I am trying to create a Windows Phone 7.1 application, basically a currency converter. I am using `DownloadStringAsync` method to get a short string containing the exchange rate from a specific website. I tested in Visual Studio 2010, `DownloadString` worked just fine. But not for the phone application. What do I need to do here? I can't really make much sense of it.

```
Partial Public Class MainPage
Inherits PhoneApplicationPage
Dim webClient As New System.Net.WebClient
Dim a As String
Dim b As String
Dim result As String = Nothing
' Constructor
Public Sub New()
    InitializeComponent()
End Sub

Private Sub Button1_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles Button1.Click
    a = "USD"
    b = "GBP"
    webClient = New WebClient
    Dim result As String = webClient.DownloadStringAsync(New Uri("http://rate-exchange.appspot.com/currency?from=" + a + "&to=" + b) as String)
    TextBox1.Text = result
End Sub

```

End Class