For and Foreach Loops — Iterating Collections

Loops let you execute the same block of code repeatedly — iterating over a list of items, processing each record in a database result, or building output from a collection. C# has four loop constructs, and the two you will use most often are for (index-based, full counter control) and foreach (clean iteration over any collection). In production ASP.NET Core code, LINQ methods often replace explicit loops for data transformation, but understanding loops is essential for understanding what LINQ does under the hood.

The For Loop #

// Syntax: for (initialiser; condition; iterator)
for (int i = 0; i < 5; i++)
    Console.Write($"{i} ");   // 0 1 2 3 4

// Iterating an array with index
string[] fruits = { "Apple", "Banana", "Cherry" };
for (int i = 0; i < fruits.Length; i++)
    Console.WriteLine($"[{i}] {fruits[i]}");
// [0] Apple
// [1] Banana
// [2] Cherry

// Reverse iteration
for (int i = fruits.Length - 1; i >= 0; i--)
    Console.Write(fruits[i] + " ");   // Cherry Banana Apple

// Step by 2
for (int i = 0; i <= 10; i += 2)
    Console.Write($"{i} ");   // 0 2 4 6 8 10

// Nested for — e.g. multiplication table
for (int r = 1; r <= 3; r++)
{
    for (int c = 1; c <= 3; c++)
        Console.Write($"{r * c,4}");
    Console.WriteLine();
}
//    1   2   3
//    2   4   6
//    3   6   9

The Foreach Loop #

// Works on any IEnumerable<T> — arrays, List<T>, LINQ queries, etc.
string[] fruits = { "Apple", "Banana", "Cherry" };

foreach (string fruit in fruits)
    Console.WriteLine(fruit);

// With a List<T>
var numbers = new List<int> { 10, 20, 30, 40, 50 };
int sum = 0;
foreach (int n in numbers)
    sum += n;
Console.WriteLine($"Sum: {sum}");   // Sum: 150

// foreach with index using LINQ
foreach ((int index, string fruit) in fruits.Select((f, i) => (i, f)))
    Console.WriteLine($"[{index}] {fruit}");

// break — exit the loop early
foreach (int n in numbers)
{
    if (n == 30) break;          // stops when n is 30
    Console.Write($"{n} ");      // 10 20
}

// continue — skip the current iteration
foreach (int n in numbers)
{
    if (n % 20 == 0) continue;  // skip multiples of 20
    Console.Write($"{n} ");     // 10 30 50
}

Break and Continue #

Common Mistakes #

Mistake 1 — Modifying a collection inside foreach (InvalidOperationException) #

❌ Wrong:

var items = new List<int> { 1, 2, 3, 4 };
foreach (int item in items)
    if (item % 2 == 0) items.Remove(item);  // InvalidOperationException!

✅ Correct — use LINQ to build a new collection:

items = items.Where(x => x % 2 != 0).ToList();   // ✓ filters without mutating during iteration

Mistake 2 — Off-by-one in the for loop condition #

❌ Wrong — i <= array.Length throws IndexOutOfRangeException on last iteration.

✅ Correct — always use i < array.Length.