Awaiting events to be fired from task list

Question

I am starting several tasks using Task.Run() . Within each of these tasks, I define an event handler for InstallCompleted . Then, I await for these tasks to finish. The tricky bit is that the installModule.Install() method seen below returns immediately because internally it calls a OneWay WCF service call.

So, how do I await until all instances of installModule in each task above have fired their respective InstallCompleted events? If there is a better way to accomplish this, I'm all ears.

var installTasks = new List<Task>();

foreach (var targetMachine in vm.TargetMachines)
{
    var installTask = Task.Run(() =>
    {
        foreach (var module in Modules)
        {
            var installModule = module as IInstallModule;  // copy closure variable
            if (installModule == null)
                continue;

            installModule.InstallCompleted += (s, e) =>
            {
                //TODO set some flag indicating this install is complete
            };

            var ip = new Progress<InstallProgress>();
            installModule.Install(targetMachine.MachineName, ip);
        }
    });

    installTasks.Add(installTask);
}

await Task.WhenAll(installTasks);  // unblocks immediately because installModule.Install() above returns immediately

Answer 1

Whenever you're interoperating async with another kind of asynchronous API , you can use TaskCompletionSource<T> as an asynchronous one-time signal. Personally, I like to do this as an extension method:

public static Task<InstallCompletedResult> InstallAsync(
    this IInstallModule module, string machineName,
    IProgress<InstallProgress> progress = null)
{
  var tcs = new TaskCompletionSource<InstallCompletedResult>();
  InstallCompletedEventHandler handler = null;
  handler = (s, e) =>
  {
    module.InstallCompleted -= handler;
    if (e.Exception != null)
      tcs.TrySetException(e.Exception);
    else
      tcs.TrySetResult(e.Result);
  };
  module.InstallCompleted += handler;
  module.Install(machineName, progress);
  return tcs.Task;
}

Once you have the interop piece in, consuming it is quite straightforward:

var installTask = Task.Run(async () =>
{
  var tasks = new List<Task>();
  foreach (var module in Modules)
  {
    var installModule = module as IInstallModule;  // copy closure variable
    if (installModule == null)
      continue;

    var ip = new Progress<InstallProgress>();
    tasks.Add(installModule.InstallAsync(targetMachine.MachineName, ip));
  }
});

Or, more simply:

var installTask = Task.Run(async () =>
{
  var tasks = Modules.OfType<IInstallModule>()
      .Select(module => module.InstallAsync(targetMachine.MachineName,
          new Progress<InstallProgress>()));
  await Task.WhenAll(tasks);
});