public static async Task CopyToAsync(
  this Stream input, Stream output,
  CancellationToken cancellationToken = default(CancellationToken))
{
  byte[] buffer = new byte[0x1000];  // 4 KiB
  while (true) {
    cancellationToken.ThrowIfCancellationRequested();
    int bytesRead = await input.ReadAsync(buffer, 0, buffer.Length);
    if (bytesRead == 0) break;
 
    cancellationToken.ThrowIfCancellationRequested();
    await output.WriteAsync(buffer, 0, bytesRead);
  }
}

public static /*async*/ Task CopyToAsync(
  this Stream input, Stream output,
  CancellationToken cancellationToken = default(CancellationToken))
{
  return CopyToAsyncTasks(input, output, cancellationToken).ToTask();
}
private static IEnumerable<Task> CopyToAsyncTasks(
  Stream input, Stream output,
  CancellationToken cancellationToken)
{
  byte[] buffer = new byte[0x1000];  // 4 KiB
  while (true) {
    cancellationToken.ThrowIfCancellationRequested();
    var bytesReadTask = input.ReadAsync(buffer, 0, buffer.Length);
    yield return bytesReadTask;
    if (bytesReadTask.Result == 0) break;
 
    cancellationToken.ThrowIfCancellationRequested();
    yield return output.WriteAsync(buffer, 0, bytesReadTask.Result);
  }
}

public /*async*/ void DoSomethingAsync() {
  DoSomethingAsyncTasks().ToTask();
}
public /*async*/ Task DoSomethingAsync() {
  return DoSomethingAsyncTasks().ToTask();
}
public /*async*/ Task<String> DoSomethingAsync() {
  return DoSomethingAsyncTasks().ToTask<String>();
}

// C#5
public async Task<String> DoSomethingAsync() {
  while (…) {
    foreach (…) {
      return "Result";
    }
  }
}
 
// C#4; DoSomethingAsync() is necessary but omitted here.
private IEnumerable<Task> DoSomethingAsyncTasks() {
  while (…) {
    foreach (…) {
      yield return TaskEx.FromResult("Result");
      goto END;
    }
  }
END: ;
}

public static Task<TResult> FromResult<TResult>(TResult resultValue) {
  var completionSource = new TaskCompletionSource<TResult>();
  completionSource.SetResult(resultValue);
  return completionSource.Task;
}

private abstract class VoidResult { }
 
public static Task ToTask(this IEnumerable<Task> tasks) {
  return ToTask<VoidResult>(tasks);
}

// BAD CODE !
public static Task<TResult> ToTask<TResult>(this IEnumerable<Task> tasks)
{
  var tcs = new TaskCompletionSource<TResult>();
  Task.Factory.StartNew(() => {
    Task last = null;
    try {
      foreach (var task in tasks) {
        last = task;
        task.Wait();
      }
 
      // Set the result from the last task returned, unless no result is requested.
      tcs.SetResult(
        last == null || typeof(TResult) == typeof(VoidResult)
          ? default(TResult) : ((Task<TResult>) last).Result);
 
    } catch (AggregateException aggrEx) {
      // If task.Wait() threw an exception it will be wrapped in an Aggregate; unwrap it.
      if (aggrEx.InnerExceptions.Count != 1) tcs.SetException(aggrEx);
      else if (aggrEx.InnerException is OperationCanceledException) tcs.SetCanceled();
      else tcs.SetException(aggrEx.InnerException);
    } catch (OperationCanceledException cancEx) {
      tcs.SetCanceled();
    } catch (Exception ex) {
      tcs.SetException(ex);
    }
  });
  return tcs.Task;
}

