Run sequence of tasks, one after the other

Question

I have a sequence of tasks, where each one depends on the output of the previous one. I\'d like to represent this as a single Task object, whose result is the o

Answer 1

The easy way (using Microsoft.Bcl.Async):

static async Task AggregateAsync(
    this IEnumerable items,
    TState initial,
    Func> makeTask)
{
  var state = initial;
  foreach (var item in items)
    state = await makeTask(state, item);
  return state;
}

The hard way:

static Task AggregateAsync(
    this IEnumerable items,
    TState initial,
    Func> makeTask)
{
  var tcs = new TaskCompletionSource();
  tcs.SetResult(initial);
  Task ret = tcs.Task;
  foreach (var item in items)
  {
    var localItem = item;
    ret = ret.ContinueWith(t => makeTask(t.Result, localItem)).Unwrap();
  }
  return ret;
}

Note that error handling is more awkward with the "hard" way; an exception from the first item will be wrapped in an AggregateException by each successive item. The "easy" way does not wrap exceptions like this.