Refactor Item collections for thread safety and performance

Replaces static Item.ItemList and related collections with thread-safe data structures using ConcurrentDictionary and ImmutableHashSet. Adds thread-safe helpers for marking items for deconstruction and managing item lists. Updates all usages of Item.ItemList and DeconstructItems to use new APIs, improving performance and safety in multi-threaded contexts. Also refactors MeleeWeapon and Projectile impact queues to use ConcurrentQueue, and updates related logic throughout the codebase.

.gitignore

@@ -60,3 +60,5 @@ Deploy/DeployAll/PrivateKey.*
 #Rider
 *.DotSettings.user
 .vscode/settings.json
+.vscode/launch.json
+.vscode/tasks.json

Barotrauma/BarotraumaClient/ClientSource/GameSession/CrewManager.cs

@@ -867,7 +867,7 @@ namespace Barotrauma
                         {
                             foreach (var stackedItem in item.GetStackedItems())
                             {
-                                Item.DeconstructItems.Add(stackedItem);
+                                Item.MarkForDeconstruction(stackedItem);
                             }
                             HintManager.OnItemMarkedForDeconstruction(order.OrderGiver);
                         }
@@ -875,7 +875,7 @@ namespace Barotrauma
                         {
                             foreach (var stackedItem in item.GetStackedItems())
                             {
-                                Item.DeconstructItems.Remove(stackedItem);
+                                Item.UnmarkForDeconstruction(stackedItem);
                             }
                         }
                     }

Barotrauma/BarotraumaClient/ClientSource/Items/Inventory.cs

@@ -1891,7 +1891,7 @@ namespace Barotrauma
                        
                     }
                 }
-                else if (Item.DeconstructItems.Contains(item) &&
+                else if (Item.IsMarkedForDeconstruction(item) &&
                     OrderPrefab.Prefabs.TryGet(Tags.DeconstructThis, out OrderPrefab deconstructOrder))
                 {
                     DrawSideIcon(deconstructOrder.SymbolSprite, Direction.Right, TextManager.Get("tooltip.markedfordeconstruction"), GUIStyle.Red, out bool mouseOn);

Barotrauma/BarotraumaClient/ClientSource/Map/MapEntity.cs

@@ -471,11 +471,11 @@ namespace Barotrauma
 
                         if (item0 == null && item1 != null)
                         {
-                            item0 = Item.ItemList.Find(it => it.GetComponent<ConnectionPanel>()?.DisconnectedWires.Contains(wire) ?? false);
+                            item0 = Item.ItemList.FirstOrDefault(it => it.GetComponent<ConnectionPanel>()?.DisconnectedWires.Contains(wire) ?? false);
                         }
                         else if (item0 != null && item1 == null)
                         {
-                            item1 = Item.ItemList.Find(it => it.GetComponent<ConnectionPanel>()?.DisconnectedWires.Contains(wire) ?? false);
+                            item1 = Item.ItemList.FirstOrDefault(it => it.GetComponent<ConnectionPanel>()?.DisconnectedWires.Contains(wire) ?? false);
                         }
                         if (item0 != null && item1 != null && SelectedList.Contains(item0) && SelectedList.Contains(item1))
                         {

Barotrauma/BarotraumaServer/ServerSource/Networking/RespawnManager.cs

@@ -279,7 +279,7 @@ namespace Barotrauma.Networking
             var shuttleGaps = Gap.GapList.FindAll(g => RespawnShuttles.Contains(g.Submarine) && g.ConnectedWall != null);
             shuttleGaps.ForEach(g => Spawner.AddEntityToRemoveQueue(g));
 
-            var dockingPorts = Item.ItemList.FindAll(i => RespawnShuttles.Contains(i.Submarine) && i.GetComponent<DockingPort>() != null);
+            var dockingPorts = Item.ItemList.Where(i => RespawnShuttles.Contains(i.Submarine) && i.GetComponent<DockingPort>() != null).ToList();
             dockingPorts.ForEach(d => d.GetComponent<DockingPort>().Undock());
 
             if (!IsShuttleInsideLevel || DateTime.Now > teamSpecificState.DespawnTime)

Barotrauma/BarotraumaShared/SharedSource/Characters/AI/Objectives/AIObjectiveCleanupItem.cs

@@ -119,7 +119,7 @@ namespace Barotrauma
 
         protected override bool CheckObjectiveState()
         {
-            if (item.IgnoreByAI(character) || Item.DeconstructItems.Contains(item))
+            if (item.IgnoreByAI(character) || Item.IsMarkedForDeconstruction(item))
             {
                 Abandon = true;
             }

Barotrauma/BarotraumaShared/SharedSource/Characters/AI/Objectives/AIObjectiveCleanupItems.cs

