Question

I’m facing a deadlock, my code structure is similar to this:

private delegate void UpdateControlDelegate(string value);  public void UpdateControl(string value) {     if (txtAddress.InvokeRequired)     {         txtAddress.Invoke(new UpdateControlDelegate(UpdateControl), value);     }     else     {         txtAddress.Text = value; // This is in GroupBox1         txtValue.Text = value; // This is in GroupBox2     } }  class ThreadHandler {     List<string> _list = new List<string>();     object _criticalSection = new object();      public ThreadHandler()     {         new Thread(new ThreadStart(Run)).Start();     }      public static ThreadHandler _threadHandler = null;      public static ThreadHandler GetThreadHandler()     {         if (_threadHandler == null)         {             _threadHandler = new ThreadHandler();         }          return _threadHandler;     }      public void Run()     {         while (true)         {             // some code              lock (_criticalSection)             {                 foreach (string str in _list)                 {                     // some Code                 }             }              // some code             Thread.Sleep(SomeTime);         }     }      public void AddItem(string item)     {         lock (_criticalSection)         {             _list.Add(item);         }     }      public void RemoveItem(string item)     {         lock (_criticalSection)         {             _list.Remove(item);         }     }  } 

But using the same code, I just modified the UpdateControl method like this:

private delegate void UpdateControlDelegate(string value);  public void UpdateControl(string value) {     if (InvokeRequired)     {         BeginInvoke(new UpdateControlDelegate(UpdateControl), value);     }     else     {         txtAddress.Text = value; // This is in GroupBox1         txtValue.Text = value; // This is in GroupBox2     } } 

This is working fine. What is the problem?

Answer 1

The problem is almost certainly that you’re acquiring the lock within a background thread, then calling Control.Invoke, and invoking a delegate (on the UI thread) which tries to acquire the same lock. It can’t do that, because the other thread holds the lock – and will keep holding the lock while it waits for the UI operation to complete.

Admittedly there’s no locking within the UpdateControl method you’ve posted, but I suspect that’s not the complete code – and you haven’t shown where you’re using AddItem or RemoveItem.

I note that GetThreadHandler() isn’t thread-safe, by the way – that looks like a bug to me…