9 files changed, 143 insertions, 9 deletions
diff --git a/src/SMAPI.Installer/InteractiveInstaller.cs b/src/SMAPI.Installer/InteractiveInstaller.cs
index 30cb10b4..f9239604 100644
--- a/src/SMAPI.Installer/InteractiveInstaller.cs
+++ b/src/SMAPI.Installer/InteractiveInstaller.cs
@@ -87,11 +87,16 @@ namespace StardewModdingApi.Installer
             string GetInstallPath(string path) => Path.Combine(installDir.FullName, path);
 
             // common
+            yield return GetInstallPath("0Harmony.dll");
+            yield return GetInstallPath("0Harmony.pdb");
             yield return GetInstallPath("Mono.Cecil.dll");
             yield return GetInstallPath("Newtonsoft.Json.dll");
             yield return GetInstallPath("StardewModdingAPI.exe");
             yield return GetInstallPath("StardewModdingAPI.config.json");
             yield return GetInstallPath("StardewModdingAPI.metadata.json");
+            yield return GetInstallPath("StardewModdingAPI.Toolkit.dll");
+            yield return GetInstallPath("StardewModdingAPI.Toolkit.pdb");
+            yield return GetInstallPath("StardewModdingAPI.xml");
             yield return GetInstallPath("System.ValueTuple.dll");
             yield return GetInstallPath("steam_appid.txt");
 
diff --git a/src/SMAPI/Constants.cs b/src/SMAPI/Constants.cs
index 31744ccd..532112ff 100644
--- a/src/SMAPI/Constants.cs
+++ b/src/SMAPI/Constants.cs
@@ -106,6 +106,9 @@ namespace StardewModdingAPI
         /// <summary>The target game platform.</summary>
         internal static Platform Platform { get; } = EnvironmentUtility.DetectPlatform();
 
+        /// <summary>The game's assembly name.</summary>
+        internal static string GameAssemblyName => Constants.Platform == Platform.Windows ? "Stardew Valley" : "StardewValley";
+
 
