Semiprime.cs

public class Semiprime
{
      static public bool Semiprime1(int number)
      {
          var divisors = new List<int>();

          for (var i = 2; i <= Math.Sqrt(number); i++)
          {
              if (number % i == 0)
              {
                  divisors.Add(i);
                  divisors.Add(number / i);
              }
          }

          return divisors.Count == 2 && divisors.All(IsPrime);
      }

      static public bool Semiprime2(int number)
      {
          var hadTwoDivisors = false;
          for (var i = 2; i <= Math.Sqrt(number); i++)
          {
              if (number % i == 0)
              {
                  if (hadTwoDivisors || !IsPrime(i) || !IsPrime(number / i))
                      return false;

                  hadTwoDivisors = true;
              }
          }

          return hadTwoDivisors;
      }

      static public bool Semiprime3(int number)
      {
          var divisors = GetDivisors(number).ToArray();

          return divisors.Count() == 2 && divisors.All(IsPrime);
      }

      static public IEnumerable<int> GetDivisors(int number)
      {
          for (var i = 2; i <= Math.Sqrt(number); i++)
          {
              if (number % i == 0)
              {
                  yield return i;
                  yield return number / i;
              }
          }
      }

      private static bool IsPrime(int number)
      {
          if (number == 1)
              return false;

          for (var i = 2; i <= Math.Sqrt(number); i++)
          {
              if (number % i == 0)
                  return false;
          }
          return true;
      }
}