异步方法不是等待

Question

In this thread , I could solve my problem to wait a async method in the constructor, using an event. In this case, the event uses a delegate without parameters.

But now I need to wait the view model finishes to wait the async method to can continue.

I have this code in my main view model:

public void printMethod()
{
    SecondViewModel mySeocViewModel = new SecondViewModel(myParameter);
    SecondView mySecondView = new SecondView();
    mySecondView.DataContext = mySeocViewModel;

    //I have to wait until it finished to can print the user control
    //The problem is that this point is reached before mySecondViewModel finish.
}

The code in my second view model:

public docFacturasViewModel(MyType parameter)
{
    this.GetDataFromDatabaseEvent += OnGetDataFromDatabase;

    GetDataFromDatabaseEvent(parameter);
}


public delegate void GetDataFromDatabaseEventHandler(MyType parameter);
public event GetDataFromDatabaseEventHandler GetDataFromDataBaseEvent;

private async void OnBuscarDatos(MyType paramter)
{
    await getDataFromDatabaseAsync(parameter);

    //Fill the data of the properties of the view model with the data from database
}


private async Task getDataFromDatabaseAsync(MyType parameter)
{
    _myResult = (await getdataAsync(parameter)).FirstOrDefault();
}

If i debug the code, it runs in this way:

• It reaches the line in the main view model to create the second view model.
• In the second view model, it reaches the line in the method OnGetDataFromDatabase the first line, the await.
• The main view model continue with the code, so it doesn't wait the result from database.
• After a time, in the second view model, the method OnGetDataFromDatabase fnish and continue with the next line of code after the await.

I don't understand why in the second view model the code doesn't wait until get the data from the database, because I am using the await keyword and all the code is the same than in the case of the post that I link at the beggining.

Thanks.

EDIT:

If the constructor of the second view model I use this:

Task.Run(() => buscarDatosAsync(paramFacturaParaImprimir)).Wait();

Then the main view model wait until the second view model finish to get the data from database.

But when I tried this method in the first case, in the case that I linked and I asked in the other post, the method is not waited, so I don't understand why in one case I have to use a delegate and in the other case I can use a task and wait it finishes.

Answer 1

When an async void method reaches an await , the control is returned to the caller. Obviously, this is not the best idea when you need the result of the operation to continue processing. Besides this, I wouldn't expect a constructor to have the side effect of creating fire-and-forget tasks.

If you need the result of the data as part of the construction of the object, I would suggest you a factory approach:

private SecondViewModel()
{
}

public static async Task<SecondViewModel> CreateAsync(MyType parameter)
{
    var result = new SecondViewModel();

    result.SomeData = await getDataFromDatabaseAsync(parameter);

    return result;
}

And you would use it like this:

public async Task printMethod()
{
    SecondViewModel mySeocViewModel = await SecondViewModel.CreateAsync(myParameter);
    SecondView mySecondView = new SecondView();
    mySecondView.DataContext = mySeocViewModel;
}

Answer 2

Assuming, that you're writing some XAML-based code, you've got a design problem here.

Do not mix view model creation and data loading. Usually you create view models and views synchronously, and load/post data in asynchronous way. So, don't wait anything inside view model constructors.

View models and views must operate properly without any data (when data properties are null s or empty collections). If you don't want to give access to some of commands until data is loaded, just disable these commands (return false from CanExecute ).