C#找出字符串中第一个字母并大写的方法

class Program{  static void Main(string[] args)  {   String aa = "%.,46,2xb1bfs.,,";   int pos = isLetter(aa);   aa = aa.Substring(pos, 1);   Console.WriteLine(aa.ToUpper());   Console.Read();  }  public static int isLetter(string validString)  {   byte[] tempbyte = System.Text.Encoding.Default.GetBytes(validString);   int pos = 0;   for (int i = 0; i < validString.Length; i++)   {    byte by = tempbyte[i];    if ((by >= 65) && (by <= 90) || ((by >= 97) && (by <= 122)))    {     break;    }    pos++;   }   return pos;  }}