对象引用未设置为对象VB.NET的实例 [英] object reference not set to an instance of an object VB.NET

问题描述

我需要一些帮助对象引用未设置为我的代码中的对象错误实例

这里是我的代码



< pre lang =vb> 公共 功能添加（std 正如 DataTable）作为 IList
尝试
listr.Add（ std.Rows）
返回 listr
Catch ex As 异常
MsgBox（ 无法加载网页& vbCrLf& ex.Message）
结束 尝试

结束 功能

解决方案

整个想法都很糟糕。当然，您必须初始化 listr 。但是，从函数返回它的想法是错误的。如果你的函数在同一个类的同一个实例中使用，你已经有了这个实例，如果你从外面调用这个函数，你还要添加什么？您必须向调用者已知的集合实例添加内容。即使您在调用中创建集合的实例，也是错误的。换句话说，你必须重新考虑你班级提供的界面，而不仅仅是实施。



当然，不仅 listr 可以为null，但也可以 std 。你需要自己找出来。每次出现这种情况时，你都不能问这样的问题。您没有显示带有对象引用未设置为对象实例消息的异常位置。



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

i need some help object reference not set to an instance of an object error in my code
here is my code

Public Function Add(std As DataTable) As IList
      Try
          listr.Add(std.Rows)
          Return listr
      Catch ex As Exception
          MsgBox("Can't load Web page" & vbCrLf & ex.Message)
      End Try

  End Function

解决方案

The whole idea is bad. Of course, you have to initialize listr. But then, the idea of returning it from the function is wrong. If your function is used in the same instance of the same class, you already have this instance, and if you call this function from outside, what are you adding to? You have to add something to a collection instance which is already known to the caller. Even if you create an instance of a collection inside a call, it would be wrong. In other words, you have to rethink your interface provided by your class, not just implementation.

Of course, not only listr could be null, but also std. You need to find it out by yourself. You cannot ask such questions every time such situation appears. You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

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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