字符串去重，过滤非数字或数字

string a = "abcabcd";
char[] b = a.ToArray().Distinct().ToArray();
string c = string.Join("", b);

ArrayList alist = new ArrayList();
char[] strs = a.ToArray();
for (int i = 0; i < strs.Length; i++)
{
    if (!alist.Contains(strs[i]))
    {
         alist.Add(strs[i]);
     }
}

/// 去掉字符串中的数字  
public static string RemoveNumber(string key)  
{  
      return Regex.Replace(key, @"\d", "");  
}  


//去掉字符串中的非数字
public static string RemoveNotNumber(string key)  
{  
    return Regex.Replace(key, @"[^\d]*", "");  
}