Question

I have cleaned and shortened a snippet of my code for a multiuser game to show what I would like to accomplish. So here it is:

    public class Subject {
        public List<IObject> Objects = new List<IObject>();
    }

    public interface IOpenable
    {
        void Open(Subject by, params string[] p);
        void Close(Subject by, params string[] p);
        bool IsOpen { get; }
    }

    public interface IObject {
        void Pickup(Subject by, params string[] p);
        void Drop(Subject by, params string[] p);
    }

    public class Box : IObject, IOpenable
    {
        bool opened = false;
        Subject owner = null;

        public void Pickup(Subject subject, params string[] p) {
            subject.Objects.Add(this);
            this.owner = subject;
        }

        public void Drop(Subject subject, params string[] p)
        {
            subject.Objects.Remove(this);
            this.owner = null;
        }

        public void Open(Subject by, params string[] p)
        {
            this.opened = true;
        }

        public void Close(Subject by, params string[] p)
        {
            this.opened = false;
        }

        public bool IsOpen { get { return opened; } }
    }

What I would like to know is:
How to prevent some user (executing code from another thread) to open a Box that is currently being Picked up by some other user.
I’ve thought of some ways but I think that people here often come up with clever ideas, that could make me avoid some stupid design issue.

EDIT: As suggested in the answers, to use the lock keyword in the open method: this is not exactly what I want, I’ll try to explain with what is allowed and what not:

The network requests we get as input are somehow async and get out of order if issued fast.

• (1) User 1 issues command PICKUP BOX
• (2) User 1 issues command OPEN BOX
• (3) User 1 issues command CLOSE BOX
• (4) User 2 issues command OPEN BOX
• (5) User 2 issues command LOOK BOX
• (6) User 1 issues command OPEN BOX

We get this order:

2,3,1,5,4,6

2 - allow
3 - allow
1 - allow [remains in execution and has not set the owner]
5(comes in between 1) - allow
4(comes in between 1) - disallow (not because already open but because 1 is in execution)
6(comes in between 1) - allow since it is from user 1, and he is currently picking it up

Thanks!

Answer 1

My solution to this is:

Monitor.TryEnter(obj)

Similar to lock but not blocking.

So we lock an object when a user start picking it up,

and when Opening the box do a check that says:

if (this.subject == subject || Monitor.TryEnter(obj))