是检查它是否存在。

这是mos可能会话[ID] 或会话[param] 返回null。然后，假设它不为null，尝试通过调用其成员 ToString（）方法来解除它，这会抛出此异常，因为实际上这是null。



原因可能有所不同。你真的应该自己发现它;每次发生这种情况时你都不能问这些问题。



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/变量通过使用及其实例（非静态）成员进行解引用，这需要此成员/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。但是如果下一个记录功能的条件对象引用未设置为对象的实例则会出错。



有时候，你不能这样做在调试器下，由一个或另一个原因。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 。



祝你好运，

-SA

Server Error in '/' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:


Line 32:
Line 33: string reportName = Session["ID"].ToString();
Line 34: string reportParams = Session["param"].ToString();
Line 35:
Line 36: string[] reportServerURL = reportName.Split('?');

What I have tried:

Tried If(reportParams !=null
{
string reportParams = Session["param"].ToString();
}

解决方案

The error is pretty clear. Your Session objects have are null, they is no value stored in Session with the name ID or param.

You should always check if a Session variable exists before trying to use it.

It's most likely that Session["ID"] or Session["param"] return null. Then you, assuming it's not null, try to derefence it by calling its member ToString() method, which throws this exception, because actually this is null.

There reason can be different. You really should detect it by yourself; you cannot ask such questions every time such situations happen.

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 dereferences 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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