How to chain a task to a previous instance of it?

问题

Looking to chain a task to a previous instance if it exists. Currently, both are executed at the same time.

Initial code that works for one task :

    private async void MenuMediaAddFiles_OnClick(object sender, RoutedEventArgs e)
    {
        var dialog = GetDefaultOpenFileDialog();
        using (dialog)
        {
            if (dialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                using (var progress = new SimpleProgress(this))
                {
                    int addFiles = await _context.AddFiles(dialog.FileNames, progress);
                    Console.WriteLine("Files added: {0}", addFiles);
                }
            }
        }
    }

A failed attempt to make it work :

    Task<int> _files;
    private async void MenuMediaAddFiles_OnClick(object sender, RoutedEventArgs e)
    {
        var dialog = GetDefaultOpenFileDialog();
        using (dialog)
        {
            if (dialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                using (var progress = new SimpleProgress(this))
                {
                    int addFiles;
                    Task<int> files = _context.AddFiles(dialog.FileNames, progress);
                    if (_files == null)
                    {
                        _files = files;
                    }
                    else
                    {
                        var task1 = await _files.ContinueWith(task => _context.AddFiles(dialog.FileNames, new SimpleProgress(this)));
                    }

                    addFiles = await _files;
                    Console.WriteLine("Files added: {0}", addFiles);
                }
            }
        }
    }

回答1:

You were pretty close, but there were a few things that needed to be modified:

private Task<int> previousTask = Task.FromResult(0);
private async void MenuMediaAddFiles_OnClick(object sender, RoutedEventArgs e)
{
    var dialog = GetDefaultOpenFileDialog();
    using (dialog)
    {
        if (dialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
        {
            using (var progress = new SimpleProgress(this))
            {
                previousTask = previousTask.ContinueWith(t =>
                    _context.AddFiles(dialog.FileNames, progress))
                    .UnWrap(); ;

                int addFiles = await previousTask;
                Console.WriteLine("Files added: {0}", addFiles);
            }
        }
    }
}

Things to note:

• Rather than having the previous task be null sometimes, it was easier to initialize it to an already completed task (Task.FromResult(0)). This avoids the null check code.

• You were calling AddFiles twice. You shouldn't have been calling it before the if, and you weren't ever assigning the task to the instance field inside the if.

• I used UnWrap instead of await to turn the Task<Task<int>> into a Task<int>. Both work, but in this case I felt UnWrap made its intentions clearer.

• Note that since the entire event handler will be running in the UI thread there's no need to synchronize access to previousTask, if it doesn't, you'd need to do some locking.

来源：https://stackoverflow.com/questions/16285569/how-to-chain-a-task-to-a-previous-instance-of-it