c# - Correct way to handle locking of classes shared across threads -

i have cleaned , shortened snippet of code multiuser game show accomplish. here 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 { { return opened; } }     } 

what know is: how prevent user (executing code thread) open box being picked other user. i've thought of ways think people here come clever ideas, make me avoid stupid design issue.

edit: suggested in answers, use lock keyword in open method: not want, i'll try explain allowed , not:

the network requests input somehow async , 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 box
• (6) user 1 issues command open box

we order:

2,3,1,5,4,6

2 - allow 3 - allow 1 - allow [remains in execution , has not set owner] 5(comes in between 1) - allow 4(comes in between 1) - disallow (not because open because 1 in execution) 6(comes in between 1) - allow since user 1, , picking 

thanks!

you can use lock statement prevent 2 threads accessing open , close. prevent race condition on checking if box opened, can change open() tryopen(), , return false if box open. there ways don't have return boolean, simplest.

if 1 thread reaches lock statement while thread inside lock statement, second thread wait until first thread exits lock statement before continuing.

private object locker=new object(); public bool tryopen(subject by, params string[] p) {     lock(locker)     {         if(this.opened)             return false;         this.opened = true;         return true;     } }  public void close(subject by, params string[] p) {        lock(locker)     {         this.opened = false;     } }