方法一：

public static string Reverse(string name)
{
     if (String.IsNullOrEmpty(name))
       {
           throw new Exception("字符串不能为空！");
       }
    StringBuilder sb = new StringBuilder(name.Length);
    for (int i = name.Length-1; i >= 0; i--)
     {
        sb.Append(name[i]);
    }
        return sb.ToString();
}

方法二：.NET3.5以上

public static string Reverse(string name)
{
     char[] reverse = name.Reverse().ToArray();

     return new string(reverse);
}

方法三：二分法

public static string Reverse(string name)
{
   if (String.IsNullOrEmpty(name))
   {
      throw new Exception("字符串不能为空！");
   }
  char[] nm = name.ToCharArray();
  for (int i = 0; i < (nm.Length-1 )/ 2; i++)
  {
     char q = nm[i];
     nm[i]= nm[nm.Length - 1 - i];
    nm[nm.Length - 1 - i] = q;
  }

  return new string(nm);

}