         /*********
         ** Internal methods
diff --git a/src/SMAPI/Framework/Patching/GameLocationPatch.cs b/src/SMAPI/Framework/Patching/GameLocationPatch.cs
new file mode 100644
index 00000000..c0216b8f
--- /dev/null
+++ b/src/SMAPI/Framework/Patching/GameLocationPatch.cs
@@ -0,0 +1,60 @@
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Reflection;
+using Harmony;
+using StardewValley;
+using xTile.Tiles;
+
+namespace StardewModdingAPI.Framework.Patching
+{
+    /// <summary>A Harmony patch for the <see cref="GameLocation.updateSeasonalTileSheets"/> method.</summary>
+    internal class GameLocationPatch : IHarmonyPatch
+    {
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>A unique name for this patch.</summary>
+        public string Name => $"{nameof(GameLocation)}.{nameof(GameLocation.updateSeasonalTileSheets)}";
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Apply the Harmony patch.</summary>
+        /// <param name="harmony">The Harmony instance.</param>
+        public void Apply(HarmonyInstance harmony)
+        {
+            MethodInfo method = AccessTools.Method(typeof(GameLocation), nameof(GameLocation.updateSeasonalTileSheets));
+            MethodInfo prefix = AccessTools.Method(this.GetType(), nameof(GameLocationPatch.Prefix));
+
+            harmony.Patch(method, new HarmonyMethod(prefix), null);
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>An implementation of <see cref="GameLocation.updateSeasonalTileSheets"/> which correctly handles custom map tilesheets.</summary>
+        /// <param name="__instance">The location instance being patched.</param>
+        [SuppressMessage("ReSharper", "InconsistentNaming", Justification = "Argument name is defined by Harmony.")]
+        private static bool Prefix(GameLocation __instance)
+        {
+            if (!__instance.IsOutdoors || __instance.Name.Equals("Desert"))
+                return false;
+            foreach (TileSheet tilesheet in __instance.Map.TileSheets)
+            {
+                string imageSource = tilesheet.ImageSource;
+                string imageFile = Path.GetFileName(imageSource);
+                if (imageFile.StartsWith("spring_") || imageFile.StartsWith("summer_") || imageFile.StartsWith("fall_") || imageFile.StartsWith("winter_"))
+                {
+                    string imageDir = Path.GetDirectoryName(imageSource);
+                    if (string.IsNullOrWhiteSpace(imageDir))
+                        imageDir = "Maps";
+                    tilesheet.ImageSource = Path.Combine(imageDir, Game1.currentSeason + "_" + imageFile.Split('_')[1]);
+                }
+            }
+
+            return false;
+        }
+    }
+}
diff --git a/src/SMAPI/Framework/Patching/GamePatcher.cs b/src/SMAPI/Framework/Patching/GamePatcher.cs
new file mode 100644
index 00000000..71ca8e55
--- /dev/null
+++ b/src/SMAPI/Framework/Patching/GamePatcher.cs
@@ -0,0 +1,45 @@
+using System;
+using Harmony;
+
+namespace StardewModdingAPI.Framework.Patching
+{
+    /// <summary>Encapsulates applying Harmony patches to the game.</summary>
+    internal class GamePatcher
+    {
+        /*********
+        ** Properties
+        *********/
+        /// <summary>Encapsulates monitoring and logging.</summary>
+        private readonly IMonitor Monitor;
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="monitor">Encapsulates monitoring and logging.</param>
+        public GamePatcher(IMonitor monitor)
+        {
+            this.Monitor = monitor;
+        }
+
+        /// <summary>Apply all loaded patches to the game.</summary>
+        /// <param name="patches">The patches to apply.</param>
+        public void Apply(params IHarmonyPatch[] patches)
+        {
+            HarmonyInstance harmony = HarmonyInstance.Create("io.smapi");
+            foreach (IHarmonyPatch patch in patches)
+            {
+                try
+                {
+                    patch.Apply(harmony);
+                }
+                catch (Exception ex)
+                {
+                    this.Monitor.Log($"Couldn't apply runtime patch '{patch.Name}' to the game. Some SMAPI features may not work correctly. See log file for details.", LogLevel.Error);
+                    this.Monitor.Log(ex.GetLogSummary(), LogLevel.Trace);
+                }
+            }
+        }
+    }
+}
diff --git a/src/SMAPI/Framework/Patching/IHarmonyPatch.cs b/src/SMAPI/Framework/Patching/IHarmonyPatch.cs
new file mode 100644
index 00000000..cb42f40e
--- /dev/null
+++ b/src/SMAPI/Framework/Patching/IHarmonyPatch.cs
@@ -0,0 +1,15 @@
+using Harmony;
+
+namespace StardewModdingAPI.Framework.Patching
+{
+    /// <summary>A Harmony patch to apply.</summary>
+    internal interface IHarmonyPatch
+    {
+        /// <summary>A unique name for this patch.</summary>
+        string Name { get; }
+
+        /// <summary>Apply the Harmony patch.</summary>
+        /// <param name="harmony">The Harmony instance.</param>
+        void Apply(HarmonyInstance harmony);
+    }
+}
diff --git a/src/SMAPI/Program.cs b/src/SMAPI/Program.cs
index 28523722..4ede45d5 100644
--- a/src/SMAPI/Program.cs
+++ b/src/SMAPI/Program.cs
@@ -24,6 +24,7 @@ using StardewModdingAPI.Framework.ModData;
 using StardewModdingAPI.Framework.Models;
 using StardewModdingAPI.Framework.ModHelpers;
 using StardewModdingAPI.Framework.ModLoading;
+using StardewModdingAPI.Framework.Patching;
 using StardewModdingAPI.Framework.Reflection;
 using StardewModdingAPI.Framework.Serialisation;
 using StardewModdingAPI.Internal;
@@ -32,7 +33,7 @@ using StardewModdingAPI.Toolkit.Serialisation;
 using StardewModdingAPI.Toolkit.Serialisation.Converters;
 using StardewModdingAPI.Toolkit.Utilities;
 using StardewValley;
-using Keys = System.Windows.Forms.Keys;
+using Keys = Microsoft.Xna.Framework.Input.Keys;
 using Monitor = StardewModdingAPI.Framework.Monitor;
 using SObject = StardewValley.Object;
 using ThreadState = System.Threading.ThreadState;
@@ -141,6 +142,11 @@ namespace StardewModdingAPI
             };
             this.EventManager = new EventManager(this.Monitor, this.ModRegistry);
 
