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//字符串转Base64
byte[] b = System.Text.Encoding.Default.GetBytes(textBox4.Text);    
textBox5.Text = Convert.ToBase64String(b);
 //Base64字符转字符串
byte[] c = Convert.FromBase64String(textBox5.Text);
textBox4.Text = System.Text.Encoding.Default.GetString(c); 

private FileStream Base64strToFile(String Base64str,string path)
{
    byte[] b = Convert.FromBase64String(Base64str);//逆base64
    FileStream file;
    using (FileStream fs = new FileStream(path,FileMode.Create,FileAccess.Write))
    {
        fs.Write(b, 0, b.Length);
        fs.Flush();
        file = fs;
    }
    return file;
}

public static string FileToBase64String(string filename)//图像文件转二进制
{
    //Bitmap bmp = new Bitmap("");
    ////this.pictureBox1.Image = bmp;
     FileStream fs = new FileStream(filename, FileMode.Open);
    //StreamWriter sw = new StreamWriter(fs);
    //bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
    byte[] arr = new byte[fs.Length];
    fs.Position = 0;
    fs.Read(arr, 0, (int)fs.Length);
    fs.Close();
    String strbaser64 = Convert.ToBase64String(arr);
    return strbaser64;
}