PRIME NUMBER Find in C#

using System; 

class MainClass { 
public static void Main() { 
int num; 
int i; 
int factor; 
bool isprime; 


for(num = 2; num < 20; num++) { 
isprime = true; 
factor = 0; 


for(i=2; i <= num/2; i++) { 
if((num % i) == 0) { 

isprime = false; 
factor = i; 
} 
} 

if(isprime) 
Console.WriteLine(num + " is prime."); 
else 
Console.WriteLine("Largest factor of " + num + 
" is " + factor); 
} 

} 
}