+            // apply game patches
+            new GamePatcher(this.Monitor).Apply(
+                new GameLocationPatch()
+            );
+
             // init JSON parser
             JsonConverter[] converters = {
                 new StringEnumConverter<Buttons>(),
@@ -348,14 +354,7 @@ namespace StardewModdingAPI
                 Console.ResetColor();
                 Program.PressAnyKeyToExit(showMessage: true);
             }
-
-            // get game assembly name
-            const string gameAssemblyName =
-#if SMAPI_FOR_WINDOWS
-                "Stardew Valley";
-#else
-                "StardewValley";
-#endif
+            string gameAssemblyName = Constants.GameAssemblyName;
 
             // game not present
             if (Type.GetType($"StardewValley.Game1, {gameAssemblyName}", throwOnError: false) == null)
diff --git a/src/SMAPI/StardewModdingAPI.csproj b/src/SMAPI/StardewModdingAPI.csproj
index 7eb5f95a..2df3e63f 100644
--- a/src/SMAPI/StardewModdingAPI.csproj
+++ b/src/SMAPI/StardewModdingAPI.csproj
@@ -53,6 +53,10 @@
     <ApplicationIcon>icon.ico</ApplicationIcon>
   </PropertyGroup>
   <ItemGroup>
+    <Reference Include="0Harmony, Version=1.0.9.1, Culture=neutral, processorArchitecture=MSIL">
+      <SpecificVersion>False</SpecificVersion>
+      <HintPath>..\lib\0Harmony.dll</HintPath>
+    </Reference>
     <Reference Include="Mono.Cecil, Version=0.9.6.0, Culture=neutral, PublicKeyToken=0738eb9f132ed756, processorArchitecture=MSIL">
       <HintPath>..\packages\Mono.Cecil.0.9.6.4\lib\net45\Mono.Cecil.dll</HintPath>
       <Private>True</Private>
@@ -103,6 +107,8 @@
     <Compile Include="Framework\ContentManagers\GameContentManager.cs" />
     <Compile Include="Framework\ContentManagers\IContentManager.cs" />
     <Compile Include="Framework\ContentManagers\ModContentManager.cs" />
+    <Compile Include="Framework\Patching\GamePatcher.cs" />
+    <Compile Include="Framework\Patching\IHarmonyPatch.cs" />
     <Compile Include="Framework\Serialisation\ColorConverter.cs" />
     <Compile Include="Framework\Serialisation\PointConverter.cs" />
     <Compile Include="Framework\Serialisation\RectangleConverter.cs" />
@@ -279,6 +285,7 @@
     <Compile Include="Framework\Monitor.cs" />
     <Compile Include="Metadata\InstructionMetadata.cs" />
     <Compile Include="Mod.cs" />
+    <Compile Include="Framework\Patching\GameLocationPatch.cs" />
     <Compile Include="PatchMode.cs" />
     <Compile Include="GamePlatform.cs" />
     <Compile Include="Program.cs" />
diff --git a/src/lib/0Harmony.dll b/src/lib/0Harmony.dll
new file mode 100644
index 00000000..63619429
--- /dev/null
+++ b/src/lib/0Harmony.dll
diff --git a/src/lib/0Harmony.pdb b/src/lib/0Harmony.pdb
new file mode 100644
index 00000000..d7a4c67c
--- /dev/null
+++ b/src/lib/0Harmony.pdb