C#检查字符串是否为全字母短句

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Demo {
   public class Program {
      public static void Main(string []arg) {
         string str = "The quick brown fox jumps over the lazy dog";
         Console.WriteLine("{0}: /"{1}/" is pangram", checkPangram(str), str);
         Console.ReadKey();
      }

      static bool checkPangram(string str) {
         return str.ToLower().Where(ch => Char.IsLetter(ch)).GroupBy(ch => ch).Count() == 26;
      }
   }
}