string tempStr = File.ReadAllText(@"C:\Documents and Settings\Administrator\桌面\Test.txt", Encoding.GetEncoding("GB2312"));//读取tx
                string pattern = @"\d";
                
                var temp_list=Regex.Matches(tempStr,pattern).Cast<Match>().GroupBy(a=>a.Value).Select(a=>new {count=a.Count(),text=a.Key}).OrderByDescending(a=>a.count).ThenByDescending(a=>a.text).ToList();
                /*
                 +        [0]    { count = 12, text = "0" }    <Anonymous Type>
                +        [1]    { count = 10, text = "2" }    <Anonymous Type>
                +        [2]    { count = 8, text = "1" }    <Anonymous Type>
                +        [3]    { count = 7, text = "7" }    <Anonymous Type>
                +        [4]    { count = 6, text = "9" }    <Anonymous Type>
                +        [5]    { count = 6, text = "8" }    <Anonymous Type>
                +        [6]    { count = 6, text = "6" }    <Anonymous Type>
                +        [7]    { count = 6, text = "3" }    <Anonymous Type>
                +        [8]    { count = 5, text = "4" }    <Anonymous Type>
                +        [9]    { count = 4, text = "5" }    <Anonymous Type>

                 */

------解决方案--------------------

使用Linq可以很方便的对字符串进行这样的统计，比如下面的代码：   

string str = "8043283672469056709847421622639010367832281012053917805921902110075074";

   var q = from c in str.ToArray<char>() orderby c group c by c into cs
           select new
           {
               Str = cs.Key,
               Count = cs.Count()
           };