Error: System.InvalidOperationException: Collection was modified; enumeration operation may not execute

This error:

System.InvalidOperationException: Collection was modified; enumeration operation may not execute

happens when you modify a collection while iterating over it (especially with foreach).

Why it happens

foreach uses an internal enumerator, and if the collection changes (add/remove/update structure) during iteration, the enumerator becomes invalid — so .NET throws this exception.

Common fixes

1. Iterate over a copy (most common fix)

foreach (var item in myList.ToList())
{
    if (condition)
    {
        myList.Remove(item);
    }
}

ToList() creates a snapshot, so modifications don’t affect iteration.

2. Use for loop instead of foreach

for (int i = 0; i < myList.Count; i++)
{
    if (condition)
    {
        myList.RemoveAt(i);
        i--; // adjust index after removal
    }
}

3. Loop in reverse (safe removal)

for (int i = myList.Count - 1; i >= 0; i--)
{
    if (condition)
    {
        myList.RemoveAt(i);
    }
}

4. If multithreading is involved

Make sure no other thread modifies the collection while you're iterating. Use locking or thread-safe collections.

⚠️ Key takeaway

• ❌ Don’t modify collection inside foreach
• ✅ Use ToList(), for, or reverse iteration