Find the nearest next prime to a given integer:
for example: int n = 45, result = 47;
int n = 8, result = 11
int GetNearestNextPrime(int n)
{
int result = -1;
for (int I = n+1; I < INT_MAX; I++)
{
   if (isPrime(i)){
       result = I;
       break;
   }
}
return result;
}

bool isPrime(int n)
{
var primes = new List<int>();
for (int I = 2; I <= n; I++)
{
    bool composite  = false;
    for (int k =0 ; k < primes.Count; k++)
    {
       if (I % primes[k]  == 0){
           composite = true;
           break;
        }
    }
     if (composite == false){
         primes.Add(I);
     }
}
return primes.Contains(n);
}

Another way is to use the sieve of Eratosthenes to filter out prime:

This is a very simple way to filter out the prime numbers upto n.

for i ranging from 2 to square root of n:

     for j ranging from multiples of i upto n

          mark the number as not prime

those that remain unmarked are the primes.    

            int count = 0;

            for (int i = 0; i < primes.Count; i++)

                for (int j = 0; j <= i; j++)

                    if (primes[i] != primes[j] && primes.Contains(primes[i] + primes[j]) && primes[i] + primes[j] <= n)

                    {

                        Console.WriteLine("{0},{1}", primes[j], primes[i]);

                        count++;

                    }

            Console.Write("Count={0}", count);