Function for creating a list of prime numbers

protected void Button1_Click(object sender, EventArgs e)
{
  //SPECIFYING NUMBER RANGE
  for (int i = 1; i <= 40; i++)
  {
      bool a = CheckNo(Convert.ToInt32(i));
      if (a == true)
      {
         Response.Write(i + " " + "Number is Prime" + " "); 
      }
      else
      {
         Response.Write(i + " " + "Number is Not Prime" + " );
       }
   }
}

//FUNCTION FOR DETECTING PRIME NUMBERS
protected bool CheckNo(int number)
{
  bool Flag = true; 
  if (number == 1 && number == 2) 
  {
    Flag = false; 
   }
  for (int i = number - 1; i >= 2; i--)
  {
    if (number % i == 0) 
    {
      Flag = false; break;
    }
  }
  return Flag;
}