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I have a complicate problem.

I am using .net 4.0 MVC 4, add References about Microsoft BLC for using async, await.

and wrote

await System.Threading.Tasks.TaskEx.WhenAll(tl);
for wait Threads work ends of course.
But error pops out here.
Error   13  The type or namespace name 'TaskEx' does not exist in the namespace 'System.Threading.Tasks' (are you missing an assembly reference?)   
I tested many times another Project on another Solution, but it's work. not like this.
Any ideas for little help?
whole Method here.
public async void SendEmailEwsAsync(string subject, string mailBody, string fileName)
            List<System.Threading.Tasks.Task> tl = new List<System.Threading.Tasks.Task>();
            foreach (var kvp in Receivers)
                EmailMessage mail = CreateMailEws(subject, mailBody);
                mail.ToRecipients.Add(kvp.Value);
                if (!this.TestFlag)
                    mail.Attachments.AddFileAttachment(string.Format(fileName, EmailNamePair[kvp.Value]), kvp.Key);
                tl.Add(TaskSendAsync(mail));
                this.CurrentCursor++;
            await System.Threading.Tasks.TaskEx.WhenAll(tl); //error here
        catch (Exception e)
            throw e;
        finally
            this.IsEnd = true;
    private System.Threading.Tasks.Task TaskSendAsync(EmailMessage mail)
        Action action = delegate()
            mail.Save(new FolderId(WellKnownFolderName.Drafts, new Mailbox("[email protected]"))); 
            mail.SendAndSaveCopy(new FolderId(WellKnownFolderName.SentItems, new Mailbox("[email protected]"))); 
        System.Threading.Tasks.Task task = new System.Threading.Tasks.Task(action);
        task.Start();
        return task;
whole References here
Project References List
Thank you for see this.
                If you mean ASP.NET MVC, you can't use Microsoft.Bcl.Async on ASP.NET. Even if you get it to compile (which is possible), it will not behave properly at runtime.
– Stephen Cleary
                Mar 15, 2016 at 15:25
                @StephenCleary for real? Actually, I did add References about MVC in this project, but it's A kind of common library in my another MVC project. so I remove MVC reference, but still doesn't work.
– cleanUser_999
                Mar 16, 2016 at 3:04
                To be clear, Microsoft.Bcl.Async cannot be used on ASP.NET at all. To use async on ASP.NET, you have to upgrade to at least .NET 4.5.
– Stephen Cleary
                Mar 16, 2016 at 11:23
You do not need to reference TaskEx, and you can do WaitAll instead of WhenAll 
// Wait for all tasks in list to complete
Task.WaitAll(tl);
                Thx for your comment! one question, is that exactly same work? between TaskEx.WhenAll() and Task.WaitAll() ?
– cleanUser_999
                Mar 15, 2016 at 6:39
                The WhenAll() method will actually create a new, awaitable task that you can await, while the WaitAll() doesen't. Fine grained difference there, and all depends of course of what you want to. There is a good answer here: stackoverflow.com/questions/6123406/waitall-vs-whenall
– Pedro G. Dias
                Mar 15, 2016 at 6:45
                I already have Syncronous method. so I want make Asyncronous one. Seem like WaitAll() is Block type right? but still I thanks for your attention.
– cleanUser_999
                Mar 15, 2016 at 6:51
        
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