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C# Academy · Lesson

Common Iteration Mistakes

Modifying while iterating.

Common Iteration Mistakes is a free C# Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Iteration Pitfalls

Loops are simple, but a few mistakes trip up beginners again and again.

In this lesson we look at the most common iteration errors and how to avoid them.

Modifying While Iterating

The biggest mistake: changing a collection while you foreach over it.

Adding or removing items during iteration throws an InvalidOperationException at runtime.

foreach (var n in list)
{
    if (n < 0) list.Remove(n); // ERROR at runtime
}

The Safe Fix

To remove while looping, iterate over a copy, or collect items first and remove them after.

Looping over list.ToList() gives you a separate snapshot to walk.

var list = new List<int> { 1, -2, 3, -4 };
foreach (var n in list.ToList())
{
    if (n < 0) list.Remove(n);
}
Console.WriteLine(string.Join(",", list));

Off-by-One in for Loops

With a manual for loop, using <= instead of < reads past the end.

Valid indexes go from 0 to Length - 1, so the condition should be i < Length.

for (int i = 0; i <= arr.Length; i++) // BUG: <= goes too far
{
    Console.WriteLine(arr[i]);
}

IndexOutOfRange

That off-by-one causes an IndexOutOfRangeException.

foreach avoids this entirely because it never uses an index. Prefer it when you only need to read items.

string[] arr = { "a", "b", "c" };
for (int i = 0; i < arr.Length; i++) // correct: < Length
{
    Console.WriteLine(arr[i]);
}

Expecting foreach to Change Items

Assigning to the loop variable does not change the collection, and for value types it is a copy anyway.

To update items, use a for loop with indexing.

int[] nums = { 1, 2, 3 };
for (int i = 0; i < nums.Length; i++)
{
    nums[i] = nums[i] * 10;
}
Console.WriteLine(string.Join(",", nums));

Null Collection

Calling foreach on a null reference throws a NullReferenceException.

An empty collection is fine, but null is not. Check or initialize before looping.

List<int> data = null;
if (data != null)
{
    foreach (var x in data) Console.WriteLine(x);
}

Variable Scope Confusion

The loop variable exists only inside the loop body.

Trying to use it after the loop is a compile error, because it is out of scope once the loop ends.

foreach (var n in nums)
{
    Console.WriteLine(n);
}
// Console.WriteLine(n); // ERROR: n not in scope here

Forgetting Braces

Without braces, only the next single statement belongs to the loop.

This can silently break logic. Always use braces for multi-line bodies to keep intent clear.

foreach (var n in nums)
    Console.WriteLine(n);
    Console.WriteLine("runs once, not per item");

Infinite Risk in while

Unlike foreach, a while loop can run forever if its condition never becomes false.

Always make sure something inside the loop moves toward the exit condition.

int i = 0;
while (i < 3)
{
    Console.WriteLine(i);
    i++; // forgetting this would loop forever
}

Picking the Right Loop

Read every item, no index needed → foreach.

Need the index or must modify in place → for. Repeat until a condition changes → while.

Quick Check

Test what you learned about iteration mistakes.

Recap

You learned to avoid modifying a collection during foreach, off-by-one errors with <=, null collections, and scope confusion.

Choose foreach to read, for to modify by index, and while for condition-based loops with a clear exit.

Frequently asked questions

Is the “Common Iteration Mistakes” lesson free?

Yes — the full text of “Common Iteration Mistakes” is free to read here on the web, and the C# Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.

What will I learn in “Common Iteration Mistakes”?

Modifying while iterating. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Common Iteration Mistakes” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C# Academy lesson?

Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. The foreach Loop
  2. Iterating Collections
  3. break and continue
  4. Common Iteration Mistakes
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