Implementing RegEx Timeout in .NET 4

Question

Platform: Silverlight 4, .NET 4

With the .NET 4.5 Developer preview the RegEx class has been enhanced to allow setting of a Timeout value which would prevent the RegEx engine from hanging the UI if there are issues with the pattern matching.

Requesting suggestions to implement a similar functionality in a .NET 4 Silverlight application.

Thanks in advance.

Answer 1

Generic example:

public static R WithTimeout<R>(Func<R> proc, int duration)
{
  var wh = proc.BeginInvoke(null, null);

  if (wh.AsyncWaitHandle.WaitOne(duration))
  {
    return proc.EndInvoke(wh);
  }

  throw new TimeOutException();
}

Usage:

var r = WithTimeout(() => regex.Match(foo), 1000);

Update:

As pointed out by Christian.K, the async thread will still continue running.

Here is one where the thread will terminate:

public static R WithTimeout<R>(Func<R> proc, int duration)
{
  var reset = new AutoResetEvent(false);
  var r = default(R);
  Exception ex = null;

  var t = new Thread(() =>
  {
    try
    {
      r = proc();
    }
    catch (Exception e)
    {
      ex = e;
    }
    reset.Set();
  });

  t.Start();

  // not sure if this is really needed in general
  while (t.ThreadState != ThreadState.Running)
  {
    Thread.Sleep(0);
  }

  if (!reset.WaitOne(duration))
  {
    t.Abort();
    throw new TimeoutException();
  }

  if (ex != null)
  {
    throw ex;
  }

  return r;
}

Update:

Fixed above snippet to deal with exceptions correctly.

Answer 2

It is not that simple - but it can be done using two threads with the first doing the regex, the second killing the first thread if itruns too long. This is problematic in itself, though.