@@ -114,7 +114,7 @@ namespace Barotrauma
                 if (!allowUnloading) { return false; }
                 if (requireValidContainer && !IsValidContainer(item.Container, character)) { return false; }
             }
-            if (ignoreItemsMarkedForDeconstruction && Item.DeconstructItems.Contains(item)) { return false; }
+            if (ignoreItemsMarkedForDeconstruction && Item.IsMarkedForDeconstruction(item)) { return false; }
             if (!item.HasAccess(character)) { return false; }
             if (character != null && !IsItemInsideValidSubmarine(item, character)) { return false; }
             if (item.HasBallastFloraInHull) { return false; }

Barotrauma/BarotraumaShared/SharedSource/Characters/AI/Order.cs

@@ -440,7 +440,7 @@ namespace Barotrauma
 
             if (Identifier == Tags.DeconstructThis && item.AllowDeconstruct)
             {
-                if (item.AllowDeconstruct && !Item.DeconstructItems.Contains(item) &&
+                if (item.AllowDeconstruct && !Item.IsMarkedForDeconstruction(item) &&
                     //only allow deconstructing if there are no deconstruction recipes (= deconstructing yields nothing), or deconstruction recipes that
                     (item.Prefab.DeconstructItems.None() ||
                     item.Prefab.DeconstructItems.Any(deconstructItem =>
@@ -454,7 +454,7 @@ namespace Barotrauma
             }
             else if (Identifier == Tags.DontDeconstructThis)
             {
-                if (Item.DeconstructItems.Contains(item)) { return true; }
+                if (Item.IsMarkedForDeconstruction(item)) { return true; }
             }
 
             ImmutableArray<Identifier> targetItems = GetTargetItems(option);

Barotrauma/BarotraumaShared/SharedSource/Characters/Character.cs

@@ -2761,10 +2761,11 @@ namespace Barotrauma
             }
             int itemsPerFrame = IsOnPlayerTeam ? 100 : 10;
             int checkedItemCount = 0;
-            for (int i = 0; i < itemsPerFrame && itemIndex < Item.ItemList.Count; i++, itemIndex++)
+            var cachedItems = Item.GetCachedItemList();
+            for (int i = 0; i < itemsPerFrame && itemIndex < cachedItems.Count; i++, itemIndex++)
             {
                 checkedItemCount++;
-                var item = Item.ItemList[itemIndex];
+                var item = cachedItems[itemIndex];
                 if (!item.IsInteractable(this)) { continue; }
                 if (ignoredItems != null && ignoredItems.Contains(item)) { continue; }
                 if (item.Submarine == null) { continue; }
@@ -2800,10 +2801,10 @@ namespace Barotrauma
                 }
             }
             targetItem = _foundItem;
-            bool completed = itemIndex >= Item.ItemList.Count - 1;
+            bool completed = itemIndex >= cachedItems.Count - 1;
             if (HumanAIController.DebugAI && checkedItemCount > 0 && targetItem != null && StopWatch.ElapsedMilliseconds > 1)
             {
-                var msg = $"Went through {checkedItemCount} of total {Item.ItemList.Count} items. Found item {targetItem.Name} in {StopWatch.ElapsedMilliseconds} ms. Completed: {completed}";
+                var msg = $"Went through {checkedItemCount} of total {cachedItems.Count} items. Found item {targetItem.Name} in {StopWatch.ElapsedMilliseconds} ms. Completed: {completed}";
                 if (StopWatch.ElapsedMilliseconds > 5)
                 {
                     DebugConsole.ThrowError(msg);

Barotrauma/BarotraumaShared/SharedSource/DebugConsole.cs

@@ -1478,7 +1478,7 @@ namespace Barotrauma
                     newItemName = args[2];
                 }
 
-                var oldItem = Item.ItemList.FindAll(it => it.Name == args[0]).ElementAtOrDefault(itemIndex);
+                var oldItem = Item.ItemList.Where(it => it.Name == args[0]).ElementAtOrDefault(itemIndex);
                 if (oldItem == null)
                 {
                     ThrowError($"Could not find an item with the name {args[0]} (index {itemIndex}).");
@@ -1852,7 +1852,7 @@ namespace Barotrauma
 
             commands.Add(new Command("power", "power: Immediately powers up the submarine's nuclear reactor.", (string[] args) =>
             {
-                Item reactorItem = Item.ItemList.Find(i => i.GetComponent<Reactor>() != null);
+                Item reactorItem = Item.ItemList.FirstOrDefault(i => i.GetComponent<Reactor>() != null);
                 if (reactorItem == null) { return; }
 
                 var reactor = reactorItem.GetComponent<Reactor>();

