Merge branch 'develop' into stable

18 files changed, 90 insertions, 78 deletions

diff --git a/src/SMAPI/Framework/StateTracking/ChestTracker.cs b/src/SMAPI/Framework/StateTracking/ChestTracker.cs
index 65f58ee7..c33a7498 100644
--- a/src/SMAPI/Framework/StateTracking/ChestTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/ChestTracker.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using StardewModdingAPI.Framework.StateTracking.Comparers;
 using StardewModdingAPI.Framework.StateTracking.FieldWatchers;
@@ -18,10 +19,10 @@ namespace StardewModdingAPI.Framework.StateTracking
         private readonly IDictionary<Item, int> StackSizes;
 
         /// <summary>Items added since the last update.</summary>
-        private readonly HashSet<Item> Added = new HashSet<Item>(new ObjectReferenceComparer<Item>());
+        private readonly HashSet<Item> Added = new(new ObjectReferenceComparer<Item>());
 
         /// <summary>Items removed since the last update.</summary>
-        private readonly HashSet<Item> Removed = new HashSet<Item>(new ObjectReferenceComparer<Item>());
+        private readonly HashSet<Item> Removed = new(new ObjectReferenceComparer<Item>());
 
         /// <summary>The underlying inventory watcher.</summary>
         private readonly ICollectionWatcher<Item> InventoryWatcher;
@@ -84,7 +85,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         /// <summary>Get the inventory changes since the last update, if anything changed.</summary>
         /// <param name="changes">The inventory changes, or <c>null</c> if nothing changed.</param>
         /// <returns>Returns whether anything changed.</returns>
-        public bool TryGetInventoryChanges(out SnapshotItemListDiff changes)
+        public bool TryGetInventoryChanges([NotNullWhen(true)] out SnapshotItemListDiff? changes)
         {
             return SnapshotItemListDiff.TryGetChanges(added: this.Added, removed: this.Removed, stackSizes: this.StackSizes, out changes);
         }
diff --git a/src/SMAPI/Framework/StateTracking/Comparers/EquatableComparer.cs b/src/SMAPI/Framework/StateTracking/Comparers/EquatableComparer.cs
index a96ffdb6..9d8559b4 100644
--- a/src/SMAPI/Framework/StateTracking/Comparers/EquatableComparer.cs
+++ b/src/SMAPI/Framework/StateTracking/Comparers/EquatableComparer.cs
@@ -15,7 +15,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Comparers
         /// <returns>true if the specified objects are equal; otherwise, false.</returns>
         /// <param name="x">The first object to compare.</param>
         /// <param name="y">The second object to compare.</param>
-        public bool Equals(T x, T y)
+        public bool Equals(T? x, T? y)
         {
             if (x == null)
                 return y == null;
diff --git a/src/SMAPI/Framework/StateTracking/Comparers/GenericEqualsComparer.cs b/src/SMAPI/Framework/StateTracking/Comparers/GenericEqualsComparer.cs
index cc1d6553..41b17e10 100644
--- a/src/SMAPI/Framework/StateTracking/Comparers/GenericEqualsComparer.cs
+++ b/src/SMAPI/Framework/StateTracking/Comparers/GenericEqualsComparer.cs
@@ -14,7 +14,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Comparers
         /// <returns>true if the specified objects are equal; otherwise, false.</returns>
         /// <param name="x">The first object to compare.</param>
         /// <param name="y">The second object to compare.</param>
-        public bool Equals(T x, T y)
+        public bool Equals(T? x, T? y)
         {
             if (x == null)
                 return y == null;
diff --git a/src/SMAPI/Framework/StateTracking/Comparers/ObjectReferenceComparer.cs b/src/SMAPI/Framework/StateTracking/Comparers/ObjectReferenceComparer.cs
index ef9adafb..e6ece854 100644
--- a/src/SMAPI/Framework/StateTracking/Comparers/ObjectReferenceComparer.cs
+++ b/src/SMAPI/Framework/StateTracking/Comparers/ObjectReferenceComparer.cs
@@ -14,7 +14,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Comparers
         /// <returns>true if the specified objects are equal; otherwise, false.</returns>
         /// <param name="x">The first object to compare.</param>
         /// <param name="y">The second object to compare.</param>
-        public bool Equals(T x, T y)
+        public bool Equals(T? x, T? y)
         {
             return object.ReferenceEquals(x, y);
         }
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableListWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableListWatcher.cs
index 32ec8c7e..256370ce 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableListWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableListWatcher.cs
@@ -17,10 +17,10 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         private HashSet<TValue> LastValues;
 