// 很牛逼，但是我们还没有。
public static Task<TResult> ToTask<TResult>(this IEnumerable<Task> tasks)
{
  var taskScheduler =
    SynchronizationContext.Current == null
      ? TaskScheduler.Default : TaskScheduler.FromCurrentSynchronizationContext();
  var tcs = new TaskCompletionSource<TResult>();
  var taskEnumerator = tasks.GetEnumerator();
  if (!taskEnumerator.MoveNext()) {
    tcs.SetResult(default(TResult));
    return tcs.Task;
  }
 
  taskEnumerator.Current.ContinueWith(
    t => ToTaskDoOneStep(taskEnumerator, taskScheduler, tcs, t),
    taskScheduler);
  return tcs.Task;
}
private static void ToTaskDoOneStep<TResult>(
  IEnumerator<Task> taskEnumerator, TaskScheduler taskScheduler,
  TaskCompletionSource<TResult> tcs, Task completedTask)
{
  var status = completedTask.Status;
  if (status == TaskStatus.Canceled) {
    tcs.SetCanceled();
 
  } else if (status == TaskStatus.Faulted) {
    tcs.SetException(completedTask.Exception);
 
  } else if (!taskEnumerator.MoveNext()) {
    // 设置最后任务返回的结果，直至无需结果为止。
    tcs.SetResult(
      typeof(TResult) == typeof(VoidResult)
        ? default(TResult) : ((Task<TResult>) completedTask).Result);
 
  } else {
    taskEnumerator.Current.ContinueWith(
      t => ToTaskDoOneStep(taskEnumerator, taskScheduler, tcs, t),
      taskScheduler);
  }
}

public static Task<TResult> ToTask<TResult>(this IEnumerable<Task> tasks) {
  var taskScheduler =
    SynchronizationContext.Current == null
      ? TaskScheduler.Default : TaskScheduler.FromCurrentSynchronizationContext();
  var taskEnumerator = tasks.GetEnumerator();
  var completionSource = new TaskCompletionSource<TResult>();
 
  // Clean up the enumerator when the task completes.
  completionSource.Task.ContinueWith(t => taskEnumerator.Dispose(), taskScheduler);
 
  ToTaskDoOneStep(taskEnumerator, taskScheduler, completionSource, null);
  return completionSource.Task;
}
 
private static void ToTaskDoOneStep<TResult>(
  IEnumerator<Task> taskEnumerator, TaskScheduler taskScheduler,
  TaskCompletionSource<TResult> completionSource, Task completedTask)
{
  // Check status of previous nested task (if any), and stop if Canceled or Faulted.
  TaskStatus status;
  if (completedTask == null) {
    // This is the first task from the iterator; skip status check.
  } else if ((status = completedTask.Status) == TaskStatus.Canceled) {
    completionSource.SetCanceled();
    return;
  } else if (status == TaskStatus.Faulted) {
    completionSource.SetException(completedTask.Exception);
    return;
  }
 
  // Find the next Task in the iterator; handle cancellation and other exceptions.
  Boolean haveMore;
  try {
    haveMore = taskEnumerator.MoveNext();
 
  } catch (OperationCanceledException cancExc) {
    completionSource.SetCanceled();
    return;
  } catch (Exception exc) {
    completionSource.SetException(exc);
    return;
  }
 
  if (!haveMore) {
    // No more tasks; set the result (if any) from the last completed task (if any).
    // We know it's not Canceled or Faulted because we checked at the start of this method.
    if (typeof(TResult) == typeof(VoidResult)) {    // No result
      completionSource.SetResult(default(TResult));
 
    } else if (!(completedTask is Task<TResult>)) {   // Wrong result
      completionSource.SetException(new InvalidOperationException(
        "Asynchronous iterator " + taskEnumerator +
          " requires a final result task of type " + typeof(Task<TResult>).FullName +
          (completedTask == null ? ", but none was provided." :
            "; the actual task type was " + completedTask.GetType().FullName)));
 
    } else {
      completionSource.SetResult(((Task<TResult>) completedTask).Result);
    }
 
  } else {
    // When the nested task completes, continue by performing this function again.
    taskEnumerator.Current.ContinueWith(
      nextTask => ToTaskDoOneStep(taskEnumerator, taskScheduler, completionSource, nextTask),
      taskScheduler);
  }
}