功能需求：

1、把一張圖片（png bmp jpeg bmp gif）轉換為byte陣列存放到資料庫。

2、把從資料庫讀取的byte陣列轉換為Image物件，賦值給相應的控制元件顯示。

3、從圖片byte陣列得到對應圖片的格式，生成一張圖片儲存到磁碟上。

這裡的Image是System.Drawing.Image。

         //Get an image from file
        Image image = Image.FromFile("D:\\test.jpg");

        Bitmap bitmap = new Bitmap("D:\\test.jpg");

以下三個函式分別實現了上述三個需求：

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Linq;
using System.Text;

namespace NetUtilityLib
{
    public static class ImageHelper
    {
        /// <summary>
        /// Convert Image to Byte[]
        /// </summary>
 

        /// <param name="image"></param>
        /// <returns></returns>
        public static byte[] ImageToBytes(Image image)
        {
            ImageFormat format = image.RawFormat;
            using (MemoryStream ms = new MemoryStream())
            {
                if (format.Equals(ImageFormat.Jpeg))
                {
                    image.Save(ms, ImageFormat.Jpeg);
                }
                
 
else if (format.Equals(ImageFormat.Png))
                {
                    image.Save(ms, ImageFormat.Png);
                }
                else if (format.Equals(ImageFormat.Bmp))
                {
                    image.Save(ms, ImageFormat.Bmp);
                }
                else if (format.Equals(ImageFormat.Gif))
                {
                    image.Save(ms, ImageFormat.Gif);
                }
                else if (format.Equals(ImageFormat.Icon))
                {
                    image.Save(ms, ImageFormat.Icon);
                }
                byte[] buffer = new byte[ms.Length];
                //Image.Save()會改變MemoryStream的Position，需要重新Seek到Begin
                ms.Seek(0, SeekOrigin.Begin);
                ms.Read(buffer, 0, buffer.Length);
                return buffer;
            }
        }

        /// <summary>
        /// Convert Byte[] to Image
        /// </summary>
        /// <param name="buffer"></param>
        /// <returns></returns>
        public static Image BytesToImage(byte[] buffer)
        {
            MemoryStream ms = new MemoryStream(buffer);
            Image image = System.Drawing.Image.FromStream(ms);
            return image;
        }

        /// <summary>
        /// Convert Byte[] to a picture and Store it in file
        /// </summary>
        /// <param name="fileName"></param>
        /// <param name="buffer"></param>
        /// <returns></returns>
        public static string CreateImageFromBytes(string fileName, byte[] buffer)
        {
            string file = fileName;
            Image image = BytesToImage(buffer);
            ImageFormat format = image.RawFormat;
            if (format.Equals(ImageFormat.Jpeg))
            {
                file += ".jpeg";
            }
            else if (format.Equals(ImageFormat.Png))
            {
                file += ".png";
            }
            else if (format.Equals(ImageFormat.Bmp))
            {
                file += ".bmp";
            }
            else if (format.Equals(ImageFormat.Gif))
            {
                file += ".gif";
            }
            else if (format.Equals(ImageFormat.Icon))
            {
                file += ".icon";
            }
            System.IO.FileInfo info = new System.IO.FileInfo(file);
            System.IO.Directory.CreateDirectory(info.Directory.FullName);
            File.WriteAllBytes(file, buffer);
            return file;
        }
    }
}