Asynchronous programming is introduced in a new version of programming languages (C++, C# and JavaScript) which are part of the WinRT and .NET. If you are programming in C# you can program asynchronous by using async and await keyword. In fact when you program with async and await you actually program as ordinary synchronous code. That’s the big thing about this type of programming model.
More information about C# asynchronous programming you can found in my previous posts.
Since we have asynchronous programming model in all languages calling service operations asynchrony is not relevant at all. We can just forget it and start writing code like synchronous. To see hot new pattern is reflected in WCF lets implement simple service and simple metro style app consumer.
Implementation of the WCF Simple Service:

• Create new ASP.NET Empty Web App
• Add new Item-> WCF Service
• Name it as SimpleService.svc

Service Interface looks like the following implementation:

[ServiceContract]
public interface ISimpleService
{
    [OperationContract]
    [WebGet(UriTemplate="AllData",ResponseFormat= WebMessageFormat.Json)]
    List<string> GetSomeData();
}

Implement one operation in the service like the following code implementation:

public class SimpleService : ISimpleService
{
    public List<string> GetSomeData()
    {
        List<string> lst = new List<string>();
        for (int i = 0; i < 10; i++)
        {
            System.Threading.Thread.Sleep(100);
            lst.Add(string.Format("Item number {0}", i + 1));
        }
        return lst;
    }
}

The mentioned operation returns list of string but between populating the collection we sleep the current thread for 0,5 second in order to simulate log running operation.

It is necessary to publish the WCF service on remote host, in order to consume it. I still dont know why WCF service cannot be consumed by Metro app on the localhost. After the service is published on remoter host, we are ready to create Metro style Consumer app.

• Right click on solution item
• Add New Project
• Choose Application template from the Metro Style app
• Name it WCFConsumer

• Add service reference to C# Metro Stryle app.

<Grid x:Name="LayoutRoot" Background="#FF0C0C0C">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="151*"/>
        <ColumnDefinition Width="326*"/>
        <ColumnDefinition Width="207*"/>
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="74*"/>
        <RowDefinition Height="185*"/>
        <RowDefinition Height="68*"/>
    </Grid.RowDefinitions>
    <GridView x:Name="gridData" Grid.Column="1" Grid.Row="1" />
    <HyperlinkButton Content="HyperlinkButton"   Grid.Column="1" HorizontalAlignment="Left" Height="25" Margin="58,30,0,0" VerticalAlignment="Top" Width="126"/>
    <Button Content="Button" Grid.Column="1" HorizontalAlignment="Left" Height="40" Margin="69,8,0,0" Grid.Row="2" VerticalAlignment="Top" Width="182"/>
    <TextBox Grid.Column="2" HorizontalAlignment="Left" Height="27" Margin="21,79,0,0" Grid.Row="1" Text="TextBox" VerticalAlignment="Top" Width="161"/>
    <TextBox HorizontalAlignment="Left" Height="15" Margin="24,79,0,0" Grid.Row="1" Text="TextBox" VerticalAlignment="Top" Width="102"/>
    <Image Grid.ColumnSpan="2" Grid.Column="1" HorizontalAlignment="Left" Height="100" Margin="312,240,0,-272" Grid.Row="2" VerticalAlignment="Top" Width="100"/>
</Grid>

private void UserControl_Loaded(object sender, RoutedEventArgs e)
{
        SimpleServiceNamespace.SimpleServiceClient client = new SimpleServiceNamespace.SimpleServiceClient();
        var servicedata = client.GetSomeDataAsync();
        gridData.ItemsSource = servicedata.Result;
}

Even if GetSomeDataAsync method is asynchronous we don’t implement callback because this operation is implemented based on the async and await C# keyword and no need to do that.
Just assign the result of the servicedata to the GridView.ItemsSource.
If we run the app, we can see that the data is loaded asynchronously and UI is not blocked. We can click on buttons, tipe some texts while data is loading. This is awesome and now we can think about application logic instead of callback service implementation. This programming pattern works not just for Metro style apps, in fact it works for all .NET 4.5 based applications.