Starting Tasks after Tasks Finish

I attached the entire code, but the code I am interested in is creating tasks to combine images using the program ImageMagick.  I read in tif file names that are named using the 12 char invoice number.  I want to combine 2 together with 2 instruction pages.  Since each invoice is independent of the other, each can run in parallel.  So I read in one invoice (2 files) and combine into another tif file.  There are 10's of thousands.  Takes a long time to run.  So I have 4 tasks.  Each can handle one invoice at a time.  When each task in done, it can start another invoice.  I just do not know the formats of the Task class to use.  I have tried a lot of things.  I know that I have to have an inner loop added, but I just do not know how to do that with the Task class.  This is the code that I am talking about, but the whole program is attached.

 DirectoryInfo directoryInfo = new DirectoryInfo(@"C:\Examples\TaskImageMagick\Tiff\");
 List<FileInfo> list = directoryInfo.GetFiles().ToList();
 StringBuilder args = new StringBuilder();
 string previousFileKey = "";



 //Remove files from list, only need numbered files,then sort
 list.RemoveAll(n => n.Name == "Instructions.tif");
 list.Sort((a, b) => string.Compare(a.Name, b.Name));





 //Initialize tasks
 Task t1 = new Task(() => CombineTiff(args.ToString(), previousFileKey));
 Task t2 = new Task(() => CombineTiff(args.ToString(), previousFileKey));
 Task t3 = new Task(() => CombineTiff(args.ToString(), previousFileKey));
 Task t4 = new Task(() => CombineTiff(args.ToString(), previousFileKey));
 foreach (var item in list)
 {
 if (previousFileKey == item.Name.Substring(0, 12))
 { 
  // add to arg list
 args.Append(" +page ");
 args.Append(item.Name);
 args.Append(" +page ");
 args.Append("Instructions.tif"); 
   previousFileKey = item.Name.Substring(0, 12);
 }
 else 
  {
 //Run combine tiff process
 if (t1.IsCompleted)
 t1.Start();
 else if (t2.IsCompleted)
 t2.Start();
 else if (t3.IsCompleted)
 t3.Start();
 else if (t4.IsCompleted)
 t4.Start();
 //clear args
 args = new StringBuilder();
 // add to arg list
 args.Append(" +page ");
 args.Append(item.Name);
 args.Append(" +page ");
 args.Append("Instructions.tif");
 previousFileKey = item.Name.Substring(0, 12);
 }





}





I tried to write this out, so I hope it is understandable.

I hope someone can help and I would really appreciate it.

Thanks for the help.

Arep