Async with await do not work. Why?

2
votes

When I try to run the code below, perform synchronous operations. Why?

I get the following warning ...

Warning 1 This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.

private async void btProcessa_Click(object sender, EventArgs e)
{
    await ProcessaA();
    await ProcessaB();
}

public async Task ProcessaA()
{
    for (int i = 0; i <= 100; i++)
    {
        pbProcessoA.Value = i;
        Thread.Sleep(500);
    }
}

public async Task ProcessaB()
{
    for (int i = 0; i <= 100; i++)
    {
        pbProcessoB.Value = i;
        Thread.Sleep(500);
    }
}
2
c#task-parallel-library.net-4.5async-await
What part of the detailed message do you not understand? We need to know to answer specifically. – usr
this question looks like a dupe to Using async without await – Gennady Vanin Геннадий Ванин

2 Answers

10
votes

async does not mean "run this code on a background thread". If you want to know more about async, I have an introductory blog post, the MSDN docs are great, and there's a full guide to the Task-based Asynchronous Pattern.

If you want to do some simulation of I/O-bound (or event-based) operations, you should use Task.Delay instead of Thread.Sleep:

public async Task ProcessaA()
{
  for (int i = 0; i <= 100; i++)
  {
    pbProcessoA.Value = i;
    await Task.Delay(500);
  }
}

If you want to simulate CPU-bound operations, then you should be pushing them off to a background task via Task.Run:

public async Task ProcessaA()
{
  for (int i = 0; i <= 100; i++)
  {
    pbProcessoA.Value = i;
    await Task.Run(() => { Thread.Sleep(500); });
  }
}
2
votes

I wrote this example to a real production system. Save the entities in a database can take time. Look at this elegant simulation

class TestAsyncAwait
{
    public void GetEntities()
    {
        for (int i = 0; i < 4; i++)
        {
            var a = getEntities(i);
            saveEntitiesAsync(a);
        }
        Console.WriteLine("\nPress any key to close\n");
        Console.ReadKey();
    }

    private List<string> getEntities(int i)
    {
        Console.Write("getting Entities for id={0}...", i);
        Thread.Sleep(2000);
        var r = new List<string> { i.ToString(), " Hello!" };
        Console.WriteLine("done, has the Entities for id={0}\n", i);
        return r;
    }

    async void saveEntitiesAsync(List<string> a)
    {
        var sb = new StringBuilder();
        await Task.Run(() =>
        {
            Thread.Sleep(4000); // simulates long task
            foreach (string s in a) sb.Append(s);
        });
        // shows the thread in action
        Trace.WriteLine("saved: " + sb.ToString());
    }
}