The algorithm of C # judging prime number

A prime number is divisible only by 1 or itself, and cannot be the product of two other integers. 1, 2, 3 itself is prime, to determine whether a number is prime, just use this value divided by 2 to its root number, if there is a number divisible, then the value is not prime, return is prime. The code is as follows:

using System;
using System.Collections.Generic;
using System.Text;
namespace ExPrimeNumber
{
class PrimeNumber
{
public bool primeNumber(int n)
{
bool b = true;
if (n == 1 || n == 2)
b = true;
else
{
int sqr = Convert.ToInt32(Math.Sqrt(n));
for (int i = sqr; i > 2; i--)
{
if (n % i == 0)
{
b = false;
}
}
}
return b;
}
static void Main(string[] args)
{
Console.Write("请输入一个数：");
int n = Convert.ToInt32(Console.ReadLine ());
PrimeNumber p=new PrimeNumber ();
bool b=p.primeNumber(n);
if (b)
Console.WriteLine("{0}是素数",n);
else
Console.WriteLine("{0}不是素数",n);
}
}
}