WEB开发常用的几个函数（获取IP，MD5加密解密，HTML转义字符）

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Security.Cryptography;
using System.Text;
using System.IO;  


/// <summary>
/// Common 的摘要说明
/// </summary>
public class Common
{
    public Common()
    {
        //
        // TODO: 在此处添加构造函数逻辑
        //
    }

    /// <summary>
    /// 获取IP地址
    /// </summary>
    /// <returns></returns>
    public static string GetAddIP()
    {
        string result = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
        if (null == result || result == String.Empty)
        {
            result = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
        }

        if (null == result || result == String.Empty)
        {
            result = HttpContext.Current.Request.UserHostAddress;
        }
        return result;
    }

    #region MD5加密
    /// <summary>   
    /// MD5加密   
    /// </summary>   
    /// <param name="strSource">需要加密的字符串</param>   
    /// <returns>MD5加密后的字符串</returns>   
    public static string Md5Encrypt(string strSource)
    {
        //把字符串放到byte数组中   
        byte[] bytIn = System.Text.Encoding.Default.GetBytes(strSource);
        //建立加密对象的密钥和偏移量           
        byte[] iv = { 102, 16, 93, 156, 78, 4, 218, 32 };//定义偏移量   
        byte[] key = { 55, 103, 246, 79, 36, 99, 167, 3 };//定义密钥   
        //实例DES加密类   
        DESCryptoServiceProvider mobjCryptoService = new DESCryptoServiceProvider();
        mobjCryptoService.Key = iv;
        mobjCryptoService.IV = key;
        ICryptoTransform encrypto = mobjCryptoService.CreateEncryptor();
        //实例MemoryStream流加密密文件   
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        CryptoStream cs = new CryptoStream(ms, encrypto, CryptoStreamMode.Write);
        cs.Write(bytIn, 0, bytIn.Length);
        cs.FlushFinalBlock();
        return System.Convert.ToBase64String(ms.ToArray());
    }
    #endregion

    #region MD5解密
    /// <summary>   
    /// MD5解密   
    /// </summary>   
    /// <param name="Source">需要解密的字符串</param>   
    /// <returns>MD5解密后的字符串</returns>   
    public static string Md5Decrypt(string Source)
    {
        //将解密字符串转换成字节数组   
        byte[] bytIn = System.Convert.FromBase64String(Source);
        //给出解密的密钥和偏移量，密钥和偏移量必须与加密时的密钥和偏移量相同   
        byte[] iv = { 102, 16, 93, 156, 78, 4, 218, 32 };//定义偏移量   
        byte[] key = { 55, 103, 246, 79, 36, 99, 167, 3 };//定义密钥   
        DESCryptoServiceProvider mobjCryptoService = new DESCryptoServiceProvider();
        mobjCryptoService.Key = iv;
        mobjCryptoService.IV = key;
        //实例流进行解密   
        System.IO.MemoryStream ms = new System.IO.MemoryStream(bytIn, 0, bytIn.Length);
        ICryptoTransform encrypto = mobjCryptoService.CreateDecryptor();
        CryptoStream cs = new CryptoStream(ms, encrypto, CryptoStreamMode.Read);
        StreamReader strd = new StreamReader(cs, Encoding.Default);
        return strd.ReadToEnd();
    }
    #endregion 

    #region  HTML 转义字符

    /// <summary>
    /// 编码
    /// </summary>
    /// <param name="str"></param>
    /// <returns></returns>
    public static string Encode(string str)
    {
        str = str.Replace("&", "&amp;");
        str = str.Replace("'", "''");
        str = str.Replace("\"", "&quot;");
        str = str.Replace(" ", "&nbsp;");
        str = str.Replace("<", "&lt;");
        str = str.Replace(">", "&gt;");
        str = str.Replace("\n", "<br>");
        return str;
    }

    /// <summary>
    /// 解码
    /// </summary>
    /// <param name="str"></param>
    /// <returns></returns>
    public static string Decode(string str)
    {
        str = str.Replace("&amp;", "&");
        str = str.Replace("''", "'");
        str = str.Replace("&quot;", "\"");
        str = str.Replace("&nbsp;", " ");
        str = str.Replace("&gt;", ">");
        str = str.Replace("&lt;", "<");
        str = str.Replace("\n", "<br>");
        return str;
    }
    #endregion

}