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;
  }
}