How to make use Task.WaitAll with async method using C# - asp.net-mvc

Question

I have used Task.WaitAll inside an async method in my HomeController, but it is not waiting to complete execution of the async methods. What did I do wrong here? Instead of waiting, it immediately goes to the next statement.
HomeController.cs:
private List<Task> taskList = new List<Task>();
public ActionResult Index()
{
for (i=0; i<5; i++)
{
SendMessages();
}
Task.WaitAll(taskList.ToArray());
// call demo method
}
private async Task SendMessages()
{
var task = Task.Factory.StartNew(() => ProcessMessages()
taskList.Add(task);
}
private async Task ProcessMessages()
{
while (run for 10 minutes)
{
// data save
}
}
I have added a Task into taskList and used Task.WaitAll(taskList.ToArray()); to wait for all tasks to complete, then call demo method. But instead of waiting and executing the whole loop, it immediately goes down after Task.WaitAll(taskList.ToArray()); and executes call demo method. Why is this?

Answer 1

Task.Factory.StartNew should not be used to launch asynchronous operations, since it does not unwrap the returned Task, and instead returns a new Task that completes as soon as the asynchronous operation launches (or, more precisely, completes its synchronous part). You should use Task.Run instead.
Additionally, SendMessages should not be async. However, Index should be, allowing you to use the asynchronous WhenAll instead of the blocking WaitAll.
private List<Task> taskList = new List<Task>();
public async Task<ActionResult> Index()
{
for (i = 0; i < 5; i++)
{
SendMessages();
}
await Task.WhenAll(taskList);
// call demo method
}
private void SendMessages()
{
var task = Task.Run(() => ProcessMessagesAsync()); // here
taskList.Add(task);
}
private async Task ProcessMessagesAsync()
{
while (run for 10 minutes)
{
// data save
}
}