WebClient Upload Progress not changing

Good morning,
Ive written a simple Silverlight component which I though would upload the contents of a file via UploadStringAsync, and added a handler to update progress change, however the sub 'myWebclient_ProgressChanged' only gets fired at the end of execution, and reports back as 0%.

Not quite sure what Im doing wrong here, any ideas?

Partial Public Class MainPage
    Inherits UserControl

    Public Sub New()
        InitializeComponent()
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles Button1.Click
        Dim dlgOpen As New OpenFileDialog
        dlgOpen.Multiselect = False
        dlgOpen.Filter = "All Files|*.*"
        If dlgOpen.ShowDialog Then
            Dim myWebclient As New WebClient

            AddHandler myWebclient.UploadProgressChanged, AddressOf myWebclient_ProgressChanged

            Dim urlUI As New Uri("http://localhost:60187/Handler1.ashx")

            myWebclient.UploadStringAsync(urlUI, dlgOpen.File.OpenText.ReadToEnd)
        End If
    End Sub

    Private Sub myWebclient_ProgressChanged(ByVal sender As Object, ByVal e As UploadProgressChangedEventArgs)
        Button1.Content = e.ProgressPercentage
    End Sub

End Class

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