VB.Net中IMPORTS变量上的Object错误实例 [英] Instance of an Object error on IMPORTS variable in VB.Net

问题描述

你好



我正在使用VB.Net 2013.



我正在创建一个对象在网络服务上使用IMPORTS关键字。



例如：



IMPORTS sd = TEST.WEBSERVICE .UI< ==这是来自Web服务引用的对象。



我可以在方法中使用sd对象，然后获取属性和事件，并且它建立成功：



sd.TRANSMISSION.TRANSMITCODE =36X



然而，当我运行程序我得到了可怕的对象引用未设置为对象的实例错误。



此错误通常意味着您没有使用New关键字创建一个新的对象实例。



我认为你不必使用New关键字，因为IMPORTS声明实例化对象，或者我错了？



如何使用IMPORTS实例化sd对象？



谢谢

解决方案

问题与 Imports 无关。您只是使用类型或命名空间名称别名。这个词是进口是极具误导性的。你实际上并没有导入任何东西。可能这是一些VB历史工件。这只是为命名空间或别名（重命名）或完整命名空间或类型名称提供缩写名称。简单地说，如果你还没有这样做，你需要在下面的所有代码中使用完整类型名称（这也是一个可行的选择）。请参阅： https://msdn.microsoft.com/en-us/library/7f38zh8x.aspx [ ^ ]。



没有像在.NET中导入的东西。最接近的概念是assembly 引用，它真正定义了程序集可以使用的程序集。 进口无法改变任何必要条件;你可以把它当作某种语法糖。



现在，让我们来解决你的真正问题吧。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

Hello

I'm using VB.Net 2013.

I am creating an object using the IMPORTS keyword on a web service.

For example:

IMPORTS sd = TEST.WEBSERVICE.UI <== this is the object from the web service reference.

I can then use the sd object in a method and I get the properties and events, and it Builds successfully:

sd.TRANSMISSION.TRANSMITCODE = "36X"

However, when I run the program I get the dreaded "Object reference not set to an instance of an object" error.

This error usually means that you didn't use the New keyword to create a new instance of the object.

I didn't think you had to use the New keyword since the IMPORTS declaration instantiates the object, or am I wrong?

How do I get the sd object to instantiate using IMPORTS?

Thanks

解决方案

The problem is not related to Imports. You are just using type or namespace name alias. This word is "Imports" is highly misleading. You actually don't "import" anything. Probably this working some VB historical artifact. This is just providing a shortened name for a namespace or aliasing (renaming) or of the full namespace or type name. Simply put, if you haven't done that, you would need to use full type names in all the code below (which also would be a viable option). Please see: https://msdn.microsoft.com/en-us/library/7f38zh8x.aspx[^].

There is no something like importing in .NET. The closest notion would be assembly referencing which really defines what assemblies can be used by your assembly. Imports cannot change anything essential; you can consider it as some kind of syntactic sugar.

Now, let's get to your real problem. 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 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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