chaining array of tasks with continuation

Question

I have a Task structure that is a little bit complex(for me at least). The structure is:

(where T = Task)

T1, T2, T3... Tn. There's an array (a list of files), and the T's represent tasks created for each file. Each T has always two subtasks that it must complete or fail: Tn.1, Tn.2 - download and install. For each download (Tn.1) there are always two subtasks to try, download from two paths(Tn.1.1, Tn.1.2).

Execution would be:

First, download file: Tn1.1. If Tn.1.1 fails, then Tn.1.2 executes. If either from download tasks returns OK - execute Tn.2. If Tn.2 executed or failed - go to next Tn.

I figured the first thing to do, was to write all this structure with jagged arrays:

    private void CreateTasks()
    {     
        //main array
        Task<int>[][][] mainTask = new Task<int>[_queuedApps.Count][][];
        for (int i = 0; i < mainTask.Length; i++)
        {
            Task<int>[][] arr = GenerateOperationTasks();
            mainTask[i] = arr;
        }
    }

    private Task<int>[][] GenerateOperationTasks()
    {
        //two download tasks 
        Task<int>[] downloadTasks = new Task<int>[2];
        downloadTasks[0] = new Task<int>(() => { return 0; });
        downloadTasks[1] = new Task<int>(() => { return 0; });

        //one installation task 
        Task<int>[] installTask = new Task<int>[1] { new Task<int>(() => { return 0; }) };

        //operations Task is jagged - keeps tasks above
        Task<int>[][] operationTasks = new Task<int>[2][];
        operationTasks[0] = downloadTasks;
        operationTasks[1] = installTask;

        return operationTasks;
    }

So now I got my mainTask array of tasks, containing nicely ordered tasks just as described above. However after reading the docs on ContinuationTasks, I realise this does not help me since I must call e.g. Task.ContinueWith(Task2). I'm stumped about doing this on my mainTask array. I can't write mainTask[0].ContinueWith(mainTask[1]) because I dont know the size of the array.

If I could somehow reference the next task in the array (but without knowing its index), but cant figure out how. Any ideas? Thank you very much for your help.

Regards,

Answer 1

If you want to run some operations in sequence on a background thread, you don't need lots of Tasks, you need just one:

Task.Factory.StartNew(DownloadAndInstallFiles)

And inside that Task, you can use normal sequential control flow constructs, like foreach and ifs to do what you want. Something like:

void DownloadAndInstallFiles()
{
    foreach (var file in files)
    {
       var downloaded = Download(file, primarySource);
       if (downloaded == null)
           downloaded = Download(file, secondarySource);

       if (downloaded != null)
           Install(downloaded);
    }
}