         /// <summary>The pairs added since the last reset.</summary>
-        private readonly List<TValue> AddedImpl = new List<TValue>();
+        private readonly List<TValue> AddedImpl = new();
 
         /// <summary>The pairs removed since the last reset.</summary>
-        private readonly List<TValue> RemovedImpl = new List<TValue>();
+        private readonly List<TValue> RemovedImpl = new();
 
 
         /*********
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableWatcher.cs
index 5ca4b9f4..5f76fe0a 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableWatcher.cs
@@ -40,7 +40,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         {
             this.GetValue = getValue;
             this.Comparer = comparer;
-            this.PreviousValue = getValue();
+            this.CurrentValue = getValue();
+            this.PreviousValue = this.CurrentValue;
         }
 
         /// <summary>Update the current value if needed.</summary>
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/ImmutableCollectionWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/ImmutableCollectionWatcher.cs
index 30e6274f..84340fbf 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/ImmutableCollectionWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/ImmutableCollectionWatcher.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 
 namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
@@ -10,16 +11,16 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         ** Accessors
         *********/
         /// <summary>A singleton collection watcher instance.</summary>
-        public static ImmutableCollectionWatcher<TValue> Instance { get; } = new ImmutableCollectionWatcher<TValue>();
+        public static ImmutableCollectionWatcher<TValue> Instance { get; } = new();
 
         /// <summary>Whether the collection changed since the last reset.</summary>
         public bool IsChanged { get; } = false;
 
         /// <summary>The values added since the last reset.</summary>
-        public IEnumerable<TValue> Added { get; } = new TValue[0];
+        public IEnumerable<TValue> Added { get; } = Array.Empty<TValue>();
 
         /// <summary>The values removed since the last reset.</summary>
-        public IEnumerable<TValue> Removed { get; } = new TValue[0];
+        public IEnumerable<TValue> Removed { get; } = Array.Empty<TValue>();
 
 
         /*********
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetCollectionWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetCollectionWatcher.cs
index 21e84c47..676c9fb4 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetCollectionWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetCollectionWatcher.cs
@@ -15,10 +15,10 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         private readonly NetCollection<TValue> Field;
 
         /// <summary>The pairs added since the last reset.</summary>
-        private readonly List<TValue> AddedImpl = new List<TValue>();
+        private readonly List<TValue> AddedImpl = new();
 
         /// <summary>The pairs removed since the last reset.</summary>
-        private readonly List<TValue> RemovedImpl = new List<TValue>();
+        private readonly List<TValue> RemovedImpl = new();
 
 
         /*********
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs
index e6882f7e..f55e4cea 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs
@@ -10,6 +10,7 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
     /// <typeparam name="TSerialDict">The serializable dictionary type that can store the keys and values.</typeparam>
     /// <typeparam name="TSelf">The net field instance type.</typeparam>
     internal class NetDictionaryWatcher<TKey, TValue, TField, TSerialDict, TSelf> : BaseDisposableWatcher, IDictionaryWatcher<TKey, TValue>
+        where TKey : notnull
         where TField : class, INetObject<INetSerializable>, new()
         where TSerialDict : IDictionary<TKey, TValue>, new()
         where TSelf : NetDictionary<TKey, TValue, TField, TSerialDict, TSelf>
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/ObservableCollectionWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/ObservableCollectionWatcher.cs
index c29d2783..97aedca8 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/ObservableCollectionWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/ObservableCollectionWatcher.cs
@@ -16,13 +16,13 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         private readonly ObservableCollection<TValue> Field;
 
