source: OniSplit/Imaging/SysWriter.cs

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Runtime.InteropServices;

namespace Oni.Imaging
{
    internal class SysWriter
    {
        public static void Write(Surface surface, string filePath)
        {
            Directory.CreateDirectory(Path.GetDirectoryName(filePath));

            surface = surface.Convert(SurfaceFormat.BGRA);

            using (var bitmap = new Bitmap(surface.Width, surface.Height, PixelFormat.Format32bppArgb))
            {
                var rc = new Rectangle(0, 0, surface.Width, surface.Height);
                
                var pixels = bitmap.LockBits(rc, ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);
                Marshal.Copy(surface.Data, 0, pixels.Scan0, surface.Data.Length);
                bitmap.UnlockBits(pixels);

                switch (Path.GetExtension(filePath))
                {
                    case ".png":
                        bitmap.Save(filePath, ImageFormat.Png);
                        break;
                    case ".jpg":
                        bitmap.Save(filePath, ImageFormat.Jpeg);
                        break;
                    case ".bmp":
                        bitmap.Save(filePath, ImageFormat.Bmp);
                        break;
                    case ".tif":
                        bitmap.Save(filePath, ImageFormat.Tiff);
                        break;
                }
            }
        }
    }
}