Question

Im trying to figure out the output of this code:

Dictionary<int, MyRequest> request = new Dictionary<int, MyRequest>();

for (int i = 0; i < 1000; i++ )
{
  request.Add(i, new MyRequest() { Name = i.ToString() });
}

var ids = request.Keys.ToList();

Parallel.For(0, ids.Count, (t) =>
{
  var id = ids[t];
  var b = request[id];

  lock (b)
  {
    if (b.Name == 4.ToString())
    {
      Thread.Sleep(10000);
    }

    Console.WriteLine(b.Name);
  }
});

Console.WriteLine("done");
Console.Read();

output:

789
800
875
.
.
.
4
5
6
7
done

MyRequest is just a dummy class used for demonstration (it is not doing anything but holding values). Is my lock blocking the execution or are the last 4 being put on their own thread?

This is a .NET 4.0 demo.

UPDATE
Ok I did figure out they were on teh same thread, but i would still like to know if the lock does anything to block execution. I cant imagine it does.

Answer 1

If ids does not contain duplicates, that lock won’t block anything. But if there are duplicates in ids, then yes, there might be contention at the lock, as different threads fight for access to the same request.