         /// <summary>The pairs added since the last reset.</summary>
-        private readonly List<TValue> AddedImpl = new List<TValue>();
+        private readonly List<TValue> AddedImpl = new();
 
         /// <summary>The pairs removed since the last reset.</summary>
-        private readonly List<TValue> RemovedImpl = new List<TValue>();
+        private readonly List<TValue> RemovedImpl = new();
 
         /// <summary>The previous values as of the last update.</summary>
-        private readonly List<TValue> PreviousValues = new List<TValue>();
+        private readonly List<TValue> PreviousValues = new();
 
 
         /*********
@@ -79,7 +79,7 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>A callback invoked when an entry is added or removed from the collection.</summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event arguments.</param>
-        private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (e.Action == NotifyCollectionChangedAction.Reset)
             {
@@ -88,8 +88,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
             }
             else
             {
-                TValue[] added = e.NewItems?.Cast<TValue>().ToArray();
-                TValue[] removed = e.OldItems?.Cast<TValue>().ToArray();
+                TValue[]? added = e.NewItems?.Cast<TValue>().ToArray();
+                TValue[]? removed = e.OldItems?.Cast<TValue>().ToArray();
 
                 if (removed != null)
                 {
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
index bde43486..c4a4d0b9 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
@@ -18,7 +18,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>Get a watcher which compares values using their <see cref="object.Equals(object)"/> method. This method should only be used when <see cref="ForEquatable{T}"/> won't work, since this doesn't validate whether they're comparable.</summary>
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="getValue">Get the current value.</param>
-        public static IValueWatcher<T> ForGenericEquality<T>(Func<T> getValue) where T : struct
+        public static IValueWatcher<T> ForGenericEquality<T>(Func<T> getValue)
+            where T : struct
         {
             return new ComparableWatcher<T>(getValue, new GenericEqualsComparer<T>());
         }
@@ -26,7 +27,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>Get a watcher for an <see cref="IEquatable{T}"/> value.</summary>
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="getValue">Get the current value.</param>
-        public static IValueWatcher<T> ForEquatable<T>(Func<T> getValue) where T : IEquatable<T>
+        public static IValueWatcher<T> ForEquatable<T>(Func<T> getValue)
+            where T : IEquatable<T>
         {
             return new ComparableWatcher<T>(getValue, new EquatableComparer<T>());
         }
@@ -77,7 +79,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>Get a watcher for a net collection.</summary>
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="collection">The net collection.</param>
-        public static ICollectionWatcher<T> ForNetCollection<T>(NetCollection<T> collection) where T : class, INetObject<INetSerializable>
+        public static ICollectionWatcher<T> ForNetCollection<T>(NetCollection<T> collection)
+            where T : class, INetObject<INetSerializable>
         {
             return new NetCollectionWatcher<T>(collection);
         }
@@ -85,7 +88,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>Get a watcher for a net list.</summary>
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="collection">The net list.</param>
-        public static ICollectionWatcher<T> ForNetList<T>(NetList<T, NetRef<T>> collection) where T : class, INetObject<INetSerializable>
+        public static ICollectionWatcher<T> ForNetList<T>(NetList<T, NetRef<T>> collection)
+            where T : class, INetObject<INetSerializable>
         {
             return new NetListWatcher<T>(collection);
         }
@@ -98,6 +102,7 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <typeparam name="TSelf">The net field instance type.</typeparam>
         /// <param name="field">The net field.</param>
         public static NetDictionaryWatcher<TKey, TValue, TField, TSerialDict, TSelf> ForNetDictionary<TKey, TValue, TField, TSerialDict, TSelf>(NetDictionary<TKey, TValue, TField, TSerialDict, TSelf> field)
+            where TKey : notnull
             where TField : class, INetObject<INetSerializable>, new()
             where TSerialDict : IDictionary<TKey, TValue>, new()
             where TSelf : NetDictionary<TKey, TValue, TField, TSerialDict, TSelf>
diff --git a/src/SMAPI/Framework/StateTracking/LocationTracker.cs b/src/SMAPI/Framework/StateTracking/LocationTracker.cs
index 6d3a62bb..ff72a19b 100644
--- a/src/SMAPI/Framework/StateTracking/LocationTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/LocationTracker.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using Microsoft.Xna.Framework;
@@ -18,7 +19,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         ** Fields
         *********/
         /// <summary>The underlying watchers.</summary>
-        private readonly List<IWatcher> Watchers = new List<IWatcher>();
+        private readonly List<IWatcher> Watchers = new();
 
