async await execution windows phone 8

Question

I am new to async and await style of programming. How can I solve the following problem:

I am calling the below code first. The problem here is that the first line is awaiting which need to populate the categoriesvm.Categorieslist , which it does not, but the second line is called. (which I think is the default behaviour of await)

How can I make sure that the second line is called only when the categoriesvm.Categorieslist is populated in the first line?

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
    categoriesvm.GetCategories();                 
    BookCategories.DataContext = from vm in categoriesvm.Categorieslist select vm;
}

In the code above when I execute the first line it goes below where Categorieslist is the list I am accessing above.

public async void GetCategories()
{            
    Categorieslist = new ObservableCollection<Categories>(await PhoneClient.GetDefaultCategories());          
}

The phoneclient is below

public class PhoneClient
{   
    private static readonly HttpClient client;

    public static Uri ServerBaseUri
    {
        get { return new Uri("http://169.254.80.80:30134/api/"); }
    }

    static PhoneClient()
    {        
       client =new HttpClient();
       client.MaxResponseContentBufferSize = 256000;
       client.DefaultRequestHeaders.Add("user-agent", "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)");
    }      

    public async static Task<List<Categories>> GetDefaultCategories()
    {      
        HttpResponseMessage getresponse = await client.GetAsync(ServerBaseUri + "Categoryss");                      
        string json = await getresponse.Content.ReadAsStringAsync();         
        json = json.Replace("<br>", Environment.NewLine);
        var categories = JsonConvert.DeserializeObject<List<Categories>>(json);
        return categories.ToList();
    }
}

Answer 1

You should avoid async void . I explain this guideline in an MSDN article .

Once you change your async void method to async Task :

public async Task GetCategoriesAsync()
{            
  Categorieslist = new ObservableCollection<Categories>(await PhoneClient.GetDefaultCategories());          
}

Then you can await it as such:

protected override async void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
  await categoriesvm.GetCategoriesAsync();                 
  BookCategories.DataContext = from vm in categoriesvm.Categorieslist select vm;
}

However, I recommend doing all your VM initialization outside of your UI events - this will make your code easier to test. Take a look at my async constructors blog post for ideas.