出自：http://blog.csdn.net/marquess/article/details/2732629

程式碼：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Drawing.Imaging;

namespace base64_img
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        //圖片 轉為 base64編碼的文字
        private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog dlg = new OpenFileDialog();
            dlg.Title = "選擇要轉換的圖片";
            dlg.Filter = "Image files (*.jpg;*.bmp;*.gif)|*.jpg*.jpeg;*.gif;*.bmp|AllFiles (*.*)|*.*";
            if (DialogResult.OK == dlg.ShowDialog())
            {
                ImgToBase64String(dlg.FileName);
            }
        }
        //圖片 轉為 base64編碼的文字
        private void ImgToBase64String(string Imagefilename)
        {
            try
            {
                Bitmap bmp = new Bitmap(Imagefilename);
                this.pictureBox1.Image = bmp;
                FileStream fs = new FileStream(Imagefilename + ".txt", FileMode.Create);
                StreamWriter sw = new StreamWriter(fs);

                MemoryStream ms = new MemoryStream();
                bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
                byte[] arr = new byte[ms.Length];
                ms.Position = 0;
                ms.Read(arr, 0, (int)ms.Length);
                ms.Close();
                String strbaser64 = Convert.ToBase64String(arr);
                sw.Write(strbaser64);

                sw.Close();
                fs.Close();
                MessageBox.Show("轉換成功!");
            }
            catch (Exception ex)
            {
                MessageBox.Show("ImgToBase64String 轉換失敗/nException:" + ex.Message);
            }
        }

        //base64編碼的文字 轉為 圖片
        private void button2_Click(object sender, EventArgs e)
        {
            OpenFileDialog dlg = new OpenFileDialog();
            dlg.Title = "選擇要轉換的base64編碼的文字";
            dlg.Filter = "txt files|*.txt";
            if (DialogResult.OK == dlg.ShowDialog())
            {
                Base64StringToImage(dlg.FileName);
            }
        }
        //base64編碼的文字 轉為 圖片
        private void Base64StringToImage(string txtFileName)
        {
            try
            {
                FileStream ifs = new FileStream(txtFileName, FileMode.Open, FileAccess.Read);
                StreamReader sr = new StreamReader(ifs);

                String inputStr = sr.ReadToEnd();
                byte[] arr = Convert.FromBase64String(inputStr);
                MemoryStream ms = new MemoryStream(arr);
                Bitmap bmp = new Bitmap(ms);

                bmp.Save(txtFileName + ".jpg", System.Drawing.Imaging.ImageFormat.Jpeg);
                //bmp.Save(txtFileName + ".bmp", ImageFormat.Bmp);
                //bmp.Save(txtFileName + ".gif", ImageFormat.Gif);
                //bmp.Save(txtFileName + ".png", ImageFormat.Png);
                ms.Close();
                sr.Close();
                ifs.Close();
                this.pictureBox1.Image = bmp;
                MessageBox.Show("轉換成功！");
            }
            catch (Exception ex)
            {
                MessageBox.Show("Base64StringToImage 轉換失敗/nException：" + ex.Message);
            }
        }
    }
}
 

        void btnTest_Click(object sender, EventArgs e)
        {
            string strParams = "";
            strParams += "name=王蒙蒙";
            strParams += "&xingbie=男"; 

            System.Net.WebClient client = new System.Net.WebClient();
            string reply = client.DownloadString("http://xxx.asp?" + strParams); //http地址並返回結果
            
            byte[] arr = Convert.FromBase64String(reply.Split('|')[1]); 
            MemoryStream ms = new MemoryStream(arr);
            Bitmap bmp = new Bitmap(ms);

            string strFileName = "test.jpg";  //圖片名稱
            string Opath = @"D:/Photo";  //儲存圖片路徑
            bmp.Save(Opath+strFileName, System.Drawing.Imaging.ImageFormat.Jpeg);
            ms.Close();
        }