Declaring a list of lists

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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:

VB

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

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.

Posted 20-Aug-14 5:58am

ScottTheHunter

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2 solutions

Solution 1

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

Posted 20-Aug-14 6:05am

PhilLenoir

Comments

Sergey Alexandrovich Kryukov 20-Aug-14 12:15pm

Why the would you think the first line may not work? Why "trying" bad things? Why casting, if OP uses generics (which, by the way, I advised answering his original question)?! You see, if you are not sure, not answering is much better. It's apparent that you are not confident with this matter yourself, so why confusing the readers?

"Trying" is fine, but no one should write code by trying. The developer should understand each and every line of what one is doing. Please, don't set a bad model of behavior.

—SA

PhilLenoir 20-Aug-14 12:26pm

Sergey, if you read his question, he said he had tried "New" in various places. I don't believe there would be any problem with the first line, but depending on his version of VS there may be a parser bug. His reference to his attempts would lead me to assume that he had tried just that. I prefer to give options, that way the asker can get on and solve his problem without bouncing things backwards and forwards. If I believe I know all the facts, my answers are more direct. 1) I didn't realize that you had given an answer to a similar question from him. 2) I offered a solution which will work on my version if VS, however I never assume facts not in evidence, so the second option works. The cast would only be necessary if he had to use a generic and would ensure that his contained list was strongly typed to a list.

Sergey Alexandrovich Kryukov 20-Aug-14 12:31pm

No, you are not giving any reasonable options, you are doing what I just described: you are not sure about the matter and advise to try. If you were sitting in one room with OP, it would be fine, but suggesting bad, really bad thing (which cannot work) as an option at public forum is not really responsible way of doing thing. Why would you at least did not try writing the code?
—SA

PhilLenoir 20-Aug-14 12:34pm

Why are yopu trolling. You seem to be denying that the solution does not work. It does.

PhilLenoir 20-Aug-14 12:29pm

... and we all have to program by trying. Most especially when our toolsets are buggy (which they often are). Workarounds are rarely, if ever, best practice, but sometimes you gotta do what you gotta do.

Sergey Alexandrovich Kryukov 20-Aug-14 12:34pm

No! You can lean programming by trying (so minor part of it), but not programming. Yes, sometimes you have to deal with buggy API, but this is not the case, so it would be better if you did not excuse your own mistakes by non-existing mistakes of others. You are on the wrong path, and yet trying to advise... No good.
—SA

PhilLenoir 20-Aug-14 12:38pm

BS. I've been programming for more years than I care to mention. I do not believe that any professional programmer writes perfect, bug free code at first pass all the time. You are making personal attacks on me when I offered a solution that works. Get over yourself.

Sergey Alexandrovich Kryukov 20-Aug-14 12:57pm

It is totally irrelevant. If you have many years of experience, it only makes me more sad. You are trying to protect yourself instead of discussing the matter.
Please, enough of this discussion.
—SA

PhilLenoir 20-Aug-14 14:37pm

My solution does work. You even falied to spot that he exactly described where his error was. There are two types of people who post answers: Those of us who are collaboratively minded and those who are trying to demonstrate their supoeriority. It turns out it was exactlty as I suspected. The poster had missed the new on the first declaration. The first suggestionI proffered was exactly what was required. I had assumed that he had tried that but put it in anyway. If it failed in his environment, not mine or yours, then there had to be another reason. It's never reasonable to make personal attacks in these fora. That's exactly what happens in SO. It discourages people from asking questions and offering solutions. You say "enough of this discssion" after making a personal attack and downvoted my solution both, alternatives of which work. You can't just make insults then say enough; it's infantile. With your rep on here, I have to assume that you have helped lots of people, but that doesn't give you the right to throw out personal attacks. Shame on you.

