protected bool  IsChineseLetter(string input,int index)
    ...{

          return false;
 }

方法二：
http://hi.baidu.com/yhfd/blog/item/3222e1fca22cfb80b901a027.html
public bool IsChina(string CString)
          {
              bool BoolValue = false;
              for (int i = 0; i < CString.Length; i++)
              {
                  if (Convert.ToInt32(Convert.ToChar(CString.Substring(i, 1))) < Convert.ToInt32(Convert.ToChar(128)))
                  {
                      BoolValue = false;
                  }
                  else
                  {
                      return BoolValue = true;
                  }
              }
              return BoolValue;
          }

方法三：

        /// <summary>
        /// 判断句子中是否含有中文     宁夏大学 张冬 zd4004.blog.163.com
        /// </summary>
        /// <param >字符串</param>
        public bool WordsIScn(string words)
        {
            string TmmP;

            for (int i = 0; i < words.Length; i++)
            {
                TmmP = words.Substring(i, 1);

                byte[] sarr = System.Text.Encoding.GetEncoding("gb2312").GetBytes(TmmP);

                if (sarr.Length == 2)
                {
                    return true;
                }
            }
            return false;
        }

方法四：
for (int i=0; i<s.length; i++)
{
Regex rx = new Regex("^[\u4e00-\u9fa5]$");
if (rx.IsMatch(s[i]))
// 是
else
// 否
}
正解！
\u4e00-\u9fa5 汉字的范围。
^[\u4e00-\u9fa5]$ 汉字的范围的正则

方法五
unicodeencoding   unicodeencoding   =   new   unicodeencoding();  
  byte   []   unicodebytearray   =   unicodeencoding.getbytes(   inputstring   );  
  for(   int   i   =   0;   i   <   unicodebytearray.length;   i++   )  
  {  
  i++;  
  //如果是中文字符那么高位不为0  
  if   (   unicodebytearray[i]   !=   0   )  
  {  
  }  
  ……  

方法六
    /// <summary>
        /// 给定一个字符串，判断其是否只包含有汉字
        /// </summary>
        /// <param name="testStr"></param>
        /// <returns></returns>
        public bool IsOnlyContainsChinese(string testStr)
        {
            char[] words = testStr.ToCharArray();
            foreach (char word in words)
            {
                if ( IsGBCode(word.ToString()) || IsGBKCode(word.ToString()) )  // it is a GB2312 or GBK chinese word
                {
                    continue;
                }
                else
                {
                    return false;
                }
            }
            return true;
        }

        /// <summary>
        /// 判断一个word是否为GB2312编码的汉字
        /// </summary>
        /// <param name="word"></param>
        /// <returns></returns>
        private bool IsGBCode(string word)
        {
            byte[] bytes = Encoding.GetEncoding("GB2312").GetBytes(word);
            if (bytes.Length <= 1)  // if there is only one byte, it is ASCII code or other code
            {
                return false;
            }
            else
            {
                byte byte1 = bytes[0];
                byte byte2 = bytes[1];
                if (byte1 >= 176 && byte1 <= 247 && byte2 >= 160 && byte2 <= 254)    //判断是否是GB2312
                {
                    return true;
                }
                else
                {
                    return false;
                }
            }
        }

        /// <summary>
        /// 判断一个word是否为GBK编码的汉字
        /// </summary>
        /// <param name="word"></param>
        /// <returns></returns>
        private bool IsGBKCode(string word)
        {
            byte[] bytes = Encoding.GetEncoding("GBK").GetBytes(word.ToString());
            if (bytes.Length <= 1)  // if there is only one byte, it is ASCII code
            {
                return false;
            }
            else
            {
                byte byte1 = bytes[0];
                byte byte2 = bytes[1];
                if ( byte1 >= 129 && byte1 <= 254 && byte2 >= 64 && byte2 <= 254)     //判断是否是GBK编码
                {
                    return true;
                }
                else
                {
                    return false;
                }
            }
        }

 

        /// <summary>
        /// 判断一个word是否为Big5编码的汉字
        /// </summary>
        /// <param name="word"></param>
        /// <returns></returns>
        private bool IsBig5Code(string word)
        {
            byte[] bytes = Encoding.GetEncoding("Big5").GetBytes(word.ToString());
            if (bytes.Length <= 1)  // if there is only one byte, it is ASCII code
            {
                return false;
            }
            else
            {
                byte byte1 = bytes[0];
                byte byte2 = bytes[1];
                if ( (byte1 >= 129 && byte1 <= 254) && ((byte2 >= 64 && byte2 <= 126) || (byte2 >= 161 && byte2 <= 254)) )     //判断是否是Big5编码
                {
                    return true;
                }
                else
                {
                    return false;
                }
            }
        }

其实原理都一样。
具体的实现，还有几种，就不列举了