Barotrauma/BarotraumaShared/SharedSource/Events/MalfunctionEvent.cs

@@ -41,7 +41,7 @@ namespace Barotrauma
 
         protected override void InitEventSpecific(EventSet parentSet)
         {
-            var matchingItems = Item.ItemList.FindAll(i => i.Condition > 0.0f && targetItemIdentifiers.Contains(i.Prefab.Identifier));
+            var matchingItems = Item.ItemList.Where(i => i.Condition > 0.0f && targetItemIdentifiers.Contains(i.Prefab.Identifier)).ToList();
             int itemAmount = Rand.Range(minItemAmount, maxItemAmount, Rand.RandSync.ServerAndClient);
             for (int i = 0; i < itemAmount; i++)
             {

Barotrauma/BarotraumaShared/SharedSource/Events/Missions/AbandonedOutpostMission.cs

@@ -111,7 +111,7 @@ namespace Barotrauma
         {
             if (!itemTag.IsEmpty)
             {
-                var itemsToDestroy = Item.ItemList.FindAll(it => it.Submarine?.Info.Type != SubmarineType.Player && it.HasTag(itemTag));
+                var itemsToDestroy = Item.ItemList.Where(it => it.Submarine?.Info.Type != SubmarineType.Player && it.HasTag(itemTag)).ToList();
                 if (!itemsToDestroy.Any())
                 {
                     DebugConsole.ThrowError($"Error in mission \"{Prefab.Identifier}\". Could not find an item with the tag \"{itemTag}\".",

Barotrauma/BarotraumaShared/SharedSource/Events/Missions/EndMission.cs

@@ -181,7 +181,7 @@ namespace Barotrauma
                 return;
             }
             destructibleItems.Clear();
-            destructibleItems.AddRange(Item.ItemList.FindAll(it => it.HasTag(destructibleItemTag)));
+            destructibleItems.AddRange(Item.ItemList.Where(it => it.HasTag(destructibleItemTag)));
             if (destructibleItems.None())
             {
                 DebugConsole.ThrowError($"Error in end mission \"{Prefab.Identifier}\". Could not find any destructible items with the tag \"{spawnPointTag}\".",

Barotrauma/BarotraumaShared/SharedSource/GameSession/CrewManager.cs

@@ -128,7 +128,7 @@ namespace Barotrauma
                         {
                             foreach (var stackedItem in item.GetStackedItems())
                             {
-                                Item.DeconstructItems.Add(stackedItem);
+                                Item.MarkForDeconstruction(stackedItem);
                             }
 #if CLIENT
                             HintManager.OnItemMarkedForDeconstruction(order.OrderGiver);
@@ -138,7 +138,7 @@ namespace Barotrauma
                         {
                             foreach (var stackedItem in item.GetStackedItems())
                             {
-                                Item.DeconstructItems.Remove(stackedItem);
+                                Item.UnmarkForDeconstruction(stackedItem);
                             }
                         }
                     }

Barotrauma/BarotraumaShared/SharedSource/Items/Components/Holdable/MeleeWeapon.cs

@@ -3,6 +3,7 @@ using FarseerPhysics.Dynamics;
 using FarseerPhysics.Dynamics.Contacts;
 using Microsoft.Xna.Framework;
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -24,7 +25,7 @@ namespace Barotrauma.Items.Components
 
         private readonly HashSet<Entity> hitTargets = new HashSet<Entity>();
 
-        private readonly Queue<Fixture> impactQueue = new Queue<Fixture>();
+        private readonly ConcurrentQueue<Fixture> impactQueue = new ConcurrentQueue<Fixture>();
 
         public Character User { get; private set; }
 
@@ -190,17 +191,16 @@ namespace Barotrauma.Items.Components
         {
             if (!item.body.Enabled)
             {
-                impactQueue.Clear();
+                while (impactQueue.TryDequeue(out _)) { } // Clear queue
                 return;
             }
             if (picker == null || !picker.HeldItems.Contains(item))
             {
-                impactQueue.Clear();
+                while (impactQueue.TryDequeue(out _)) { } // Clear queue
                 IsActive = false;
             }
-            while (impactQueue.Count > 0)
+            while (impactQueue.TryDequeue(out var impact))
             {
-                var impact = impactQueue.Dequeue();
                 HandleImpact(impact);
             }
             //in case handling the impact does something to the picker
@@ -300,7 +300,7 @@ namespace Barotrauma.Items.Components
 
         private void RestoreCollision()
         {
-            impactQueue.Clear();
+            while (impactQueue.TryDequeue(out _)) { } // Clear queue
             item.body.FarseerBody.OnCollision -= OnCollision;
             item.body.CollisionCategories = Physics.CollisionItem;
             item.body.CollidesWith = Physics.DefaultItemCollidesWith;