Sergey Alexandrovich Kryukov 20-Aug-14 16:36pm

Okay, sorry for the words "not working". I should have said "unacceptable".
Please, I'm not ready to discuss any personal characteristics of any people; this is not an appropriate place for that. You have no idea who an I and accuse me in a totally baseless way, out of your frustration, nothing else.

Please understand: I criticize your answer to protect the readers from being badly mislead. And no one should be free from critiques, only we should not move to personal characterization. I can communicate with you only if you stop it.

Good luck,
—SA

PhilLenoir 20-Aug-14 16:52pm

Sergey, can you point out where I attacked you. I have attacked your behaviour. If I'm frustrated it because this is what I said:

"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))"

This is exactly the solution. Where am I on the wrong path? I had to assume that this was not working for the poster? I'm trying to give quick solutions to help a fellow professional. I have no problem if you want to add some education on error trapping and pointers to technical documents. Genuinely, good for you. But downvoting and criticizing a viable solution and actually the correct solution and imposing your style of answer on the rest of us is uncalled for. I say again that you have done good work on here, but putting the boot in to my solution is not fair game.

Sergey Alexandrovich Kryukov 20-Aug-14 17:05pm

All my criticism is about Dim SeriesList as New List (Of Object). This is misleading and really, really bad. Generics are created to avoid type case, and you artificially introduce the type cases where it is already eliminated. First sample if perfectly fine, but the fact that you say "If this doesn't work, I'd try" tells me that you are not confident enough to answer the question. If I was so non-confident, I would not allow myself to answer, and if I did the mistake of answering this way, I would accept the criticism. Every regular user here knows that in such cases I admit my mistakes and say "thank you" for one who pointed out my mistake.

I'm ready to consider you not attacking me; I'm generally not the one who plays offended card. I'm just not ready to discuss anything about "There are two types of people who post answers: Those of us who are collaboratively minded and those who are trying to demonstrate their supoeriority". This not just baseless (again, you don't know me to say anything); but, first of all, this is a wrong place for such discussion, I just cannot accept it. Probably you are not getting it: I want to avoid anything negative by all means, but only if nobody tries to take out my right for criticism. Criticism is one of the main driving forces of every progress and the way to help people to avoid many frustrations.

—SA

PhilLenoir 21-Aug-14 12:30pm

Sergey, maybe the answer could have been worded better, but I stand by the solution. Here were my thought processes. I saw that the outer list had not been initialized. It's an unusual nested declaration, so I did a quick test in VS 2010, the compiler was happy. The poster said he had tried "New" everywhere he could think of. Now we've all been caught looking for a problem in the wrong place, but what if he'd tried the declaration and it didn't work. I have no access or information about his environment and I'm certainly not going to test every variant of VS builds, yet alone other non VS compiler options. So I typed the right answer but added the caveat. Now my assumption is that the poster will see the first line and thinks "Oh, crap, I didn't try that", problem solved with a tight strongly typed declaration. But if he reads it and he had already tried that, he's no further forward. Maybe it has never happened to you and it doesn't happen often , but sometimes a toolset misbehaves and you have to find a quick and dirty solution. The second declaration is just such a suggestion. Maybe you would have been happier if I'd typed that all out, but I do tend to assume that the reader has a brain and coding standards.

Now when you assume that I don't know that the the first declaration is correct you not only assume facts not in evidence, but I did use the words "The code should read ...". Next you did impune my morality "(Yes, and be careful with wrong answers, like Solution 1, where I can even see some ethical problem :-( )" For you to question my ethics was outrageous. Now maybe you can see why this made me so angry. I submit that the answer was not wrong and that an honourable approach on your part would have been to suggest your own solution without impugning me or mine.

We seem to have had a communication breakdown here. I don't mind criticism when it's justified. In fact I'm my own harshest critic and accept that more verbose wording might have avoided this spat, but the essence of the solution was 100% and so were my motivations. The, in my view, unjustified criticism still irks and the personal sleight was salt on the wound.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2014-08-20T06:10:00

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