//a new list to store items that are colliding with the bulletist<QT.IQuadTreeItem> collideItems = new List<QT.IQuadTreeItem>(10);            foreach (Actors.Projectile item in projectileItems)            {                collideItems.Clear();                //will put all quadtreeitems which collide with projectile item in list                quadTree.Query(item.BoundingBox, ref collideItems);                                                /////////////////////////////////                //This is the area which im trying to find all the zombie-only items that are in the collide list (the list includes the projectile                 //item) and it is fussing about not being able to convert the projectile to a zombie.                 foreach (Actors.Enemies.Zombie collideQTItem in collideItems)                {                    //here is code to deal the projectile damage to the zombie                }            }

foreach (var zombie in collideItems.OfType<Actors.Enemies.Zombie>()){    ...}