Barotrauma/BarotraumaShared/SharedSource/Items/Components/Projectile.cs

@@ -5,6 +5,7 @@ using FarseerPhysics.Dynamics.Contacts;
 using FarseerPhysics.Dynamics.Joints;
 using Microsoft.Xna.Framework;
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -72,7 +73,7 @@ namespace Barotrauma.Items.Components
 
         public const float WaterDragCoefficient = 0.1f;
 
-        private readonly Queue<Impact> impactQueue = new Queue<Impact>();
+        private readonly ConcurrentQueue<Impact> impactQueue = new ConcurrentQueue<Impact>();
 
         private bool removePending;
 
@@ -840,9 +841,8 @@ namespace Barotrauma.Items.Components
                     DisableProjectileCollisions();
                 }
             }
-            while (impactQueue.Count > 0)
+            while (impactQueue.TryDequeue(out var impact))
             {
-                var impact = impactQueue.Dequeue();
                 HandleProjectileCollision(impact.Fixture, impact.Normal, impact.LinearVelocity);
             }
 

Barotrauma/BarotraumaShared/SharedSource/Items/Components/Signal/Wire.cs

@@ -723,11 +723,11 @@ namespace Barotrauma.Items.Components
 
             if (item0 == null && item1 != null)
             {
-                item0 = Item.ItemList.Find(it => it.GetComponent<ConnectionPanel>()?.DisconnectedWires.Contains(this) ?? false);
+                item0 = Item.ItemList.FirstOrDefault(it => it.GetComponent<ConnectionPanel>()?.DisconnectedWires.Contains(this) ?? false);
             }
             else if (item0 != null && item1 == null)
             {
-                item1 = Item.ItemList.Find(it => it.GetComponent<ConnectionPanel>()?.DisconnectedWires.Contains(this) ?? false);
+                item1 = Item.ItemList.FirstOrDefault(it => it.GetComponent<ConnectionPanel>()?.DisconnectedWires.Contains(this) ?? false);
             }
 
             if (item0 == null || item1 == null || nodes.Count == 0) { return; }

Barotrauma/BarotraumaShared/SharedSource/Items/Item.cs

@@ -14,7 +14,9 @@ using Barotrauma.Extensions;
 using Barotrauma.MapCreatures.Behavior;
 using MoonSharp.Interpreter;
 using System.Collections.Immutable;
+using System.Threading;
 using Barotrauma.Abilities;
+using HarmonyLib;
 
 #if CLIENT
 using Microsoft.Xna.Framework.Graphics;
@@ -27,57 +29,172 @@ namespace Barotrauma
         #region Lists
 
         /// <summary>
-        /// A list of every item that exists somewhere in the world. Note that there can be a huge number of items in the list, 
+        /// Thread-safe dictionary of all items by ID.
-        /// and you probably shouldn't be enumerating it to find some that match some specific criteria (unless that's done very, very sparsely or during initialization).
         /// </summary>
-        public static readonly List<Item> ItemList = new List<Item>();
+        private static readonly ConcurrentDictionary<ushort, Item> _itemDictionary = new ConcurrentDictionary<ushort, Item>();
 
-        private static readonly HashSet<Item> _dangerousItems = new HashSet<Item>();
+        /// <summary>
+        /// Provides thread-safe enumeration over all items.
+        /// </summary>
+        public static ICollection<Item> ItemList => _itemDictionary.Values;
+
+        /// <summary>
+        /// Thread-safe item lookup by ID.
+        /// </summary>
+        public static Item GetItemById(ushort id)
+        {
+            _itemDictionary.TryGetValue(id, out var item);
+            return item;
+        }
+
+        // Thread-safe optimized item collections using Immutable + atomic swap pattern
+        private static volatile ImmutableHashSet<Item> _dangerousItems = ImmutableHashSet<Item>.Empty;
+        private static volatile ImmutableHashSet<Item> _repairableItems = ImmutableHashSet<Item>.Empty;
+        private static volatile ImmutableHashSet<Item> _cleanableItems = ImmutableHashSet<Item>.Empty;
+        private static volatile ImmutableHashSet<Item> _sonarVisibleItems = ImmutableHashSet<Item>.Empty;
+        private static volatile ImmutableHashSet<Item> _turretTargetItems = ImmutableHashSet<Item>.Empty;
+        private static volatile ImmutableHashSet<Item> _chairItems = ImmutableHashSet<Item>.Empty;
+
+        // DeconstructItems uses ConcurrentDictionary to simulate a thread-safe HashSet
+        private static readonly ConcurrentDictionary<Item, byte> _deconstructItems = new ConcurrentDictionary<Item, byte>();
 
         public static IReadOnlyCollection<Item> DangerousItems => _dangerousItems;
 
