Do you have to put Task Run in a method to make it async

Question

I m trying to understand async await in the simplest form   I want to create a very simple method that adds two numbers for the sake of this example  granted  it s no processing time at all  it s just a matter of formulating an example here   Example 1  private async Task DoWork1Async         int result   1   2      Example 2  private async Task DoWork2Async         Task Run       gt                int result   1   2              If I await DoWork1Async   will the code run synchronously or asynchronously   Do I need to wrap the sync code with Task Run to make the method awaitable AND asynchronous so as not to block the UI thread   I m trying to figure out if my method is a Task or returns Task lt T gt  do I need to wrap the code with Task Run to make it asynchronous   Stupid question I m sure but I see examples on the net where people are awaiting code that has nothing async within and not wrapped in a Task Run or StartNew

User · Answer

When you use Task Run to run a method  Task gets a thread from threadpool to run that method  So from the UI thread s perspective  it is  asynchronous  as it doesn t block UI thread This is fine for desktop application as you usually don t need many threads to take care of user interactions   However  for web application each request is serviced by a thread-pool thread and thus the number of active requests can be increased by saving such threads  Frequently using threadpool threads to simulate async operation is not scalable for web applications   True Async doesn t necessarily involving using a thread for I O operations  such as file   DB access etc  You can read this to understand why I O operation doesn t need threads  http   blog stephencleary com 2013 11 there-is-no-thread html  In your simple example it is a pure CPU-bound calculation  so using Task Run is fine

User · Answer

First  let s clear up some terminology   asynchronous   async  means that it may yield control back to the calling thread before it starts  In an async method  those  yield  points are await expressions   This is very different than the term  asynchronous   as  mis used by the MSDN documentation for years to mean  executes on a background thread    To futher confuse the issue  async is very different than  awaitable   there are some async methods whose return types are not awaitable  and many methods returning awaitable types that are not async   Enough about what they aren t  here s what they are    The async keyword allows an asynchronous method  that is  it allows await expressions   async methods may return Task  Task lt T gt   or  if you must  void  Any type that follows a certain pattern can be awaitable  The most common awaitable types are Task and Task lt T gt     So  if we reformulate your question to  how can I run an operation on a background thread in a way that it s awaitable   the answer is to use Task Run   private Task lt int gt  DoWorkAsync      No async because the method does not need await     return Task Run      gt          return 1   2             But this pattern is a poor approach  see below    But if your question is  how do I create an async method that can yield back to its caller instead of blocking   the answer is to declare the method async and use await for its  yielding  points   private async Task lt int gt  GetWebPageHtmlSizeAsync       var client   new HttpClient      var html   await client GetAsync  http   www example com       return html Length      So  the basic pattern of things is to have async code depend on  awaitables  in its await expressions  These  awaitables  can be other async methods or just regular methods returning awaitables  Regular methods returning Task Task lt T gt  can use Task Run to execute code on a background thread  or  more commonly  they can use TaskCompletionSource lt T gt  or one of its shortcuts  TaskFactory FromAsync  Task FromResult  etc   I don t recommend wrapping an entire method in Task Run  synchronous methods should have synchronous signatures  and it should be left up to the consumer whether it should be wrapped in a Task Run   private int DoWork       return 1   2     private void MoreSynchronousProcessing          Execute it directly  synchronously   since we are also a synchronous method    var result   DoWork             private async Task DoVariousThingsFromTheUIThreadAsync          I have a bunch of async work to do  and I am executed on the UI thread    var result   await Task Run      gt  DoWork               I have an async await intro on my blog  at the end are some good followup resources  The MSDN docs for async are unusually good  too

User · Answer

One of the most important thing to remember when decorating a method with async is that at least there is one await operator inside the method  In your example  I would translate it as shown below using TaskCompletionSource   private Task lt int gt  DoWorkAsync           create a task completion source       the type of the result value must be the same       as the type in the returning Task     TaskCompletionSource lt int gt  tcs   new TaskCompletionSource lt int gt         Task Run      gt                int result   1   2            set the result to TaskCompletionSource         tcs SetResult result                 return the Task     return tcs Task     private async void DoWork         int result   await DoWorkAsync

[c#] Do you have to put Task.Run in a method to make it async?

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