文字列が完全な単語であるかどうかを確認する C# プログラム

string str = "The quick brown fox jumps over the lazy dog";

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;
      }
   }
}

True: "The quick brown fox jumps over the lazy dog" is pangram