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

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

You should use a class instead.

Answer 2

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 3

Backpointer HAS to exist before creating a Backpointer, because you can't have a Backpointer without another Backpointer (which would then need another Backpointer and on and on). You simply can't create a Backpointer based on the way you've created it, because, as a struct, Backpointer can never be null.

In other words, it's impossible to create a Backpointer with this code. The compiler knows that, and so it forces you to make something that would work logically.

Answer 4

Structs are stored by value. In this case, your struct stores within itself another instance of the same struct. That struct stores within itself another struct and so on. Therefore this is impossible. It is like saying that every person in the world must have 1 child. There is no way this is possible.

What you need to use is a class. Classes store by reference, which means that it does not store the class within itself, it only stores a reference to that class.

Answer 5

A CLR struct is by definition a value type . What this means in your context is that the compiler needs to know the exact layout of the type. However, it cannot know how to layout a type which contains an instance of itself - does that sound reasonable? Change the struct to class (which makes your BackPointer to a reference type ) and you'll see it's gonna work out of the box. The reason is that an instance of any reference type has always has the same layout - it is basically just a "pointer" to some location of the managed heap. I strongly recommend to read on a bit about the basics of C# or CLI type system.