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    /// <summary>
    /// ÓÃÓÚÎı¾ºÍBase64±àÂëÎı¾µÄ»¥Ïàת»» ºÍ Byte[]ºÍBase64±àÂëÎı¾µÄ»¥Ïàת»»
    /// </summary>
    class Base64Change
    {
        /// <summary>
        /// ½«ÆÕͨÎı¾×ª»»³ÉBase64±àÂëµÄÎı¾
        /// </summary>
        /// <param name="value">ÆÕͨÎı¾</param>
        /// <returns></returns>
        public string StringToBase64String(String value)
        {
            byte[] binBuffer =(new UnicodeEncoding()).GetBytes(value);
            int base64ArraySize = (int)Math.Ceiling(binBuffer.Length / 3d) * 4;
            char[] charBuffer = new char[base64ArraySize];
            Convert.ToBase64CharArray(binBuffer, 0, binBuffer.Length, charBuffer, 0);
            string s = new string(charBuffer);
            return s;
        }

        /// <summary>
        /// ½«Base64±àÂëµÄÎı¾×ª»»³ÉÆÕͨÎı¾
        /// </summary>
        /// <param name="base64">Base64±àÂëµÄÎı¾</param>
        /// <returns></returns>
        public string Base64StringToString(string base64)
        {
            char[] charBuffer = base64.ToCharArray();
            byte[] bytes = Convert.FromBase64CharArray(charBuffer, 0, charBuffer.Length);
            return (new UnicodeEncoding()).GetString(bytes);
        }
       
        /// <summary>
        /// ½«Byte[]ת»»³ÉBase64±àÂëÎı¾
        /// </summary>
        /// <param name="binBuffer">Byte[]</param>
        /// <returns></returns>
        public string toBase64(byte[] binBuffer)
        {
            int base64ArraySize = (int)Math.Ceiling(binBuffer.Length / 3d) * 4;
            char[] charBuffer = new char[base64ArraySize];
            Convert.ToBase64CharArray(binBuffer, 0, binBuffer.Length, charBuffer, 0);
            string s = new string(charBuffer);
            return s;
        }

        /// <summary>
        /// ½«Base64±àÂëÎı¾×ª»»³ÉByte[]
        /// </summary>
        /// <param name="base64">Base64±àÂëÎı¾</param>
        /// <returns></returns>
        public Byte[] Base64ToBytes(string base64)
        {
            char[] charBuffer = base64.ToCharArray();
            byte[] bytes = Convert.FromBase64CharArray(charBuffer, 0, charBuffer.Length);
            return bytes;
        }
    }

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Îı¾×ª»»
            Base64Change bc = new Base64Change();
            string s = "Òä·ç";

            string bs = bc.StringToBase64String(s);//bsΪת»»³ÉµÄbase64±àÂëÎı¾

            s = bc.Base64StringToString(bs);//sΪ»¹Ô­ºóµÄÎı¾"Òä·ç"

Byte[]ת»»

            Base64Change bc=new Base64Change();

            FileStream fs = new FileStream("d:\\a.bmp", FileMode.Open, FileAccess.ReadWrite);
            byte[] bt=new byte[fs.Length];
            fs.Read(bt, 0, (int)fs.Length);
            fs.Close();
            string s = bc.toBase64(bt);//sΪת»»³ÉµÄbase64±àÂëÎı¾

            byte[] b=bc.Base64ToBytes(s);//bΪ»¹Ô­ºóµÄByte[]
            FileStream f = new FileStream("d:\\b.bmp", FileMode.Create, FileAccess.Write);
            f.Write(b, 0, b.Length);
            f.Close();