Branches and Loops

The familiar control structures

C# has the C-family control structures: if/else, switch, for, while, do ... while, foreach, break, continue and return. Conditions must be bool; there is no implicit conversion from int, so if (count) does not compile. A switch statement does not allow fall-through between non-empty sections; each section ends with break, return, throw or goto case. foreach works on anything that provides an enumerator (arrays, lists, dictionaries, strings, LINQ queries) and is the default way to walk a collection; you cannot add to or remove from a List<T> while iterating it with foreach, which throws InvalidOperationException. Use for when you need the index or to iterate backwards, for example to remove items safely.

Choosing a loop

Use foreach for whole collections, for when the index matters, while when the end is unknown.

Loops in practice

Removing items while looping is done backwards with for, or with RemoveAll.

var scores = new List<int> { 72, 45, 90, 38, 66 };

foreach (var s in scores)
{
    Console.WriteLine(s >= 50 ? $"{s} pass" : $"{s} fail");
}

for (int i = scores.Count - 1; i >= 0; i--)
{
    if (scores[i] < 50) scores.RemoveAt(i);   // safe: index shifts do not affect earlier items
}

// simpler and clearer
scores.RemoveAll(s => s < 50);

int attempts = 0;
while (attempts < 3 && !TryConnect())
{
    attempts++;
}

static bool TryConnect() => Random.Shared.Next(3) == 0;

No accidental fall-through

In C, forgetting break silently runs the next case. C# makes that a compile error, but it also means you must stack labels explicitly (case 'a': case 'e':) to share a body.

Quick check: What happens if you remove an item from a List<T> inside a foreach over that list?

• It works and skips the removed item
• The loop restarts from the beginning
• An InvalidOperationException is thrown
• It compiles to a for loop automatically