//图片 转为    base64编码的文本

        private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog dlg = new OpenFileDialog();
            dlg.Multiselect = true;
            dlg.Title = "选择要转换的图片";
            dlg.Filter = "Image files (*.jpg;*.bmp;*.gif;*.png)|*.jpg*.jpeg;*.gif;*.bmp|AllFiles (*.*)|*.*";
            if (DialogResult.OK == dlg.ShowDialog())
            {
                for (int i = 0; i < dlg.FileNames.Length; i++)
                {
                    ImgToBase64String(dlg.FileNames[i].ToString());
                }
            }
        }
        //图片 转为    base64编码的文本
        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("转换成功!");
            }
            catch (Exception ex)
            {
                MessageBox.Show("ImgToBase64String 转换失败\nException:" + ex.Message);
            }
        }

        //base64编码的文本 转为    图片
        private void button2_Click(object sender, EventArgs e)
        {
            OpenFileDialog dlg = new OpenFileDialog();
            dlg.Multiselect = true;
            dlg.Title = "选择要转换的base64编码的文本";
            dlg.Filter = "txt files|*.txt";
            if (DialogResult.OK == dlg.ShowDialog())
            {
                for (int i = 0; i < dlg.FileNames.Length; i++)
                {
                    Base64StringToImage(dlg.FileNames[i].ToString());
                }
                
            }
        }
        //base64编码的文本 转为    图片
        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.pictureBox2.Image = bmp;
                if (File.Exists(txtFileName))
                {
                    File.Delete(txtFileName);
                }
                //MessageBox.Show("转换成功！");
            }
            catch (Exception ex)
            {
                MessageBox.Show("Base64StringToImage 转换失败\nException：" + ex.Message);
            }
        }

C#中图片与BASE64码互相转换