Unsafe structs in C#

Question

I wanted to try c#'s unsafe 'feature' by creating simple structs (Vector, Particle).

SITUATION

I have this 2 structs and want to inject position and velocity vectors into my Particle struct. As a test I wanted to print out position's X value, but somehow I'm getting random values.

I have the following code here

Vector

public readonly struct Vector
{
    public int X { get; }
    public int Y { get; }

    public Vector(int x, int y)
    {
        X = x;
        Y = y;
    }
}

Particle

public unsafe struct Particle
{
    private Vector* mPosition;
    private Vector* mVelocity;

    public Particle(Vector position, Vector velocity = default)
    {
        mPosition = &position; // here is x 10
        mVelocity = &velocity;
    }

    public int GetPosX()
    {
        return mPosition->X; // but here not
    }
}

Program

public class Program
{
    private static void Main(string[] args)
    {
        var pos = new Vector(10, 5);
        var obj = new Particle(pos);

        Console.WriteLine(obj.GetPosX()); // prints random value
    }
}

PROBLEM

It prints a random value instead of 10.

Answer 1

class Program {
    static void Main (string [ ] args) {
        unsafe {
            Vector pos = new Vector(10, 5);
            Particle obj = new Particle(&pos);
            // &pos is at position 0xabcdef00 here.
            // obj.mPosition has a different value here. It points to a different address? Or am I misunderstanding something
            Console.WriteLine(obj.GetPosX( ));
        }
    }
}

public struct Vector {
    public int X;
    public int Y;

    public Vector (int x, int y) {
        X = x;
        Y = y;
    }
}

public unsafe struct Particle {
    private Vector* mPosition;

    public Particle (Vector *position) {
        mPosition = position; // here is x 10
    }

    public int GetPosX ( ) {
        return mPosition->X; // still 10 here
    }
}

This works for me. Please... do not ask me why it does. You will notice that I didn't change that much. Just calling Particle with *pos instead of pos. For some reason that fixes the problem. You have to wrap the code with unsafe then and change the constructor for Particle obviously.

I could speculate about why it works, but I'd rather not. Maybe the pointer changes when you pass pos as a parameter for some reason?

Answer 2

You could not take the ref with right value.

Create a variable like int posX = 10;

And you can take the reference with variable. You take the compile time reference and read the runtime reference.

Don't use pointers without fixed. C# stack performance ise very good. You dont need this.

Usually pointers use with linking (C/Cpp dynamic library linking etc). If you have large structs (30 bytes and greater) then you can use the ref parameter tag.