TaskHelper.cs

public static class TaskHelper {
    public static Task CompletedTask = AsCompletedTask(true);
    public static Task<T> AsCompletedTask<T>(T result) {
        TaskCompletionSource<T> precompletedSource = new TaskCompletionSource<T>();
        precompletedSource.SetResult(result);
        return precompletedSource.Task;
    }
    public static void TrySetResultTask<TResult>(this TaskCompletionSource<TResult> tcs, Task<TResult> task) {
        if (task == null) {
            throw new ArgumentNullException("task");
        }

        task.ContinueWith(t => {
            tcs.TrySetResult(t.Result);
        });
    }
    public static void SetResultTask<TResult>(this TaskCompletionSource<TResult> tcs, Task<TResult> task) {
        if (task == null) {
            throw new ArgumentNullException("task");
        }

        task.ContinueWith(t => {
            tcs.SetResult(t.Result);
        });
    }
}