문자열에서 단어의 발생 횟수를 계산하는 C# 프로그램

string str = "Hello World! Hello!";

while ((a = str1.IndexOf(pattern, a)) != -1) {
   a += pattern.Length;
   count++;
}

using System;
class Program {
   static void Main() {
      string str = "Hello World! Hello!";
      Console.WriteLine("Occurrence:"+Check.CheckOccurrences(str, "Hello"));
   }
}
public static class Check {
   public static int CheckOccurrences(string str1, string pattern) {
      int count = 0;
      int a = 0;
      while ((a = str1.IndexOf(pattern, a)) != -1) {
         a += pattern.Length;
         count++;
      }
      return count;
   }
}

Occurrence:2