How to auto generate image name without opening dialogue box in my silverlight application

private void SaveImage_Click(object sender, RoutedEventArgs e)
        {
            //string strImageID = getImageID();
            // Gets the current captured raw bitmap
            var rawImage = SelectedSnapshot;

            // If image is selected in the Snapshots ListBox then set it as a save target
            if (Snapshots.SelectedItem != null)
                rawImage = (WriteableBitmap)Snapshots.SelectedItem;

            if (rawImage != null)
            {
                // Init the Save File Dialog
               SaveFileDialog saveDialog = new SaveFileDialog();
                saveDialog.GetType().GetMethod("set_DefaultFileName").Invoke(saveDialog, new object[] { "FileName.jpg" });
               saveDialog.Filter = "Image Files (*.jpg, *.jpeg)|*.jpg;*.jpeg|All Files (*.*)|*.*";
                saveDialog.DefaultExt = ".jpg";
               saveDialog.FilterIndex = 1;




                // Show save dialog to the user
                if ((bool)saveDialog.ShowDialog())
                {
                    using (Stream stream = saveDialog.OpenFile())
                    {
                        // Convert raw captured bitmap to the image that Image Tools understand with the extension method
                        var image = rawImage.ToImage();
                        // Declare jpeg encoder
                        var encoder = new JpegEncoder();
                        // Set the image quality
                        encoder.Quality = 90;


                        // Finally encode raw bitmap and save it as a jpg image
                       encoder.Encode(image, stream);
                       // Close the stream
                        stream.Close();
                    }
                }
           }
        }