-        private static readonly List<Item> _repairableItems = new List<Item>();
-
         /// <summary>
         /// Items that have one more more Repairable component
         /// </summary>
         public static IReadOnlyCollection<Item> RepairableItems => _repairableItems;
 
-        private static readonly List<Item> _cleanableItems = new List<Item>();
-
         /// <summary>
         /// Items that may potentially need to be cleaned up (pickable, not attached to a wall, and not inside a valid container)
         /// </summary>
         public static IReadOnlyCollection<Item> CleanableItems => _cleanableItems;
 
-        private static readonly HashSet<Item> _deconstructItems = new HashSet<Item>();
-
         /// <summary>
-        /// Items that have been marked for deconstruction
+        /// Items that have been marked for deconstruction. Thread-safe collection.
         /// </summary>
-        public static HashSet<Item> DeconstructItems => _deconstructItems;
+        public static ICollection<Item> DeconstructItems => _deconstructItems.Keys;
-
-        private static readonly List<Item> _sonarVisibleItems = new List<Item>();
 
         /// <summary>
         /// Items whose <see cref="ItemPrefab.SonarSize"/> is larger than 0
         /// </summary>
         public static IReadOnlyCollection<Item> SonarVisibleItems => _sonarVisibleItems;
 
-        private static readonly List<Item> _turretTargetItems = new List<Item>();
-
         /// <summary>
         /// Items whose <see cref="ItemPrefab.IsAITurretTarget"/> is true.
         /// </summary>
         public static IReadOnlyCollection<Item> TurretTargetItems => _turretTargetItems;
 
-        private static readonly List<Item> _chairItems = new List<Item>();
-
         /// <summary>
         /// Items that have the tag <see cref="Tags.ChairItem"/>. Which is an oddly specific thing, but useful as an optimization for NPC AI.
         /// </summary>
         public static IReadOnlyCollection<Item> ChairItems => _chairItems;
 
+        #region Thread-safe collection helpers
+
+        /// <summary>
+        /// Atomically adds an item to an immutable set using compare-and-swap.
+        /// </summary>
+        private static void AddToImmutableSet(ref ImmutableHashSet<Item> location, Item item)
+        {
+            ImmutableHashSet<Item> original, updated;
+            do
+            {
+                original = location;
+                updated = original.Add(item);
+                if (ReferenceEquals(original, updated)) return; // Already exists
+            }
+            while (Interlocked.CompareExchange(ref location, updated, original) != original);
+        }
+
+        /// <summary>
+        /// Atomically removes an item from an immutable set using compare-and-swap.
+        /// </summary>
+        private static void RemoveFromImmutableSet(ref ImmutableHashSet<Item> location, Item item)
+        {
+            ImmutableHashSet<Item> original, updated;
+            do
+            {
+                original = location;
+                updated = original.Remove(item);
+                if (ReferenceEquals(original, updated)) return; // Doesn't exist
+            }
+            while (Interlocked.CompareExchange(ref location, updated, original) != original);
+        }
+
+        /// <summary>
+        /// Marks an item for deconstruction (thread-safe).
+        /// </summary>
+        public static void MarkForDeconstruction(Item item)
+        {
+            _deconstructItems.TryAdd(item, 0);
+        }
+
+        /// <summary>
+        /// Unmarks an item for deconstruction (thread-safe).
+        /// </summary>
+        public static void UnmarkForDeconstruction(Item item)
+        {
+            _deconstructItems.TryRemove(item, out _);
+        }
+
+        /// <summary>
+        /// Checks if an item is marked for deconstruction (thread-safe).
+        /// </summary>
+        public static bool IsMarkedForDeconstruction(Item item)
+        {
+            return _deconstructItems.ContainsKey(item);
+        }
+
+        /// <summary>
+        /// Clears all item collections (thread-safe). Used during unloading.
+        /// </summary>
+        public static void ClearAllItemCollections()
+        {
+            _itemDictionary.Clear();
+            _dangerousItems = ImmutableHashSet<Item>.Empty;
+            _repairableItems = ImmutableHashSet<Item>.Empty;
+            _cleanableItems = ImmutableHashSet<Item>.Empty;
+            _sonarVisibleItems = ImmutableHashSet<Item>.Empty;
+            _turretTargetItems = ImmutableHashSet<Item>.Empty;
+            _chairItems = ImmutableHashSet<Item>.Empty;
+            _deconstructItems.Clear();
+            while (_pendingConditionUpdates.TryDequeue(out _)) { }
+            _cachedItemList = null;
+            _cachedItemListVersion = -1;
+        }
+
+        // Cached item list for indexed access (used by AI systems)
+        private static volatile List<Item> _cachedItemList;
+        private static volatile int _cachedItemListVersion = -1;
+        private static volatile int _itemListVersion;
+
+        /// <summary>
+        /// Gets a cached list snapshot of all items for indexed access.
+        /// The list is refreshed when items are added or removed.
+        /// Thread-safe but may return slightly stale data.
+        /// </summary>
+        public static List<Item> GetCachedItemList()
+        {
+            int currentVersion = _itemListVersion;
+            if (_cachedItemList == null || _cachedItemListVersion != currentVersion)
+            {
+                _cachedItemList = _itemDictionary.Values.ToList();
+                _cachedItemListVersion = currentVersion;
+            }
+            return _cachedItemList;
+        }
+
+        /// <summary>
+        /// Called when items are added or removed to invalidate the cached list.
+        /// </summary>
+        private static void InvalidateCachedItemList()
+        {
+            Interlocked.Increment(ref _itemListVersion);
+        }
+
+        #endregion
+
         #endregion
 
