Prime Number in C#

public static bool IsPrime(int number)
{
    if (number <= 1) return false;
    if (number == 2) return true;
    if (number % 2 == 0) return false;

    var boundary = (int)Math.Floor(Math.Sqrt(number));

    for (int i = 3; i <= boundary; i+=2)
        if (number % i == 0)
            return false;

    return true;        
}

using System;
public class Program
{
    static void Main(string[] args)
    {
        var results = GenerateSieve(1000);
        var i=0;
        foreach (var item in results)
        {
            if(item) Console.Write(i + " ");
            i++;
        }
    }
 
    static bool[] GenerateSieve(int num)
    {
        // Creating an array indicating whether numbers are prime.
        bool[] isPrime = new bool[num + 1];
        for (int i = 2; i <= num; i++) isPrime[i] = true;
 
        // Removing out multiples.
        for (int i = 2; i <= num; i++)
        {
            // Check if i is prime.
            if (isPrime[i])
            {
                // Eliminate multiples of i.
                for (int j = i * 2; j <= num; j += i)
                    isPrime[j] = false;
            }
        }
        return isPrime;
    }
}