How do you create an asynchronous method in C

Question

Every blog post I ve read tells you how to consume an asynchronous method in C   but for some odd reason never explain how to build your own asynchronous methods to consume  So I have this code right now that consumes my method   private async void button1 Click object sender  EventArgs e        var now   await CountToAsync 1000       label1 Text   now ToString        And I wrote this method that is CountToAsync   private Task lt DateTime gt  CountToAsync int num   1000        return Task Factory StartNew      gt                for  int i   0  i  lt  num  i                          Console WriteLine    0    i                    ContinueWith x   gt  DateTime Now       Is this  the use of Task Factory  the best way to write an asynchronous method  or should I write this another way

User · Answer

One very simple way to make a method asynchronous is to use Task.Yield() method. As MSDN states:

You can use await Task.Yield(); in an asynchronous method to force the method to complete asynchronously.

Insert it at beginning of your method and it will then return immediately to the caller and complete the rest of the method on another thread.

private async Task<DateTime> CountToAsync(int num = 1000)
{
    await Task.Yield();
    for (int i = 0; i < num; i++)
    {
        Console.WriteLine("#{0}", i);
    }
    return DateTime.Now;
}

User · Answer

If you didn t want to use async await inside your method  but still  quot decorate quot  it so as to be able to use the await keyword from outside  TaskCompletionSource cs  public static Task lt T gt  RunAsync lt T gt  Func lt T gt  function         if  function    null  throw new ArgumentNullException    function           var tcs   new TaskCompletionSource lt T gt          ThreadPool QueueUserWorkItem     gt                           try                         T result   function                tcs SetResult result                        catch Exception exc    tcs SetException exc                 return tcs Task      From here and here  To support such a paradigm with Tasks  we need a way to retain the Task fa  ade and the ability to refer to an arbitrary asynchronous operation as a Task  but to control the lifetime of that Task according to the rules of the underlying infrastructure that   s providing the asynchrony  and to do so in a manner that doesn   t cost significantly  This is the purpose of TaskCompletionSource   I saw it s also used in the  NET source  e g  WebClient cs       HostProtection ExternalThreading   true        ComVisible false       public Task lt string gt  UploadStringTaskAsync Uri address  string method  string data                   Create the task to be returned         var tcs   new TaskCompletionSource lt string gt  address               Setup the callback event handler         UploadStringCompletedEventHandler handler   null          handler    sender  e    gt  HandleCompletion tcs  e   args    gt  args Result  handler   webClient  completion    gt  webClient UploadStringCompleted -  completion           this UploadStringCompleted    handler              Start the async operation          try   this UploadStringAsync address  method  data  tcs             catch                       this UploadStringCompleted -  handler              throw                        Return the task that represents the async operation         return tcs Task         Finally  I also found the following useful   I get asked this question all the time  The implication is that there must be some thread somewhere that   s blocking on the I O call to the external resource  So  asynchronous code frees up the request thread  but only at the expense of another thread elsewhere in the system  right  No  not at all  To understand why asynchronous requests scale  I   ll trace a  simplified  example of an asynchronous I O call  Let   s say a request needs to write to a file  The request thread calls the asynchronous write method  WriteAsync is implemented by the Base Class Library  BCL   and uses completion ports for its asynchronous I O  So  the WriteAsync call is passed down to the OS as an asynchronous file write  The OS then communicates with the driver stack  passing along the data to write in an I O request packet  IRP   This is where things get interesting  If a device driver can   t handle an IRP immediately  it must handle it asynchronously  So  the driver tells the disk to start writing and returns a    pending    response to the OS  The OS passes that    pending    response to the BCL  and the BCL returns an incomplete task to the request-handling code  The request-handling code awaits the task  which returns an incomplete task from that method and so on  Finally  the request-handling code ends up returning an incomplete task to ASP NET  and the request thread is freed to return to the thread pool   Introduction to Async Await on ASP NET If the target is to improve scalability  rather than responsiveness   it all relies on the existence of an external I O that provides the opportunity to do that

User · Answer

I don t recommend StartNew unless you need that level of complexity   If your async method is dependent on other async methods  the easiest approach is to use the async keyword   private static async Task lt DateTime gt  CountToAsync int num   10      for  int i   0  i  lt  num  i            await Task Delay TimeSpan FromSeconds 1           return DateTime Now      If your async method is doing CPU work  you should use Task Run   private static async Task lt DateTime gt  CountToAsync int num   10      await Task Run      gt          return DateTime Now      You may find my async await intro helpful

[c#] How do you create an asynchronous method in C#?

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