Iterating and accumulating

foreach: visit everything

foreach (int p in primes)
    Console.WriteLine(p);

foreach walks every element in order, no index, no bounds risk. It cannot modify the array structure, and you don't get a position. When you need the position — for:

for (int i = 0; i < primes.Length; i++)
    Console.WriteLine($"{i}: {primes[i]}");

Note i < Length, not <=. This loop shape appears in every challenge you'll ship.

Accumulation: the four patterns

Almost every array exercise is one of these loops in disguise:

int sum = 0;                    // 1. fold: one running value
foreach (int p in primes) sum += p;

int max = primes[0];            // 2. best-so-far
foreach (int p in primes) if (p > max) max = p;

int count = 0;                  // 3. counter with predicate
foreach (int p in primes) if (p % 2 == 0) count++;

int[] copy = new int[primes.Length];   // 4. transform
for (int i = 0; i < primes.Length; i++) copy[i] = primes[i] * 2;

The best-so-far loop assumes a non-empty array — on empty input primes[0] throws. Guard with Length == 0 (or seed from int.MinValue when a sentinel is honest). These four shapes plus a guard clause cover min/max/average/count/filter/map — the entire Module 7 practice set is them, and LINQ (Module 17) later names them.