如何避免C＃中的StackOverflowException？

Question

I'm fairly new to C# and thought I'd start with the tutorials here.

I have done the first 3, and was researching more about C# classes and the various ways you can use them. It then dawned on me that all my code was in one class (MainForm, as I'm using SharpDevelop) and I should try and organise it better. However, when I try to refer to code in other classes, I get a StackOverflowException. I have instantiated the class (eg CheckAnswers checkanswers = new checkanswers; ), what's going wrong? I can post the code if need be, but my guess is that this is some n00b error that is fairly easy to diagnose.

EDIT:

Code - https://docs.google.com/open?id=0B9vVcIham8NHMGlNaDk3MTZ6ejA

There's quite a bit. This is only the code that I've written myself, not the form config stuff.

Answer 1

You're making mutual-recursion here.

In MainForm you implicitly call the CheckAnswers constructor at line 23 (it's an initialization of a non-static variable, meaning it'll run when MainForm is constructed).
In CheckAnswers you create a MainForm instance in a similar way (line 193).

That'll cause a stack overflow - MainForm creates a CheckAnswers object which Creates a MainForm object which creates a CheckAnswers object which...

Edit: If you need to make two way communication between the classes (which I advice against in most cases), this won't work. The CheckAnswers object hold a reference to a different instance MainForm, an so does the instance of MainForm. This will simply not work (as well as cause an infinite recursion as see).
One way to do what you want is to send an instance of MainForm to the CheckAnswers constructor, and in line 23 use "this" as the parameter for the constructor and initialize mainform (in CheckAnswers) using that parameter. Something like this:

public partial class MainForm : Form
{
    CheckAnswers checkanswers;
    ...
    public MainForm()
    {
        checkanswers = new CheckAnswers(this);
        ...
    }
}
public class CheckAnswers // Not sure why you inherit MainForm here, but it's not a good idea, as someone already stated
{
    MainForm mainform;

    public CheckAnswers (MainForm main)
    {
        mainform = main;
    }
    ...
}

Answer 2

CheckAnswers class derives from MainForm class. So when you call to constructor of CheckAnswers class, it creates instance of this class with all internal members of base class (MainForm). That's why this line of code is called again:

Line 23: CheckAnswers checkanswers = new CheckAnswers();

And again, instance is created and this line of code is called again and again recursively until StackOverflow exception occurs.

Basically, this code represents the same problem as in your code:

class BaseClass
{
    DerivedClass myObject = new DerivedClass(); // this is a recursive call
}

class DerivedClass : BaseClass
{   
}

Answer 3

To communicate, you should pass as parameter an instance of existing object, not create a new one each time (otherwise there will no be communication between the concrete objects).

So here is an example that represents the idea of what you need to do instead:

public class CheckAnswers
{
    MainForm mainform;
    public CheckAnswers (MainForm  form)
    { 
        this.mainform = form;
    }

   ...
}

In the MainForm class you will have to pass "itself" as parameter to constructor of CheckAnswers like this:

CheckAnswers checkanswers = new CheckAnswers(this);