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public interface IImageService
{
    Stream GetShrinkedStream(string filePath, int width, int height);
}

using System.IO;
using Android.Graphics;
[assembly: Dependency(typeof(ImageService))]
public class ImageService : IImageService
{
    public Stream GetShrinkedStream(string filePath, int width, int height)
    {
        Stream stream = null;
        try
        { 
            //画像を圧縮する
            Bitmap bitmap = ImageService.GetShrinkedBitmap(filePath, width, height);

            //Streamを取得する
            stream = new MemoryStream();
            bitmap.Compress(Android.Graphics.Bitmap.CompressFormat.Png, 100, stream);  // this is the diff between iOS and Android
            stream.Position = 0;
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine +
                                    ex.StackTrace);
        }
        return stream;
    }

    /// <summary>
    /// 縮小したBitmapを取得する
    /// </summary>
    /// <param name="source">ファイルパス</param>
    /// <param name="width">縮小サイズ（幅）</param>
    /// <param name="height">縮小サイズ（高さ）</param>
    /// <returns></returns>
    private static Bitmap GetShrinkedBitmap(string filePath, int width, int height)
    {
        Bitmap bitmap = null;
        try
        {
            //読み込み用のオプションオブジェクトを生成
            //この値をtrueにするとメモリに画像を読み込まず、画像のサイズ情報だけを取得することができます。
            var options = new BitmapFactory.Options { InJustDecodeBounds = true };

            //画像ファイル読み込み
            //オプションのInJustDecodeBoundsがtrueのため実際の画像はメモリに読み込まれません。
            ImageService.GetBitmap(filePath, options);

            //引数の縦幅に合わせた画像サイズを再計算します。
            //The with and height of the elements will be used to decode the image
            if (width > 0 && height > 0)
            {
                options.InSampleSize = ImageService.CalculateInSampleSize(options, width, height);
            }
            else
            {
                options.InSampleSize = ImageService.CalculateInSampleSize(options, options.OutWidth, options.OutHeight);
            }
            //読み込む画像の階調を指定して読み込む
            options.InPreferredConfig = Bitmap.Config.Argb8888;

            //画像をメモリに読み込む場合はfalseを指定
            options.InJustDecodeBounds = false;

            //これで指定した縮尺で画像を読み込めます。
            //容量も小さくなるのでOutOfMemoryが発生しにくいです。
            bitmap = ImageService.GetBitmap(filePath, options);

        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine +
                                    ex.StackTrace);
        }
        return bitmap;
    }

    private static Bitmap GetBitmap(string filePath, BitmapFactory.Options options)
    {
        Bitmap bitmap = null;
        try
        {
            if (!String.IsNullOrEmpty(filePath))
            {
                bitmap = BitmapFactory.DecodeFile(filePath, options);
            }
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine +
                                    ex.StackTrace);
        }
        return bitmap;
    }

    private static int CalculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight)
    {
        // Raw height and width of image
        float height = options.OutHeight;
        float width = options.OutWidth;
        var inSampleSize = 1D;

        if (height > reqHeight || width > reqWidth)
        {
            int halfHeight = (int)(height / 2);
            int halfWidth = (int)(width / 2);

            // Calculate a inSampleSize that is a power of 2 - the decoder will use a value that is a power of two anyway.
            while ((halfHeight / inSampleSize) > reqHeight && (halfWidth / inSampleSize) > reqWidth)
            {
                inSampleSize *= 2;
            }
        }
        return (int)inSampleSize;
    }
}

if (this.imgPicture.Source.GetType() == typeof(FileImageSource))
{
    FileImageSource fis = (FileImageSource)this.imgPicture.Source;
    //Androidの場合、画像を圧縮する
    if (Device.RuntimePlatform == Device.Android)
    {
        Stream stream = DependencyService.Get<IImageService>().GetShrinkedStream(
                                fis.File,
                                (int)(this.imgPicture.WidthRequest),
                                (int)(this.imgPicture.HeightRequest)
                                );
        //ストリームをバイト配列に変換する。
        //このバイト配列をDB等に保存します。
        byte[] smallSize = ImgConverter.GetByteArrayFromStream(stream);

        //元サイズとの比較を出力
        byte[] normalSize = DependencyService.Get<IFileService>().GetBytesFromFile(fis.File);
        Debug.WriteLine("●normalSize Length=" + normalSize.Length.ToString() +
                         " StringLength=" + System.Convert.ToBase64String(normalSize).Length.ToString());
        Debug.WriteLine("●smallSize Length=" + smallSize.Length.ToString() +
                         " StringLength=" + System.Convert.ToBase64String(smallSize).Length.ToString());
    }
}