         public new ItemPrefab Prefab => base.Prefab as ItemPrefab;
@@ -179,7 +296,12 @@ namespace Barotrauma
 
         private bool transformDirty = true;
 
-        private static readonly List<Item> itemsWithPendingConditionUpdates = new List<Item>();
+        private static readonly ConcurrentQueue<Item> _pendingConditionUpdates = new ConcurrentQueue<Item>();
+
+        /// <summary>
+        /// Flag to avoid duplicate enqueue for pending condition updates.
+        /// </summary>
+        private volatile bool _hasPendingConditionUpdate;
 
         private float lastSentCondition;
         private float sendConditionUpdateTimer;
@@ -845,11 +967,11 @@ namespace Barotrauma
                 isDangerous = value; 
                 if (!value)
                 {
-                    _dangerousItems.Remove(this);
+                    RemoveFromImmutableSet(ref _dangerousItems, this);
                 }
                 else
                 {
-                    _dangerousItems.Add(this);
+                    AddToImmutableSet(ref _dangerousItems, this);
                 }
             }
         }
@@ -1398,12 +1520,13 @@ namespace Barotrauma
             }
 
             InsertToList();
-            ItemList.Add(this);
+            _itemDictionary.TryAdd(ID, this);
-            if (Prefab.IsDangerous) { _dangerousItems.Add(this); }
+            InvalidateCachedItemList();
-            if (Repairables.Any()) { _repairableItems.Add(this); }
+            if (Prefab.IsDangerous) { AddToImmutableSet(ref _dangerousItems, this); }
-            if (Prefab.SonarSize > 0.0f) { _sonarVisibleItems.Add(this); }
+            if (Repairables.Any()) { AddToImmutableSet(ref _repairableItems, this); }
-            if (Prefab.IsAITurretTarget) { _turretTargetItems.Add(this); }
+            if (Prefab.SonarSize > 0.0f) { AddToImmutableSet(ref _sonarVisibleItems, this); }
-            if (Prefab.Tags.Contains(Barotrauma.Tags.ChairItem)) { _chairItems.Add(this); }
+            if (Prefab.IsAITurretTarget) { AddToImmutableSet(ref _turretTargetItems, this); }
+            if (Prefab.Tags.Contains(Barotrauma.Tags.ChairItem)) { AddToImmutableSet(ref _chairItems, this); }
             CheckCleanable();
 
             DebugConsole.Log("Created " + Name + " (" + ID + ")");
@@ -1756,14 +1879,11 @@ namespace Barotrauma
                 Prefab.PreferredContainers.Any() &&
                 (container == null || container.HasTag(Barotrauma.Tags.AllowCleanup)))
             {
-                if (!_cleanableItems.Contains(this))
+                AddToImmutableSet(ref _cleanableItems, this);
-                {
-                    _cleanableItems.Add(this);
-                }
             }
             else
             {
-                _cleanableItems.Remove(this);
+                RemoveFromImmutableSet(ref _cleanableItems, this);
             }
         }
 
@@ -2294,9 +2414,10 @@ namespace Barotrauma
                 {
                     needsConditionUpdate = true;
                 }
-                if (needsConditionUpdate && !itemsWithPendingConditionUpdates.Contains(this))
+                if (needsConditionUpdate && !_hasPendingConditionUpdate)
                 {
-                    itemsWithPendingConditionUpdates.Add(this);
+                    _hasPendingConditionUpdate = true;
+                    _pendingConditionUpdates.Enqueue(this);
                 }
             }
 
