Private textDialog As SaveFileDialog
Public Sub New()
    InitializeComponent()
    textDialog = New SaveFileDialog()
    textDialog.Filter = "Text Files | *.txt"
    textDialog.DefaultExt = "txt"
End Sub

Private Sub button1_Click(ByVal sender As Object, ByVal e As RoutedEventArgs)
    Dim result As System.Nullable(Of Boolean) = textDialog.ShowDialog()
    If result = True Then
        Dim fileStream As System.IO.Stream = textDialog.OpenFile()
        Dim sw As New System.IO.StreamWriter(fileStream)
        sw.WriteLine("Writing some text in the file.")
        sw.Flush()
        sw.Close()
    End If
End Sub

SaveFileDialog textDialog;
public Page()
{
    InitializeComponent();
    textDialog = new SaveFileDialog();
    textDialog.Filter = "Text Files | *.txt";
    textDialog.DefaultExt = "txt";
 }

private void button1_Click(object sender, RoutedEventArgs e)
{
    bool? result = textDialog.ShowDialog();
    if (result == true)
    {
        System.IO.Stream fileStream = textDialog.OpenFile();
        System.IO.StreamWriter sw = new System.IO.StreamWriter(fileStream);
        sw.WriteLine("Writing some text in the file.");
        sw.Flush();
        sw.Close();
    }
}

<Grid x:Name="LayoutRoot" Background="White">
     <Button Height="100" Width="100" Content="Save Text File" x:Name="button1" 
        Click="button1_Click"  />
</Grid>