How to call an async method from a getter or setter

Question

What d be the most elegant way to call an async method from a getter or setter in C    Here s some pseudo-code to help explain myself   async Task lt IEnumerable gt  MyAsyncMethod         return await DoSomethingAsync       public IEnumerable MyList       get                  call MyAsyncMethod   here

User · Answer

There is no technical reason that async properties are not allowed in C   It was a purposeful design decision  because  asynchronous properties  is an oxymoron   Properties should return current values  they should not be kicking off background operations   Usually  when someone wants an  asynchronous property   what they really want is one of these    An asynchronous method that returns a value  In this case  change the property to an async method  A value that can be used in data-binding but must be calculated retrieved asynchronously  In this case  either use an async factory method for the containing object or use an async InitAsync   method  The data-bound value will be default T  until the value is calculated retrieved  A value that is expensive to create  but should be cached for future use  In this case  use AsyncLazy from my blog or AsyncEx library  This will give you an awaitable property    Update  I cover asynchronous properties in one of my recent  async OOP  blog posts

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You can use Task like this    public int SelectedTab                       get   gt  selected tab              set                               selected tab   value                   new Task async      gt                                        await newTab ScaleTo 0 8                      Start

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I think that we can await for the value just returning first null and then get the real value  so in the case of Pure MVVM  PCL project for instance   I think the following is the most elegant solution   private IEnumerable myList  public IEnumerable MyList     get              if myList    null           InitializeMyList          return myList           set                myList   value          NotifyPropertyChanged              private async void InitializeMyList        MyList   await AzureService GetMyList

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When I ran into this problem  trying to run an async method synchronicity from either a setter or a constructor got me into a deadlock on the UI thread  and using an event handler required too many changes in the general design  The solution was  as often is  to just write explicitly what I wanted to happen implicitly  which was to have another thread handle the operation and to get the main thread to wait for it to finish   string someValue null  var t   new Thread      gt someValue   SomeAsyncMethod   Result   t Start    t Join      You could argue that I abuse the framework  but it works

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Since your  async property  is in a viewmodel  you could use AsyncMVVM   class MyViewModel   AsyncBindableBase       public string Title               get                       return Property Get GetTitleAsync                        private async Task lt string gt  GetTitleAsync                                 It will take care of the synchronization context and property change notification for you

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I think my example below may follow  Stephen-Cleary  s approach but I wanted to give a coded example   This is for use in a data binding context for example Xamarin   The constructor of the class - or indeed the setter of another property on which it is dependent - may call an async void that will populate the property on completion of the task without the need for an await or block   When it finally gets a value it will update your UI via the NotifyPropertyChanged mechanism   I m not certain about any side effects of calling a aysnc void from a constructor   Perhaps a commenter will elaborate on error handling etc   class MainPageViewModel   INotifyPropertyChanged       IEnumerable myList       public event PropertyChangedEventHandler PropertyChanged       public MainPageViewModel                  MyAsyncMethod               public IEnumerable MyList               set                       if  myList    value                                myList   value                   if  PropertyChanged    null                                        PropertyChanged this  new PropertyChangedEventArgs  MyList                                                       get                       return myList                       async void MyAsyncMethod                 MyList   await DoSomethingAsync

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Necromancing   In  NET Core NetStandard2  you can use Nito AsyncEx AsyncContext Run instead of System Windows Threading Dispatcher InvokeAsync    class AsyncPropertyTest        private static async System Threading Tasks Task lt int gt  GetInt string text                await System Threading Tasks Task Delay 2000           System Threading Thread Sleep 2000           return int Parse text               public static int MyProperty               get                       int x   0                  https   stackoverflow com questions 6602244 how-to-call-an-async-method-from-a-getter-or-setter                https   stackoverflow com questions 41748335 net-dispatcher-for-net-core                https   github com StephenCleary AsyncEx             Nito AsyncEx AsyncContext Run async delegate                                  x   await GetInt  123                                 return x                        public static void Test                 System Console WriteLine System DateTime Now ToString  dd MM yyyy HH mm ss fff             System Console WriteLine MyProperty           System Console WriteLine System DateTime Now ToString  dd MM yyyy HH mm ss fff                 If you simply chose System Threading Tasks Task Run or System Threading Tasks Task lt int gt  Run  then it wouldn t work

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I review all answer but all have a performance issue   for example in    string  Title  public string Title       get               if   Title    null                           Deployment Current Dispatcher InvokeAsync async      gt    Title   await getTitle                          return  Title            set               if  value     Title                         Title   value              RaisePropertyChanged  Title                        Deployment Current Dispatcher InvokeAsync async         Title   await getTitle         use dispatcher which is not a good answer   but there is a simple solution  just do it   string  Title      public string Title               get                       if   Title    null                                   Task Run     gt                                          Title   getTitle                        RaisePropertyChanged  Title                                                return                            return  Title                    set                       if  value     Title                                 Title   value                  RaisePropertyChanged  Title

User · Answer

You can always call an asynchronous method without await if you simply want to fire and forget  But it becomes tricky when you need to wait for the result and do something with the result  I had to call an async method in a property setter in an ASP NET Core Blazor Server Side component  I needed to get a result from that async method and do something else  I was stuck as I could not even use the Task Result property in Blazor context  So  I solved the problem by calling the  ContinueWith method which worked great  Here is a sample snippet I used      public bool AProperty               get   gt   aProperty          set                       if   aProperty   value                                 aProperty  value                   myService UpdateAReocrdAsync   ContinueWith previousTask   gt                                        int numberOfRowsAffected   previousTask Result                                          if  numberOfRowsAffected  gt  0                                                toastService ShowSuccess   quot Total  numberOfRowsAffected  rows updated  quot                                              else                                               toastService ShowError  quot No record was updated  quot

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I really needed the call to originate from the get method  due to my decoupled architecture  So I came up with the following implementation   Usage  Title is in a ViewModel or an object you could statically declare as a page resource  Bind to it and the value will get populated without blocking the UI  when getTitle   returns   string  Title  public string Title       get               if   Title    null                           Deployment Current Dispatcher InvokeAsync async      gt    Title   await getTitle                          return  Title            set               if  value     Title                         Title   value              RaisePropertyChanged  Title

User · Answer

I thought  GetAwaiter   GetResult   was exactly the solution to this problem  no  eg   string  Title  public string Title       get               if   Title    null                            Title   getTitle   GetAwaiter   GetResult                      return  Title            set               if  value     Title                         Title   value              RaisePropertyChanged  Title

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You can change the proerty to Task lt IEnumerable gt   and do something like   get       Task lt IEnumerable gt  Run async    gt          return await getMyList                and use it like await MyList

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You can t call it asynchronously  since there is no asynchronous property support  only async methods   As such  there are two options  both taking advantage of the fact that asynchronous methods in the CTP are really just a method that returns Task lt T gt  or Task      Make the property return a Task lt T gt  public Task lt IEnumerable gt  MyList       get                   Just call the method          return MyAsyncMethod              Or      Make the property blocking public IEnumerable MyList       get                   Block via  Result          return MyAsyncMethod   Result

[c#] How to call an async method from a getter or setter?

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