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C# Base64编码class Base64Helper
{
    /// <summary>
    /// Base64加密，采用utf8编码方式加密
    /// </summary>
    /// <param name="source">待加密的明文</param>
    /// <returns>加密后的字符串</returns>
    public static string Base64Encode(string source)
    {
        return Base64Encode(Encoding.UTF8, source);
    }

    /// <summary>
    /// Base64加密
    /// </summary>
    /// <param name="encodeType">加密采用的编码方式</param>
    /// <param name="source">待加密的明文</param>
    /// <returns></returns>
    public static string Base64Encode(Encoding encodeType, string source)
    {
        string encode = string.Empty;
        byte[] bytes = encodeType.GetBytes(source);
        try
        {
            encode = Convert.ToBase64String(bytes);
        }
        catch
        {
            encode = source;
        }
        return encode;
    }

    /// <summary>
    /// Base64解密，采用utf8编码方式解密
    /// </summary>
    /// <param name="result">待解密的密文</param>
    /// <returns>解密后的字符串</returns>
    public static string Base64Decode(string result)
    {
        return Base64Decode(Encoding.UTF8, result);
    }

    /// <summary>
    /// Base64解密
    /// </summary>
    /// <param name="encodeType">解密采用的编码方式，注意和加密时采用的方式一致</param>
    /// <param name="result">待解密的密文</param>
    /// <returns>解密后的字符串</returns>
    public static string Base64Decode(Encoding encodeType, string result)
    {
        string decode = string.Empty;
        byte[] bytes = Convert.FromBase64String(result);
        try
        {
            decode = encodeType.GetString(bytes);
        }
        catch
        {
            decode = result;
        }
        return decode;
    }
}

C# 文件与二进制流/// <summary>
/// 将文件转换为byte数组
/// </summary>
/// <param name="path">文件地址</param>
/// <returns>转换后的byte数组</returns>
public static byte[] File2Bytes(string path)
{
　　if (!System.IO.File.Exists(path))
　　{
　　　　return new byte[0];
　　}

　　FileInfo fi = new FileInfo(path);
　　byte[] buff = new byte[fi.Length];

　　FileStream fs = fi.OpenRead();
　　fs.Read(buff, 0, Convert.ToInt32(fs.Length));
　　fs.Close();

　　return buff;
}

/// <summary>
/// 将byte数组转换为文件并保存到指定地址
/// </summary>
/// <param name="buff">byte数组</param>
/// <param name="savepath">保存地址</param>
public static void Bytes2File(byte[] buff, string savepath)
{
　　if (System.IO.File.Exists(savepath))
　　{
　　　　System.IO.File.Delete(savepath);
　　}

　　FileStream fs = new FileStream(savepath, FileMode.CreateNew);
　　BinaryWriter bw = new BinaryWriter(fs);
　　bw.Write(buff, 0, buff.Length);
　　bw.Close();
　　fs.Close();
}

C# MD5加密public static string MD5Encrypt(string strText)
{
　　MD5 md5 = new MD5CryptoServiceProvider();
　　byte[] result = md5.ComputeHash(System.Text.Encoding.Default.GetBytes(strText));
　　return System.Text.Encoding.Default.GetString(result);
}
private static string GetMD5String(string sign)
{
　　MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
　　byte[] encryptedBytes = md5.ComputeHash(Encoding.ASCII.GetBytes(sign));
　　StringBuilder sb = new StringBuilder();
　　for (int i = 0; i < encryptedBytes.Length; i++)
　　{
　　　　sb.AppendFormat("{0:x2}", encryptedBytes[i]);
　　}
　　return sb.ToString();
}

C# string和byte[]string类型转成byte[]：
byte[] byteArray = System.Text.Encoding.Default.GetBytes ( str );
byte[]转成string：
string str = System.Text.Encoding.Default.GetString ( byteArray );

来源：https://www.cnblogs.com/az4215/p/11388172.html