@@ -2352,9 +2473,9 @@ namespace Barotrauma
         public void SendPendingNetworkUpdates()
         {
             if (!(GameMain.NetworkMember is { IsServer: true })) { return; }
-            if (!itemsWithPendingConditionUpdates.Contains(this)) { return; }
+            if (!_hasPendingConditionUpdate) { return; }
             SendPendingNetworkUpdatesInternal();
-            itemsWithPendingConditionUpdates.Remove(this);
+            _hasPendingConditionUpdate = false;
         }
 
         private void SendPendingNetworkUpdatesInternal()
@@ -2383,21 +2504,35 @@ namespace Barotrauma
         public static void UpdatePendingConditionUpdates(float deltaTime)
         {
             if (GameMain.NetworkMember is not { IsServer: true }) { return; }
-            for (int i = 0; i < itemsWithPendingConditionUpdates.Count; i++)
+            
+            int count = _pendingConditionUpdates.Count;
+            for (int i = 0; i < count; i++)
             {
-                var item = itemsWithPendingConditionUpdates[i];
+                if (!_pendingConditionUpdates.TryDequeue(out var item)) { break; }
+                
                 if (item == null || item.Removed)
                 {
-                    itemsWithPendingConditionUpdates.RemoveAt(i--);
+                    item._hasPendingConditionUpdate = false;
+                    continue;
+                }
+                
+                if (item.Submarine is { Loading: true })
+                {
+                    // Re-enqueue, still loading
+                    _pendingConditionUpdates.Enqueue(item);
                     continue;
                 }
-                if (item.Submarine is { Loading: true }) { continue; }
 
                 item.sendConditionUpdateTimer -= deltaTime;
                 if (item.sendConditionUpdateTimer <= 0.0f)
                 {
                     item.SendPendingNetworkUpdatesInternal();
-                    itemsWithPendingConditionUpdates.RemoveAt(i--);
+                    item._hasPendingConditionUpdate = false;
+                }
+                else
+                {
+                    // Not ready yet, re-enqueue
+                    _pendingConditionUpdates.Enqueue(item);
                 }
             }
         }
@@ -4217,7 +4352,7 @@ namespace Barotrauma
                 }
             }
 
-            if (element.GetAttributeBool("markedfordeconstruction", false)) { _deconstructItems.Add(item); }
+            if (element.GetAttributeBool("markedfordeconstruction", false)) { _deconstructItems.TryAdd(item, 0); }
 
             float prevRotation = item.Rotation;
             if (element.GetAttributeBool("flippedx", false)) { item.FlipX(relativeToSub: false, force: true); }
@@ -4510,7 +4645,7 @@ namespace Barotrauma
                 new XAttribute("name", Prefab.OriginalName),
                 new XAttribute("identifier", Prefab.Identifier),
                 new XAttribute("ID", ID),
-                new XAttribute("markedfordeconstruction", _deconstructItems.Contains(this)));
+                new XAttribute("markedfordeconstruction", _deconstructItems.ContainsKey(this)));
 
             if (PendingItemSwap != null)
             {
@@ -4713,14 +4848,16 @@ namespace Barotrauma
 
         private void RemoveFromLists()
         {
-            ItemList.Remove(this);
+            _itemDictionary.TryRemove(ID, out _);
-            _dangerousItems.Remove(this);
+            InvalidateCachedItemList();
-            _repairableItems.Remove(this);
+            RemoveFromImmutableSet(ref _dangerousItems, this);
-            _sonarVisibleItems.Remove(this);
+            RemoveFromImmutableSet(ref _repairableItems, this);
-            _cleanableItems.Remove(this);
+            RemoveFromImmutableSet(ref _sonarVisibleItems, this);
-            _deconstructItems.Remove(this);
+            RemoveFromImmutableSet(ref _cleanableItems, this);
-            _turretTargetItems.Remove(this);
+            _deconstructItems.TryRemove(this, out _);
-            _chairItems.Remove(this);
+            RemoveFromImmutableSet(ref _turretTargetItems, this);
+            RemoveFromImmutableSet(ref _chairItems, this);
+            _hasPendingConditionUpdate = false;
             RemoveFromDroppedStack(allowClientExecute: true);
         }
 

Barotrauma/BarotraumaShared/SharedSource/Map/Entity.cs

@@ -261,7 +261,7 @@ namespace Barotrauma
                         DebugConsole.ThrowError($"Error while removing item \"{item}\"", exception);
                     }
                 }
-                Item.ItemList.Clear();
+                Item.ClearAllItemCollections();
             }
             if (Character.CharacterList.Count > 0)
             {

Barotrauma/BarotraumaShared/SharedSource/Map/Levels/Level.cs

@@ -4775,7 +4775,7 @@ namespace Barotrauma
             //     BeaconStation.FlipX();
             // }
 