 
         /*********
@@ -84,7 +85,7 @@ namespace StardewModdingAPI.Framework.StateTracking
                 this.FurnitureWatcher
             });
 
-            this.UpdateChestWatcherList(added: location.Objects.Pairs, removed: new KeyValuePair<Vector2, SObject>[0]);
+            this.UpdateChestWatcherList(added: location.Objects.Pairs, removed: Array.Empty<KeyValuePair<Vector2, SObject>>());
         }
 
         /// <summary>Update the current value if needed.</summary>
@@ -129,20 +130,20 @@ namespace StardewModdingAPI.Framework.StateTracking
         private void UpdateChestWatcherList(IEnumerable<KeyValuePair<Vector2, SObject>> added, IEnumerable<KeyValuePair<Vector2, SObject>> removed)
         {
             // remove unused watchers
-            foreach (KeyValuePair<Vector2, SObject> pair in removed)
+            foreach ((Vector2 tile, SObject? obj) in removed)
             {
-                if (pair.Value is Chest && this.ChestWatchers.TryGetValue(pair.Key, out ChestTracker watcher))
+                if (obj is Chest && this.ChestWatchers.TryGetValue(tile, out ChestTracker? watcher))
                 {
                     watcher.Dispose();
-                    this.ChestWatchers.Remove(pair.Key);
+                    this.ChestWatchers.Remove(tile);
                 }
             }
 
             // add new watchers
-            foreach (KeyValuePair<Vector2, SObject> pair in added)
+            foreach ((Vector2 tile, SObject? obj) in added)
             {
-                if (pair.Value is Chest chest && !this.ChestWatchers.ContainsKey(pair.Key))
-                    this.ChestWatchers.Add(pair.Key, new ChestTracker(chest));
+                if (obj is Chest chest && !this.ChestWatchers.ContainsKey(tile))
+                    this.ChestWatchers.Add(tile, new ChestTracker(chest));
             }
         }
     }
diff --git a/src/SMAPI/Framework/StateTracking/PlayerTracker.cs b/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
index cf49a7c1..5433ac8e 100644
--- a/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using StardewModdingAPI.Enums;
 using StardewModdingAPI.Framework.StateTracking.Comparers;
@@ -21,10 +22,10 @@ namespace StardewModdingAPI.Framework.StateTracking
         private IDictionary<Item, int> CurrentInventory;
 
         /// <summary>The player's last valid location.</summary>
-        private GameLocation LastValidLocation;
+        private GameLocation? LastValidLocation;
 
         /// <summary>The underlying watchers.</summary>
-        private readonly List<IWatcher> Watchers = new List<IWatcher>();
+        private readonly List<IWatcher> Watchers = new();
 
 
         /*********
@@ -34,7 +35,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         public Farmer Player { get; }
 
         /// <summary>The player's current location.</summary>
-        public IValueWatcher<GameLocation> LocationWatcher { get; }
+        public IValueWatcher<GameLocation?> LocationWatcher { get; }
 
