Can constructors be async

Question

I have a project where I m trying to populate some data in a constructor   public class ViewModel       public ObservableCollection lt TData gt  Data   get  set         async public ViewModel                 Data   await GetDataTask               public Task lt ObservableCollection lt TData gt  gt  GetDataTask                 Task lt ObservableCollection lt TData gt  gt  task             Create a task which represents getting the data         return task            Unfortunately  I m getting an error       The modifier async is not valid for this item   Of course  if I wrap in a standard method and call that from the constructor   public async void Foo         Data   await GetDataTask        it works fine  Likewise  if I use the old inside-out way  GetData   ContinueWith t   gt  Data   t Result     That works too  I was just wondering why we can t call await from within a constructor directly  There are probably lots of  even obvious  edge cases and reasons against it  I just can t think of any  I ve also search around for an explanation  but can t seem to find any

User · Accepted Answer

Constructor acts very similarly to a method returning the constructed type. And async method can't return just any type, it has to be either “fire and forget” void, or Task.

If the constructor of type T actually returned Task<T>, that would be very confusing, I think.

If the async constructor behaved the same way as an async void method, that kind of breaks what constructor is meant to be. After constructor returns, you should get a fully initialized object. Not an object that will be actually properly initialized at some undefined point in the future. That is, if you're lucky and the async initialization doesn't fail.

All this is just a guess. But it seems to me that having the possibility of an async constructor brings more trouble than it's worth.

If you actually want the “fire and forget” semantics of async void methods (which should be avoided, if possible), you can easily encapsulate all the code in an async void method and call that from your constructor, as you mentioned in the question.

User · Answer

Your problem is comparable to the creation of a file object and opening the file  In fact there are a lot of classes where you have to perform two steps before you can actually use the object  create   Initialize  often called something similar to Open    The advantage of this is that the constructor can be lightweight  If desired   you can change some properties before actually initializing the object  When all properties are set  the Initialize Open function is called to prepare the object to be used  This Initialize function can be async   The disadvantage is that you have to trust the user of your class that he will call Initialize   before he uses any other function of your class  In fact if you want to make your class full proof  fool proof   you have to check in every function that the Initialize   has been called   The pattern to make this easier is to declare the constructor private and make a public static function that will construct the object and call Initialize   before returning the constructed object  This way you ll know that everyone who has access to the object has used the Initialize function   The example shows a class that mimics your desired async constructor  public MyClass       public static async Task lt MyClass gt  CreateAsync                    MyClass x   new MyClass            await x InitializeAsync              return x                make sure no one but the Create function can call the constructor      private MyClass          private async Task InitializeAsync                       do the async things you wanted to do in your async constructor            public async Task lt int gt  OtherFunctionAsync int a  int b                return await        return something useful         Usage will be as follows   public async Task lt int gt  SomethingAsync            Create and initialize a MyClass object     MyClass myObject   await MyClass CreateAsync               use the created object      return await myObject OtherFunctionAsync 4  7

User · Answer

I would use something like this     public class MyViewModel                   public MyDataTable Data   get  set                public MyViewModel                                       loadData      gt  GetData                                     private async void loadData Func lt DataTable gt  load                                     try                                           MyDataTable   await Task Run load                                         catch  Exception ex                                               log                                                     private DataTable GetData                                        DataTable data                         get data and return                     return data                           This is as close to I can get for constructors

User · Answer

Some of the answers involve creating a new public method  Without doing this  use the Lazy lt T gt  class   public class ViewModel       private Lazy lt ObservableCollection lt TData gt  gt  Data       async public ViewModel                 Data   new Lazy lt ObservableCollection lt TData gt  gt  GetDataTask              public ObservableCollection lt TData gt  GetDataTask                 Task lt ObservableCollection lt TData gt  gt  task             Create a task which represents getting the data         return task GetAwaiter   GetResult              To use Data  use Data Value

User · Answer

Since it is not possible to make an async constructor  I use a static async method that returns a class instance created by a private constructor  This is not elegant but it works ok   public class ViewModel                     public ObservableCollection lt TData gt  Data   get  set                  static async method that behave like a constructor            async public static Task lt ViewModel gt  BuildViewModelAsync                          ObservableCollection lt TData gt  tmpData   await GetDataTask              return new ViewModel tmpData                        private constructor called by the async method     private ViewModel ObservableCollection lt TData gt  Data                this Data   Data

User · Answer

calling async in constructor maybe cause deadlock  please refer to http   social msdn microsoft com Forums en winappswithcsharp thread 0d24419e-36ad-4157-abb5-3d9e6c5dacf1  http   blogs msdn com b pfxteam archive 2011 01 13 10115163 aspx

User · Answer

In this particular case  a viewModel is required to launch the task and notify the view upon its completion  An  async property   not an  async constructor   is in order   I just released AsyncMVVM  which solves exactly this problem  among others   Should you use it  your ViewModel would become    public class ViewModel   AsyncBindableBase       public ObservableCollection lt TData gt  Data               get   return Property Get GetDataAsync                private Task lt ObservableCollection lt TData gt  gt  GetDataAsync                   Get the data asynchronously           Strangely enough  Silverlight is supported

User · Answer

I use this easy trick   public sealed partial class NamePage     private readonly Task  initializingTask     public NamePage            initializingTask   Init           private async Task Init                  Initialization that you need with await async stuff allowed

User · Answer

if you make constructor asynchronous  after creating an object  you may fall into problems like null values instead of instance objects  For instance   MyClass instance   new MyClass    instance Foo       null exception here   That s why they don t allow this i guess

User · Answer

I was just wondering why we can t call await from within a constructor directly    I believe the short answer is simply  Because the  Net team has not programmed this feature   I believe with the right syntax this could be implemented and shouldn t be too confusing or error prone  I think Stephen Cleary s blog post and several other answers here have implicitly pointed out that there is no fundamental reason against it  and more than that - solved that lack with workarounds  The existence of these relatively simple workarounds is probably one of the reasons why this feature has not  yet  been implemented

User · Answer

I m not familiar with the async keyword  is this specific to Silverlight or a new feature in the beta version of Visual Studio    but I think I can give you an idea of why you can t do this   If I do   var o   new MyObject    MessageBox o SomeProperty ToString       o may not be done initializing before the next line of code runs   An instantiation of your object cannot be assigned until your constructor is completed  and making the constructor asynchronous wouldn t change that so what would be the point   However  you could call an asynchronous method from your constructor and then your constructor could complete and you would get your instantiation while the async method is still doing whatever it needs to do to setup your object

User · Answer

you can use Action inside Constructor    public class ViewModel               public ObservableCollection lt TData gt  Data   get  set           public ViewModel                                       new Action async      gt                                  Data   await GetDataTask                   Invoke                       public Task lt ObservableCollection lt TData gt  gt  GetDataTask                         Task lt ObservableCollection lt TData gt  gt  task                Create a task which represents getting the data             return task

[c#] Can constructors be async?

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