Question

The following program prints:

Entered 3 Entered 4 Wait for Exited messages Exited 3 Exited 4 

Meaning that it cannot acquire an exclusive lock on resource. Why?

public class Worker     {         public void DoIt(object resource)         {             Monitor.Enter(resource);             Console.WriteLine('Entered ' + Thread.CurrentThread.ManagedThreadId);             Thread.Sleep(3000);             Monitor.Exit(resource);             Console.WriteLine('Exited ' + Thread.CurrentThread.ManagedThreadId);         }     }      class Program     {         struct Resource         {             public int A;             public int B;         }          static void Main(string[] args)         {             Resource resource;             resource.A = 0;             resource.B = 1;              var a = new Worker();             var b = new Worker();              var t1 = new Thread(() => a.DoIt(resource));             var t2 = new Thread(() => b.DoIt(resource));              t1.Start();             t2.Start();              Console.WriteLine('Wait for Exited messages');             Console.ReadLine();         }     } 

Answer 1

Your Resource is a struct. It is boxed when passed to DoIt, so each call to DoIt locks a different object. Change Resource to be a class.