声明列表列表 [英] Declaring a list of lists

问题描述

我搜索了论坛，无法找到解决方案。我正在尝试声明一个列表（List（of something）。我试图复制我找到的其他示例，但无法使其工作。这只是我的代码的一个简短片段：

I have searched the forums and can't find a solution to this one. I am trying to declare a list(of List(of something). I have tried to copy the other examples I have found but can't get it to work. Here is just a short clip of my code:

Dim SeriesList As List(Of List(Of PlotData))

   Sub Main()

       For x As Integer = 0 To 3
           Dim Series As New List(Of PlotData)
           SeriesList.Add(Series)
       Next

当我到达SeriesList.Add（系列）时，我得到一个指向此行的NulReferenceException，告诉我对象引用未设置为对象的实例并使用'new'关键字创建对象实例但是......我无法弄清楚'新'关键字的位置。我尝试的一切只会导致语法错误。



我非常感谢这里有这么多知识渊博的人所给予的所有帮助，所以，提前谢谢。

When I get to the SeriesList.Add(Series) I get a NulReferenceException pointing to this line telling me that the "Object reference not set to an instance of an object" and to "use the 'new' keyword to create an object instance." But... I can't figure out where to put the 'new' keyword. Everything I try just causes syntax errors.

I so very much appreciate all the help so willingly given by so many knowledgeable people here, so, thanks in advance.

推荐答案

除非编译器对嵌套列表声明有问题，否则我没有看到问题。代码应为：



Dim SeriesList为新列表（列表（Of PlotData））



如果这不起作用，我会尝试



Dim SeriesList作为新列表（对象）

然后我会在访问嵌套列表

I don't see the problem unless the compiler has a problem with the nested list declaration. The code should read:

Dim SeriesList as New List(Of List(Of PlotData))

If this doesn't work, I'd try

Dim SeriesList as New List (Of Object)
I would then cast back when accessing the nested lists

您应该在原始问题的页面上询问我已经回答的问题：使用一组集合（对象） [ ^ ]。



正如您所看到的，列表类型是通用的，因此它可以是任何其他类型的元素列表，因此，为什么不列出？怎么可能是个问题呢？例如，如果你需要一个字典列表或一个列表字典，那么这是另一个问题吗？ :-)



让我们看看：你已经将每个列表初始化为外部列表的元素。但是你没有初始化外部列表本身。 SeriesList 为空。



看，我不想只修复你的错误，我想要你自己思考。看起来您理解所有对象最终都需要初始化。如果必须初始化某个对象，为什么不初始化另一个？在 SeriesList 的情况下，在声明点初始化它。



这不是全部。如何提问提供足够信息的问题？



您没有显示带有对象引用未设置为对象实例消息的异常的位置。



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

You should have asked that on the page of your original question, where I already answered: Work with a collection of collections(of objects)[^].

As you could see, the list type is generic, so it can be a list of the elements of any other types, so, why not lists? How can it possibly be a problem? If you, for example, need a list of dictionaries, or a dictionary of lists, would it be another question? :-)

Let's see: you have initialized each list to become and element of the outer list. But you did not initialize the outer list itself. SeriesList is null.

Look, I don't want just to fix your bug, I want to you to think by yourself. It looks like you understand that all objects need to be initialized, eventually. If some object has to be initialized, why not initializing another one? In the case of SeriesList, initialize it at the point of declaration.

That's not all. How about asking questions providing adequate information?

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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