RT/Metro商店应用如何如何获取图片的宽高

var file = await StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///Assets/test.jpg", UriKind.RelativeOrAbsolute));
            ImageProperties imageProperties = await file.Properties.GetImagePropertiesAsync();
            canvas.Width = imageProperties.Width;
            canvas.Height = imageProperties.Height;