         /// <summary>Tracks changes to the player's skill levels.</summary>
         public IDictionary<SkillType, IValueWatcher<int>> SkillWatchers { get; }
@@ -49,7 +50,8 @@ namespace StardewModdingAPI.Framework.StateTracking
         {
             // init player data
             this.Player = player;
-            this.PreviousInventory = this.GetInventory();
+            this.CurrentInventory = this.GetInventory();
+            this.PreviousInventory = new Dictionary<Item, int>(this.CurrentInventory);
 
             // init trackers
             this.LocationWatcher = WatcherFactory.ForReference(this.GetCurrentLocation);
@@ -93,7 +95,7 @@ namespace StardewModdingAPI.Framework.StateTracking
 
         /// <summary>Get the player's current location, ignoring temporary null values.</summary>
         /// <remarks>The game will set <see cref="Character.currentLocation"/> to null in some cases, e.g. when they're a secondary player in multiplayer and transition to a location that hasn't been synced yet. While that's happening, this returns the player's last valid location instead.</remarks>
-        public GameLocation GetCurrentLocation()
+        public GameLocation? GetCurrentLocation()
         {
             return this.Player.currentLocation ?? this.LastValidLocation;
         }
@@ -101,7 +103,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         /// <summary>Get the inventory changes since the last update, if anything changed.</summary>
         /// <param name="changes">The inventory changes, or <c>null</c> if nothing changed.</param>
         /// <returns>Returns whether anything changed.</returns>
-        public bool TryGetInventoryChanges(out SnapshotItemListDiff changes)
+        public bool TryGetInventoryChanges([NotNullWhen(true)] out SnapshotItemListDiff? changes)
         {
             IDictionary<Item, int> current = this.GetInventory();
 
@@ -122,7 +124,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         public void Dispose()
         {
             this.PreviousInventory.Clear();
-            this.CurrentInventory?.Clear();
+            this.CurrentInventory.Clear();
 
             foreach (IWatcher watcher in this.Watchers)
                 watcher.Dispose();
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
index 6c9cc4f5..0d0469d7 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
@@ -17,25 +17,25 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         public GameLocation Location { get; }
 
         /// <summary>Tracks added or removed buildings.</summary>
-        public SnapshotListDiff<Building> Buildings { get; } = new SnapshotListDiff<Building>();
+        public SnapshotListDiff<Building> Buildings { get; } = new();
 
         /// <summary>Tracks added or removed debris.</summary>
-        public SnapshotListDiff<Debris> Debris { get; } = new SnapshotListDiff<Debris>();
+        public SnapshotListDiff<Debris> Debris { get; } = new();
 
         /// <summary>Tracks added or removed large terrain features.</summary>
-        public SnapshotListDiff<LargeTerrainFeature> LargeTerrainFeatures { get; } = new SnapshotListDiff<LargeTerrainFeature>();
+        public SnapshotListDiff<LargeTerrainFeature> LargeTerrainFeatures { get; } = new();
 
         /// <summary>Tracks added or removed NPCs.</summary>
-        public SnapshotListDiff<NPC> Npcs { get; } = new SnapshotListDiff<NPC>();
+        public SnapshotListDiff<NPC> Npcs { get; } = new();
 
         /// <summary>Tracks added or removed objects.</summary>
-        public SnapshotListDiff<KeyValuePair<Vector2, Object>> Objects { get; } = new SnapshotListDiff<KeyValuePair<Vector2, Object>>();
+        public SnapshotListDiff<KeyValuePair<Vector2, Object>> Objects { get; } = new();
 
         /// <summary>Tracks added or removed terrain features.</summary>
-        public SnapshotListDiff<KeyValuePair<Vector2, TerrainFeature>> TerrainFeatures { get; } = new SnapshotListDiff<KeyValuePair<Vector2, TerrainFeature>>();
+        public SnapshotListDiff<KeyValuePair<Vector2, TerrainFeature>> TerrainFeatures { get; } = new();
 
