c# - Parallel.Invoke does not wait for async methods to complete

Question

I have an application that pulls a fair amount of data from different sources. A local database, a networked database, and a web query. Any of these can take a few seconds to complete. So, first I decided to run these in parallel:

Parallel.Invoke(
   () => dataX = loadX(),
   () => dataY = loadY(),
   () => dataZ = loadZ()
);

As expected, all three execute in parallel, but execution on the whole block doesn't come back until the last one is done.

Next, I decided to add a spinner or "busy indicator" to the application. I don't want to block the UI thread or the spinner won't spin. So these need to be ran in async mode. But if I run all three in an async mode, then they in affect happen "synchronously", just not in the same thread as the UI. I still want them to run in parallel.

spinner.IsBusy = true;

Parallel.Invoke(
     async () => dataX = await Task.Run(() => { return loadX(); }),
     async () => dataY = await Task.Run(() => { return loadY(); }),
     async () => dataZ = await Task.Run(() => { return loadZ(); })
);

spinner.isBusy = false;

Now, the Parallel.Invoke does not wait for the methods to finish and the spinner is instantly off. Worse, dataX/Y/Z are null and exceptions occur later.

What's the proper way here? Should I use a BackgroundWorker instead? I was hoping to make use of the .NET 4.5 features.

Answer 1

It sounds like you really want something like:

spinner.IsBusy = true;
try
{
    Task t1 = Task.Run(() => dataX = loadX());
    Task t2 = Task.Run(() => dataY = loadY());
    Task t3 = Task.Run(() => dataZ = loadZ());

    await Task.WhenAll(t1, t2, t3);
}
finally
{
    spinner.IsBusy = false;
}

That way you're asynchronously waiting for all the tasks to complete (Task.WhenAll returns a task which completes when all the other tasks complete), without blocking the UI thread... whereas Parallel.Invoke (and Parallel.ForEach etc) are blocking calls, and shouldn't be used in the UI thread.

(The reason that Parallel.Invoke wasn't blocking with your async lambdas is that it was just waiting until each Action returned... which was basically when it hit the start of the await. Normally you'd want to assign an async lambda to Func<Task> or similar, in the same way that you don't want to write async void methods usually.)