Strange behavior with lists in foreach - unity3d

Question

I have a List that tracks "alive" objects in runtime. And when I apply some behavior in items from this list I've found something strange (at least to me), for example, let's say that I want to damage each enemy in current scene, so (Items is a List of custom class)
if (Input.GetKeyDown(KeyCode.C))
{
var enemiesWithLife = enemiesInScene.Items.FindAll(enemy => enemy.life > 1);
foreach (var enemy in enemiesWithLife)
enemy.TakeDamage(1);
}
The above code works as expected BUT if I want to apply something to ALL enemies and try something as
if (Input.GetKeyDown(KeyCode.C))
{
foreach (var enemy in enemiesInScene.Items)
enemy.TakeDamage(1);
}
When I press "C" only 1 enemy "TakeDamage" and an error about list been modified during iterating
EnemiesInScene is an ScriptableObject, so the enemies list exist only in assets
BUT if I try same code, but make an find before, as
if (Input.GetKeyDown(KeyCode.C))
{
var allEnemies = enemiesInScene.Items.FindAll(enemy => true);
foreach (var enemy in allEnemies)
enemy.TakeDamage(1);
}
Things work as expected again ... WHY?
Edit
Codes asked in comments:
public void TakeDamage(int damage)
{
if (currentLife <= 0)
return;
currentLife -= damage;
if (currentLife <= 0)
Die();
else
TakeHit();
}
EnemiesInScene is an RuntimeSet
public class RuntimeSet : ScriptableObject
{
public List<RuntimeItem> Items = new List<RuntimeItem>();
public void Add(RuntimeItem thing)
{
if (!Items.Contains(thing))
Items.Add(thing);
}
public void Remove(RuntimeItem thing)
{
if (Items.Contains(thing))
Items.Remove(thing);
}
}
Runtime item
public class RuntimeItem : LazyComponents
{
public RuntimeSet runtimeSet;
private void OnEnable()
=> runtimeSet.Add(this);
private void OnDisable()
=> runtimeSet.Remove(this);
}
And LazyComponents is only a lazy approach wrapper that inherits from monobehaviour to get some components as rigidbody2d, etc...

Answer 1

foreach uses an enumerator, and enumerators can't change the underlying collection, but can, however, change any objects referenced by an object in the collection see MSDN
2 solutions: copy the collection in Array or List (less performance)
if (Input.GetKeyDown(KeyCode.C))
{
foreach (var enemy in enemiesInScene.Items.ToArray())
enemy.TakeDamage(1);
}
or use for loop which allows the modification in collection (more performance) i recommand
for(int i = 0 ; i < enemiesInScene.Items.Length; i++)
enemiesInScene.Items[i].TakeDamage(1);

Answer 2

I think that I've found my error, enumerator runs asynchronous, so it'll change while other things happen, so
When I try it
if (Input.GetKeyDown(KeyCode.C))
{
var allEnemies = enemiesInScene.Items.FindAll(enemy => true);
foreach (var enemy in allEnemies)
enemy.TakeDamage(1);
}
AllEnemies is already filled, so any change in enemiesInScene.Items don't affect allEnemies, this is why in other case we got errors

Answer 3

for (int i = list.Count - 1; i != -1; i--)
list.RemoveAt(i); // replace with useful code that can modify this list