-            Item sonarItem = Item.ItemList.Find(it => it.Submarine == BeaconStation && it.GetComponent<Sonar>() != null);
+            Item sonarItem = Item.ItemList.FirstOrDefault(it => it.Submarine == BeaconStation && it.GetComponent<Sonar>() != null);
             if (sonarItem == null)
             {
                 DebugConsole.ThrowError($"No sonar found in the beacon station \"{beaconStationName}\"!");
@@ -4794,7 +4794,7 @@ namespace Barotrauma
                 throw new InvalidOperationException("Failed to prepare beacon station (no beacon station in the level).");
             }
 
-            List<Item> beaconItems = Item.ItemList.FindAll(it => it.Submarine == BeaconStation);
+            List<Item> beaconItems = Item.ItemList.Where(it => it.Submarine == BeaconStation).ToList();
 
             Item reactorItem = beaconItems.Find(it => it.GetComponent<Reactor>() != null);
             Reactor reactorComponent = null;
@@ -4840,7 +4840,7 @@ namespace Barotrauma
             if (BeaconStation?.Info?.BeaconStationInfo is { AllowDisconnectedWires: false }) { return; }
 
             if (disconnectWireProbability <= 0.0f) { return; }
-            List<Item> beaconItems = Item.ItemList.FindAll(it => it.Submarine == BeaconStation);
+            List<Item> beaconItems = Item.ItemList.Where(it => it.Submarine == BeaconStation).ToList();
             foreach (Item item in beaconItems.Where(it => it.GetComponent<Wire>() != null).ToList())
             {
                 if (item.NonInteractable || item.InvulnerableToDamage) { continue; }
@@ -4878,7 +4878,7 @@ namespace Barotrauma
 
             if (breakDeviceProbability <= 0.0f) { return; }
             //break powered items
-            List<Item> beaconItems = Item.ItemList.FindAll(it => it.Submarine == BeaconStation);
+            List<Item> beaconItems = Item.ItemList.Where(it => it.Submarine == BeaconStation).ToList();
             foreach (Item item in beaconItems.Where(it => it.Components.Any(c => c is Powered) && it.Components.Any(c => c is Repairable)))
             {
                 if (item.NonInteractable || item.InvulnerableToDamage) { continue; }

Barotrauma/BarotraumaShared/SharedSource/Map/Map/Location.cs

@@ -1371,7 +1371,7 @@ namespace Barotrauma
         {
             foreach (TakenItem takenItem in takenItems)
             {
-                Item item = Item.ItemList.Find(it => takenItem.Matches(it));
+                Item item = Item.ItemList.FirstOrDefault(it => takenItem.Matches(it));
                 item?.Remove();
             }
         }

Barotrauma/BarotraumaShared/SharedSource/Map/Submarine.cs

@@ -1077,7 +1077,7 @@ namespace Barotrauma
 
             Item.UpdateHulls();
 
-            List<Item> bodyItems = Item.ItemList.FindAll(it => it.Submarine == this && it.body != null);
+            List<Item> bodyItems = Item.ItemList.Where(it => it.Submarine == this && it.body != null).ToList();
             List<MapEntity> subEntities = MapEntity.MapEntityList.FindAll(me => me.Submarine == this);
 
             foreach (MapEntity e in subEntities)
@@ -1507,7 +1507,7 @@ namespace Barotrauma
 
         public List<Hull> GetHulls(bool alsoFromConnectedSubs) => GetEntities(alsoFromConnectedSubs, Hull.HullList);
         public List<Gap> GetGaps(bool alsoFromConnectedSubs) => GetEntities(alsoFromConnectedSubs, Gap.GapList);
-        public List<Item> GetItems(bool alsoFromConnectedSubs) => GetEntities(alsoFromConnectedSubs, Item.ItemList);
+        public List<Item> GetItems(bool alsoFromConnectedSubs) => GetEntities(alsoFromConnectedSubs, Item.ItemList).ToList();
         public List<WayPoint> GetWaypoints(bool alsoFromConnectedSubs) => GetEntities(alsoFromConnectedSubs, WayPoint.WayPointList);
         public List<Structure> GetWalls(bool alsoFromConnectedSubs) => GetEntities(alsoFromConnectedSubs, Structure.WallList);
 
@@ -2148,7 +2148,7 @@ namespace Barotrauma
                             DebugConsole.ThrowError("Error while removing \"" + item.Name + "\"!", e);
                         }
                     }
-                    Item.ItemList.Clear();
+                    Item.ClearAllItemCollections();
                 }
 
                 Ragdoll.RemoveAll();