         /// <summary>Tracks added or removed furniture.</summary>
-        public SnapshotListDiff<Furniture> Furniture { get; } = new SnapshotListDiff<Furniture>();
+        public SnapshotListDiff<Furniture> Furniture { get; } = new();
 
         /// <summary>Tracks changed chest inventories.</summary>
         public IDictionary<Chest, SnapshotItemListDiff> ChestItems { get; } = new Dictionary<Chest, SnapshotItemListDiff>();
@@ -68,7 +68,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
             this.ChestItems.Clear();
             foreach (ChestTracker tracker in watcher.ChestWatchers.Values)
             {
-                if (tracker.TryGetInventoryChanges(out SnapshotItemListDiff changes))
+                if (tracker.TryGetInventoryChanges(out SnapshotItemListDiff? changes))
                     this.ChestItems[tracker.Chest] = changes;
             }
         }
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
index 0908b02a..6a24ec30 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
@@ -14,7 +14,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         ** Fields
         *********/
         /// <summary>An empty item list diff.</summary>
-        private readonly SnapshotItemListDiff EmptyItemListDiff = new SnapshotItemListDiff(new Item[0], new Item[0], new ItemStackSizeChange[0]);
+        private readonly SnapshotItemListDiff EmptyItemListDiff = new(Array.Empty<Item>(), Array.Empty<Item>(), Array.Empty<ItemStackSizeChange>());
 
 
         /*********
@@ -24,14 +24,14 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         public Farmer Player { get; }
 
         /// <summary>The player's current location.</summary>
-        public SnapshotDiff<GameLocation> Location { get; } = new SnapshotDiff<GameLocation>();
+        public SnapshotDiff<GameLocation> Location { get; } = new();
 
         /// <summary>Tracks changes to the player's skill levels.</summary>
         public IDictionary<SkillType, SnapshotDiff<int>> Skills { get; } =
             Enum
                 .GetValues(typeof(SkillType))
                 .Cast<SkillType>()
-                .ToDictionary(skill => skill, skill => new SnapshotDiff<int>());
+                .ToDictionary(skill => skill, _ => new SnapshotDiff<int>());
 
         /// <summary>Get a list of inventory changes.</summary>
         public SnapshotItemListDiff Inventory { get; private set; }
@@ -45,17 +45,18 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         public PlayerSnapshot(Farmer player)
         {
             this.Player = player;
+            this.Inventory = this.EmptyItemListDiff;
         }
 
         /// <summary>Update the tracked values.</summary>
         /// <param name="watcher">The player watcher to snapshot.</param>
         public void Update(PlayerTracker watcher)
         {
-            this.Location.Update(watcher.LocationWatcher);
-            foreach (var pair in this.Skills)
-                pair.Value.Update(watcher.SkillWatchers[pair.Key]);
+            this.Location.Update(watcher.LocationWatcher!);
+            foreach ((SkillType skill, var value) in this.Skills)
+                value.Update(watcher.SkillWatchers[skill]);
 
-            this.Inventory = watcher.TryGetInventoryChanges(out SnapshotItemListDiff itemChanges)
+            this.Inventory = watcher.TryGetInventoryChanges(out SnapshotItemListDiff? itemChanges)
                 ? itemChanges
                 : this.EmptyItemListDiff;
         }
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/WatcherSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/WatcherSnapshot.cs
index cf51e040..27a891de 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/WatcherSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/WatcherSnapshot.cs
@@ -11,31 +11,31 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         ** Accessors
         *********/
         /// <summary>Tracks changes to the window size.</summary>
-        public SnapshotDiff<Point> WindowSize { get; } = new SnapshotDiff<Point>();
+        public SnapshotDiff<Point> WindowSize { get; } = new();
 
         /// <summary>Tracks changes to the current player.</summary>
-        public PlayerSnapshot CurrentPlayer { get; private set; }
+        public PlayerSnapshot? CurrentPlayer { get; private set; }
 
