How to wait until all tasks finished without block

问题:

In the following code I disable button before processing tasks and would like to enable it after all tasks are finished.

List<Task> tasks = new List<Task>();
buttonUpdateImage.Enabled = false; // disable button
foreach (OLVListItem item in cellsListView.CheckedItems)
{
    Cell c = (Cell)(item.RowObject);

    var task = Task.Factory.StartNew(() =>
    {
        Process p = new Process();
        ...
        p.Start();
        p.WaitForExit();
    });
    task.ContinueWith(t => c.Status = 0);
    tasks.Add(task);
}

Task.WaitAll(tasks.ToArray());
// enable button here

WaitAll is blocking the UI thread. How can I wait until all tasks finish and then enable the button?

回答1:

First, I'd install Microsoft.Bcl.Async which will enable the use of async-await in .NET 4.0.

Now, using the answer to this question, you can asynchronously register for process exit, with no need to use Task.Factory.StartNew:

public static class ProcessExtensions
{
    public static Task RunProcessAsync(this Process process, string fileName)
    {
        if (process == null)
            throw new ArgumentNullException(nameof(process));

        var tcs = new TaskCompletionSource<bool>();
        process.StartInfo = new ProcessStartInfo
        {
            FileName = fileName 
        };

        process.EnableRaisingEvents = true
        process.Exited += (sender, args) =>
        {
            tcs.SetResult(true);
            process.Dispose();
        };

        process.Start();
        return tcs.Task;
    }
}

Now, you can do this:

buttonUpdateImage.Enabled = false; // disable button

var tasks = cellsListView.CheckedItems.Cast<OLVListItem>()
                                      .Select(async item => 
{
    Cell cell = (Cell)item.RowObject;

    var process = new Process();
    await process.RunProcessAsync("path");

    cell.Status = 0;
});

await Task.WhenAll(tasks);
buttonUpdateImage.Enabled = true;

回答2:

List<Task> tasks = new List<Task>();
buttonUpdateImage.Enabled = false; // disable button
foreach (OLVListItem item in cellsListView.CheckedItems)
{
    Cell c = (Cell)(item.RowObject);

    var task = Task.Factory.StartNew(() =>
    {
        Process p = new Process();
        ...
        p.Start();
        p.WaitForExit();
    }).ContinueWith(t => c.Status = 0); // this modification is very important, Please note it, this will fix a problem

    tasks.Add(task);
}

// your code has been modified to just make the task object 
// is the continued task instead of the task is the original task 
// Task.WaitAll and add this instead of it.
Task.Factory.ContinueWhenAll(tasks.ToArray(), ac => buttonUpdateImage.Enabled = true;);    

Another way .....

// Note this object.
Barrier b = new Barrier(cellsListView.CheckedItems.Count, () => {
   // enable the button here
   buttonUpdateImage.Enabled = true;
});

buttonUpdateImage.Enabled = false; // disable button
foreach (OLVListItem item in cellsListView.CheckedItems)
{
    Cell c = (Cell)(item.RowObject);

    var task = Task.Factory.StartNew(() =>
    {
          Process p = new Process();
            ...
          p.Start();
          p.WaitForExit();             
     }).ContinueWith(t => {
          c.Status = 0;
          b.SignalAndWait();
     });
}

the first way is better than the second, I advise you to use the first one.

回答3:

You can create a new task to run WaitAll method,just like

Task.Factory.StartNew(() =>
{
    Task.WaitAll(tasks.ToArray());
    Action act = () =>
    {
        this.button1.Enabled = true;
    };
    this.button1.Invoke(act);
});