Loops in C#

C# has four loop forms. Choosing between them comes down to two questions: do you need to know the position of each item, and do you know in advance how many times to repeat?

List<decimal> lineTotals = new List<decimal> { 24.99m, 150m, 8.50m };

decimal orderTotal = 0m;
foreach (decimal lineTotal in lineTotals)
{
    orderTotal += lineTotal;
}

for (int i = 0; i < lineTotals.Count; i++)
{
    Console.WriteLine($"Line {i + 1}: {lineTotals[i]:N2}");
}

Console.WriteLine($"Order total: {orderTotal:N2}");

• foreach hands you each value and nothing else. There is no position, and you cannot assign to lineTotal — the loop variable is read-only.
• The for loop's three parts are: where to start, the condition to keep going, and what to do after each pass.
• It is i < Count, not i <= Count. The last valid position is Count - 1, so <= asks for an item that does not exist and throws ArgumentOutOfRangeException on the final pass.
• The output uses i + 1 because people count from one while positions start at zero.

int unitsToPack = 7;
int unitsPerBox = 3;
int boxesUsed = 0;

while (unitsToPack > 0)
{
    int packed = Math.Min(unitsPerBox, unitsToPack);
    unitsToPack -= packed;
    boxesUsed++;
}

Console.WriteLine($"Boxes needed: {boxesUsed}");   // 3

string? command;

do
{
    Console.Write("Command (type quit to stop): ");
    command = Console.ReadLine();
    Console.WriteLine($"Received: {command}");
}
while (!string.Equals(command, "quit", StringComparison.OrdinalIgnoreCase));

• while checks the condition first, so a loop whose condition starts out false never runs its body once.
• do-while checks afterwards, which suits a prompt: you have to ask before you can know whether to stop.
• Every while loop needs something inside it that moves towards the condition becoming false. Here unitsToPack shrinks each pass. Remove that line and the program does not crash — it hangs, which is harder to spot.
• command is declared before the loop because the condition below refers to it. A variable declared inside the braces would not be visible there.

List<string> skus = new List<string> { "LAMP-01", "", "STAND-04", "TIDY-09" };

foreach (string sku in skus)
{
    if (string.IsNullOrWhiteSpace(sku))
    {
        continue;    // skip this one, carry on with the next
    }

    Console.WriteLine($"Checked {sku}");

    if (sku == "STAND-04")
    {
        Console.WriteLine("Found the one we wanted.");
        break;       // stop the loop completely
    }
}

• continue abandons the current pass and starts the next one. It is a tidy way to filter without wrapping the whole body in an if.
• break leaves the loop: nothing after it in the body runs, and no further items are visited.
• In nested loops, both affect the innermost loop only. Getting out of two levels at once usually means pulling the inner loop into its own method and returning from it.