         /// <summary>Tracks changes to the time of day (in 24-hour military format).</summary>
-        public SnapshotDiff<int> Time { get; } = new SnapshotDiff<int>();
+        public SnapshotDiff<int> Time { get; } = new();
 
         /// <summary>Tracks changes to the save ID.</summary>
-        public SnapshotDiff<ulong> SaveID { get; } = new SnapshotDiff<ulong>();
+        public SnapshotDiff<ulong> SaveID { get; } = new();
 
         /// <summary>Tracks changes to the game's locations.</summary>
-        public WorldLocationsSnapshot Locations { get; } = new WorldLocationsSnapshot();
+        public WorldLocationsSnapshot Locations { get; } = new();
 
         /// <summary>Tracks changes to <see cref="Game1.activeClickableMenu"/>.</summary>
-        public SnapshotDiff<IClickableMenu> ActiveMenu { get; } = new SnapshotDiff<IClickableMenu>();
+        public SnapshotDiff<IClickableMenu> ActiveMenu { get; } = new();
 
         /// <summary>Tracks changes to the cursor position.</summary>
-        public SnapshotDiff<ICursorPosition> Cursor { get; } = new SnapshotDiff<ICursorPosition>();
+        public SnapshotDiff<ICursorPosition> Cursor { get; } = new();
 
         /// <summary>Tracks changes to the mouse wheel scroll.</summary>
-        public SnapshotDiff<int> MouseWheelScroll { get; } = new SnapshotDiff<int>();
+        public SnapshotDiff<int> MouseWheelScroll { get; } = new();
 
         /// <summary>Tracks changes to the content locale.</summary>
-        public SnapshotDiff<LocalizedContentManager.LanguageCode> Locale { get; } = new SnapshotDiff<LocalizedContentManager.LanguageCode>();
+        public SnapshotDiff<LocalizedContentManager.LanguageCode> Locale { get; } = new();
 
 
         /*********
@@ -54,7 +54,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
             // update snapshots
             this.WindowSize.Update(watchers.WindowSizeWatcher);
             this.Locale.Update(watchers.LocaleWatcher);
-            this.CurrentPlayer?.Update(watchers.CurrentPlayerTracker);
+            this.CurrentPlayer?.Update(watchers.CurrentPlayerTracker!);
             this.Time.Update(watchers.TimeWatcher);
             this.SaveID.Update(watchers.SaveIdWatcher);
             this.Locations.Update(watchers.LocationsWatcher);
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/WorldLocationsSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/WorldLocationsSnapshot.cs
index 73ed2d8f..59f94942 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/WorldLocationsSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/WorldLocationsSnapshot.cs
@@ -12,14 +12,14 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         ** Fields
         *********/
         /// <summary>A map of tracked locations.</summary>
-        private readonly Dictionary<GameLocation, LocationSnapshot> LocationsDict = new Dictionary<GameLocation, LocationSnapshot>(new ObjectReferenceComparer<GameLocation>());
+        private readonly Dictionary<GameLocation, LocationSnapshot> LocationsDict = new(new ObjectReferenceComparer<GameLocation>());
 
 
         /*********
         ** Accessors
         *********/
         /// <summary>Tracks changes to the location list.</summary>
-        public SnapshotListDiff<GameLocation> LocationList { get; } = new SnapshotListDiff<GameLocation>();
+        public SnapshotListDiff<GameLocation> LocationList { get; } = new();
 
