Question

We call a (thread-safe) static method in a multi-threaded context, but need to add returned error messages to some sort of list.

In .NET 4, a ConcurrentDictionary could be used. But what is an alternative for the allMessages object below if I need to run this in 3.5?

 using (var mre = new ManualResetEvent(false))
 {
     for (int i = 0; i < max1; i++)
     {
          ThreadPool.QueueUserWorkItem(st =>
          {
            for (int j = 0; j < max2; j++)
            {
                string errorMsg;
                // Call thread-safe static method
                SomeStaticClass.SomeStaticMethod(out errorMsg);

                // Need to collect all errorMsg in some kind of threadsafe list
                // allMessages.Add(errorMsg);

             }
             if (Interlocked.Decrement(ref pending) == 0) mre.Set();
           });
    }
    mre.WaitOne();
 }

The allMessages-object can be as simple as possible – any kind of list would do that is capable of being filled with strings in a thread-safe manner.

Answer 1

You could write your own implementation of the ICollection<T> interface, where you just delegate all calls to an private member of type List<T>, protecting every access with lock.

E.g.

public class LockedCollection<T> : ICollection<T>
{
   private readonly object _lock = new object();
   private readonly List<T> _list = new List<T>();

   public void Add(T item)
   {
      lock (_lock)
      {
         _list.Add(item);
      }
   }

   // other members ...

}

If that is worth the “effort” (and code to maintain), is the question you need to decide. You could just protect your calls to a “plain” List with a lock/monitor.

E.g.

            for (int j = 0; j < max2; j++)
            {
                string errorMsg;
                // Call thread-safe static method
                SomeStaticClass.SomeStaticMethod(out errorMsg);

                lock (allMessageLock) // allMessagesLock is an "object", defined with the same lifetime (static- or instance-member) as "allMessages".
                {
                   // Need to collect all errorMsg in some kind of threadsafe list
                   allMessages.Add(errorMsg);
                }
            }