What's wrong with this C# struct?

Question

Note: My question has several parts to it. I'd appreciate it if you would please answer each of the questions, instead of simply telling me what to do to get this to compile. :)

I'm not by any means good with C#. In fact, the reason why I don't know much about it is my class is focused on making efficient Algorithms and not really on teaching us .NET. Nevertheless all of our programs must be written in .NET and it hasn't been a problem until just now. I have the following code, but it won't compile and I don't really understand why. I have a gut feeling that this should be rewritten altogether, but before I do that, I want to know WHY this isn't allowed.

The point of the struct is to create a linked list like structure so I can add another node to the end of the "list" and then traverse and recall the nodes in reverse order

private struct BackPointer
{
    public BackPointer previous;
    public string a;
    public string b;
    public BackPointer(BackPointer p, string aa, string bb)
    {
        previous = p;
        a = aa;
        b = bb;
    }
}

then later in my code I have something to the effect of

BackPointer pointer = new BackPointer();
pointer = new BackPointer(pointer, somestring_a, somestring_b);

The compile error I'm getting is Struct member 'MyClass.BackPointer.previous' of type 'MyClass.BackPointer' causes a cycle in the struct layout

This seems to be an obvious error. It doesn't like the fact that I am passing in the struct in the constructor of the same struct. But why is that not allowed? I would imagine this code would just create a new node in the list and return this node with a pointer back to the previous node, but apparently that's not what would happen. So what would actually happen then? Lastly what is the recommended way to resolve this? I was thinking to just tell it to be unmanaged just handle my pointers manually, but I only really know how to do that in C++. I don't really know what could go wrong in C#

Answer 1

But why is that not allowed?

It's a struct - a value type. That means wherever you've got a variable of that type, that variable contains all the fields within the struct, directly inline. If something contains itself (or creates a more complicated cycle) then you clearly can't allocate enough space for it - because it's got to have enough space for all its fields and another copy of itself.

Lastly what is the recommended way to resolve this?

Write a class instead of a struct. Then the value of the variable will be a reference to an instance, not the data itself. That's how you get something close to "a pointer" in C#. (Pointers and references are different, mind you.)

I suggest you read my article on value types and reference types for more information - this is an absolutely critical topic to understand in C#.

Answer 2

That's not a pointer; it's an actual embedded struct value.
The whole point of structs is that they're (almost) never pointers.

You should use a class instead.