read local image in wp7

In my app i need to open a local image for cropping. I tried this by setting 开发者_Go百科the path of the image as source to a BitmapImage but it raising an error. How i can read and assign that local image to BitmapImage in WP Mango.

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   private WriteableBitmap ReadLocalImage(string Uri)
        {
            StreamResourceInfo sri = null;
            Uri uri = new Uri(Uri, UriKind.Relative);
            sri = Application.GetResourceStream(uri);
            BitmapImage bitmap = new BitmapImage();
            bitmap.CreateOptions = BitmapCreateOptions.None;
            bitmap.SetSource(sri.Stream);
            WriteableBitmap wb = new WriteableBitmap(bitmap);
            return wb;

        }

use this method for reading local image.