How do I remove items from a list safely while looping?

Loop backwards with for (int i = list.Count - 1; i >= 0; i--), so removing an item never shifts the ones you have not visited yet, or use list.RemoveAll(x => x.IsDead). Removing inside a foreach loop throws a different error, Collection was modified.

Which method threw it?

The stack trace names it. Besides list indexing, List.Insert, RemoveAt, string.Substring and Random.Range-based indices are the usual sources. The parameter name in the message (index, startIndex, length) tells you which argument was out of range.

How do I read the first item only when there is one?

Check list.Count > 0 first, or use a helper that returns a default. With System.Linq, list.FirstOrDefault() returns null (or default) on an empty list instead of throwing.

ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection

ConsoleRuntime exception · Scripting

ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.

The same mistake as IndexOutOfRangeException, on a List instead of an array: the index was negative or not less than Count.

Read the Console line first

Check the line in the stack trace for list[i], RemoveAt(i) or Insert(i). Log i and list.Count before it.

Common causes

Removing items in a forward loop

RemoveAt shifts every later item down by one, so a forward loop skips items and its last index runs past the shrinking Count.
Reading the first item of an empty list

list[0] fails when nothing has been added yet, for example an inventory before the first pickup.

The fix

 Fix
C# 
 // Walk backwards so removals do not shift what is left to visit
for (int i = enemies.Count - 1; i >= 0; i--)
{
    if (enemies[i].IsDead) enemies.RemoveAt(i);
}

// Or in one call
enemies.RemoveAll(e => e.IsDead);
 8 lines

Stop it coming back

Remove from lists with RemoveAll or a reverse loop, and check Count before reading a fixed index.

Questions

How do I remove items from a list safely while looping?: Loop backwards with for (int i = list.Count - 1; i >= 0; i--), so removing an item never shifts the ones you have not visited yet, or use list.RemoveAll(x => x.IsDead). Removing inside a foreach loop throws a different error, Collection was modified.
Which method threw it?: The stack trace names it. Besides list indexing, List.Insert, RemoveAt, string.Substring and Random.Range-based indices are the usual sources. The parameter name in the message (index, startIndex, length) tells you which argument was out of range.
How do I read the first item only when there is one?: Check list.Count > 0 first, or use a helper that returns a default. With System.Linq, list.FirstOrDefault() returns null (or default) on an empty list instead of throwing.

Scripts that avoid it

Related errors

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