ho provato il codice (convertendolo in vb), ma non funge:

codice:
 Private Sub Button_Click_1(sender As Object, e As RoutedEventArgs)
        Dim avatarUri As String = "http://.../prova.txt"
        Dim request As HttpWebRequest = HttpWebRequest.Create(avatarUri)
        request.BeginGetResponse(GetAvatarImageCallback,request)
    End Sub


    Sub GetAvatarImageCallback(result As IAsyncResult)
        Dim request As HttpWebRequest = result.AsyncState
        If (Not (request) Is Nothing) Then
            Try
                Dim response As WebResponse = request.EndGetResponse(result)
                Me.but1.Content = "si"
            Catch ex As Exception


            End Try
        End If
    End Sub
L'errore è:
argomento non specificato per il tipo result di public sub GetAvatarImageCallback(result As IAsyncResult)

ho provato a mettere l'argomento, ma non cambia nulla...