Using Task.Unwrap to get to the inner task

Question

I'm trying to access the inner task with Task.Unwrap and I'm getting this error:

System.InvalidCastException: Unable to cast object of type 
'System.Threading.Tasks.UnwrapPromise`1[System.Threading.Tasks.TaskExtensions+VoidResult]' 
to type 'System.Threading.Tasks.Task`1[System.Boolean]'.

To reproduce the problem:

    static void Main(string[] args)
    {
        var tcs = new TaskCompletionSource<bool>();
        tcs.SetResult(true);
        Task task1 = tcs.Task;

        Task<Task> task2 = task1.ContinueWith(
            (t) => t, TaskContinuationOptions.ExecuteSynchronously);

        Task task3 = task2.Unwrap();

        try
        {
            Task<bool> task4 = (Task<bool>)task3;

            Console.WriteLine(task4.Result.ToString());
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
        Console.ReadLine();
    }

In the real project, I'm provided with a list of Task<Task>, where each inner task is a generic task. Can I not use Unwrap to access the inner tasks and their results?

Answer 1

You can do this by using ContinueWith(), which will cast the inner Task to Task<YourType>, followed by Unwrap():

Task<bool> task4 = task2.ContinueWith(t => (Task<bool>)t.Result).Unwrap();