Lock in multi-threading

Question

class Port
{
    static readonly object locker = new object();
    List<Connection> listOfConnections = new List<Connection>

    public void MethodX()
    {
       Thread.Sleep(10000);
       lock(locker)
       {
           listOfConnections.RemoveAt(0);
       }
    }

    public void ReceiveFromSwitch()
    {
        lock(locker)
        {
           if(listOfConnections.Count == 0) listOfConnections.Add(new Connection());
           if(listOfConnections.Count == 1) MessageBox.Show("Whatever");

           new Thread(()=>MetohodX()).Start();
        }
    }
}

That's my code, two different threads call the method ReceiveFromSwitch(). My objective is to be given a messagebox "Whatever". One thread starts first. It steps into ReceiveFromSwitch, locks the resource and the second thread is waiting for the resource to be released. A connection on the list is added, it steps into MethodX() and release the method ReceiveFromSwitch for a thread in the queue. The second one steps into the method. The count equals 1, so it shows message.

It doesn't work. It gives two messages "Whatever". How can i fix it?

Silly question but are the two threads calling the method on the same instance of Port? Because the list is not static. — Rotem
– Rotem, Commented Dec 8, 2013 at 16:04
I have edited your title. Please see, "Should questions include “tags” in their titles?", where the consensus is "no, they should not". — John Saunders
– John Saunders, Commented Dec 8, 2013 at 16:57

Rotem · Accepted Answer · 2013-12-08 16:11:22Z

1

You forgot an else.

if(listOfConnections.Count == 0) listOfConnections.Add(new Connection());
else if(listOfConnections.Count == 1) MessageBox.Show("Whatever");

//or better yet
if (listOfConnections.Any())
{ 
    MessageBox.Show("Whatever");
}
else
{
    listOfConnections.Add(new Connection());
}

What's happening is the first thread enters and adds a connection to the list, and then immediately shows the message because the Count is now 1. The second thread enters, and as expected, shows the second message.

There is another problem with your code. The second thread will also trigger MethodX, and when it executes after 10 seconds, it will try to remove index 0 from an already empty list, causing an ArgumentOutOfRangeException.

answered Dec 8, 2013 at 16:11

Rotem

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1 Comment

user3076439 Over a year ago

Thanks, however it was just an example based on much longer code. The solution was deeper. Locker was doing great, but I was so confused that I suspected my incompetence was a reason of not working properly.

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