         /// <summary>The tracked locations.</summary>
         public IEnumerable<LocationSnapshot> Locations => this.LocationsDict.Values;
@@ -42,7 +42,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
             // update locations
             foreach (LocationTracker locationWatcher in watcher.Locations)
             {
-                if (!this.LocationsDict.TryGetValue(locationWatcher.Location, out LocationSnapshot snapshot))
+                if (!this.LocationsDict.TryGetValue(locationWatcher.Location, out LocationSnapshot? snapshot))
                     this.LocationsDict[locationWatcher.Location] = snapshot = new LocationSnapshot(locationWatcher.Location);
 
                 snapshot.Update(locationWatcher);
diff --git a/src/SMAPI/Framework/StateTracking/WorldLocationsTracker.cs b/src/SMAPI/Framework/StateTracking/WorldLocationsTracker.cs
index e968d79c..817a6011 100644
--- a/src/SMAPI/Framework/StateTracking/WorldLocationsTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/WorldLocationsTracker.cs
@@ -25,10 +25,10 @@ namespace StardewModdingAPI.Framework.StateTracking
         private readonly ICollectionWatcher<GameLocation> VolcanoLocationListWatcher;
 
         /// <summary>A lookup of the tracked locations.</summary>
-        private IDictionary<GameLocation, LocationTracker> LocationDict { get; } = new Dictionary<GameLocation, LocationTracker>(new ObjectReferenceComparer<GameLocation>());
+        private Dictionary<GameLocation, LocationTracker> LocationDict { get; } = new(new ObjectReferenceComparer<GameLocation>());
 
         /// <summary>A lookup of registered buildings and their indoor location.</summary>
-        private readonly IDictionary<Building, GameLocation> BuildingIndoors = new Dictionary<Building, GameLocation>(new ObjectReferenceComparer<Building>());
+        private readonly Dictionary<Building, GameLocation?> BuildingIndoors = new(new ObjectReferenceComparer<Building>());
 
 
         /*********
@@ -99,10 +99,9 @@ namespace StardewModdingAPI.Framework.StateTracking
             }
 
             // detect building interiors changed (e.g. construction completed)
-            foreach (KeyValuePair<Building, GameLocation> pair in this.BuildingIndoors.Where(p => !object.Equals(p.Key.indoors.Value, p.Value)))
+            foreach ((Building building, GameLocation? oldIndoors) in this.BuildingIndoors.Where(p => !object.Equals(p.Key.indoors.Value, p.Value)))
             {
-                GameLocation oldIndoors = pair.Value;
-                GameLocation newIndoors = pair.Key.indoors.Value;
+                GameLocation? newIndoors = building.indoors.Value;
 
                 if (oldIndoors != null)
                     this.Added.Add(oldIndoors);
@@ -187,19 +186,19 @@ namespace StardewModdingAPI.Framework.StateTracking
         ****/
         /// <summary>Add the given building.</summary>
         /// <param name="building">The building to add.</param>
-        public void Add(Building building)
+        public void Add(Building? building)
         {
             if (building == null)
                 return;
 
-            GameLocation indoors = building.indoors.Value;
+            GameLocation? indoors = building.indoors.Value;
             this.BuildingIndoors[building] = indoors;
             this.Add(indoors);
         }
 
         /// <summary>Add the given location.</summary>
         /// <param name="location">The location to add.</param>
-        public void Add(GameLocation location)
+        public void Add(GameLocation? location)
         {
             if (location == null)
                 return;
@@ -218,7 +217,7 @@ namespace StardewModdingAPI.Framework.StateTracking
 
         /// <summary>Remove the given building.</summary>
         /// <param name="building">The building to remove.</param>
-        public void Remove(Building building)
+        public void Remove(Building? building)
         {
             if (building == null)
                 return;
@@ -229,12 +228,12 @@ namespace StardewModdingAPI.Framework.StateTracking
 
         /// <summary>Remove the given location.</summary>
         /// <param name="location">The location to remove.</param>
-        public void Remove(GameLocation location)
+        public void Remove(GameLocation? location)
         {
             if (location == null)
                 return;
 
-            if (this.LocationDict.TryGetValue(location, out LocationTracker watcher))
+            if (this.LocationDict.TryGetValue(location, out LocationTracker? watcher))
             {
                 // track change
                 this.Removed.Add(location);