How to convert ImageSource to Bitmap?

I found an example, but this example uses the "Int32Rect", class of System.Windows, we can't use this library for android or ios
For bitmap, i use the System.Drawing.Common

public static System.Drawing.Bitmap BitmapSourceToBitmap2(BitmapSource srs)
        {
            int width = srs.PixelWidth;
            int height = srs.PixelHeight;
            int stride = width * ((srs.Format.BitsPerPixel + 7) / 8);
            IntPtr ptr = IntPtr.Zero;
            try
            {
                ptr = Marshal.AllocHGlobal(height * stride);
                srs.CopyPixels(new Int32Rect(0, 0, width, height), ptr, height * stride, stride);
                using (var btm = new System.Drawing.Bitmap(width, height, stride, System.Drawing.Imaging.PixelFormat.Format1bppIndexed, ptr))
                {
                    // Clone the bitmap so that we can dispose it and
                    // release the unmanaged memory at ptr
                    return new System.Drawing.Bitmap(btm);
                }
            }
            finally
            {
                if (ptr != IntPtr.Zero)
                    Marshal.FreeHGlobal(ptr);
            }
        }

So, how to convert ImageSource to Bitmap for cross-platform project?