enable nullable annotations for most of the SMAPI toolkit (#837)

21 files changed, 115 insertions, 129 deletions

diff --git a/src/SMAPI.Toolkit/Framework/Constants.cs b/src/SMAPI.Toolkit/Framework/Constants.cs
index c3a787c7..55f26582 100644
--- a/src/SMAPI.Toolkit/Framework/Constants.cs
+++ b/src/SMAPI.Toolkit/Framework/Constants.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 namespace StardewModdingAPI.Toolkit.Framework
 {
     /// <summary>Contains the SMAPI installer's constants and assumptions.</summary>
diff --git a/src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs b/src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs
index ac6fe411..4f872f1c 100644
--- a/src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs
+++ b/src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -41,7 +39,7 @@ namespace StardewModdingAPI.Toolkit.Framework.GameScanning
             IEnumerable<string> paths = this
                 .GetCustomInstallPaths()
                 .Concat(this.GetDefaultInstallPaths())
-                .Select(PathUtilities.NormalizePath)
+                .Select(path => PathUtilities.NormalizePath(path))
                 .Distinct(StringComparer.OrdinalIgnoreCase);
 
             // yield valid folders
@@ -80,10 +78,12 @@ namespace StardewModdingAPI.Toolkit.Framework.GameScanning
                 return GameFolderType.NoGameFound;
 
             // get assembly version
-            Version version;
+            Version? version;
             try
             {
                 version = AssemblyName.GetAssemblyName(executable.FullName).Version;
+                if (version == null)
+                    return GameFolderType.InvalidUnknown;
             }
             catch
             {
@@ -123,7 +123,7 @@ namespace StardewModdingAPI.Toolkit.Framework.GameScanning
                 case Platform.Linux:
                 case Platform.Mac:
                     {
-                        string home = Environment.GetEnvironmentVariable("HOME");
+                        string home = Environment.GetEnvironmentVariable("HOME")!;
 
                         // Linux
                         yield return $"{home}/GOG Games/Stardew Valley/game";
@@ -148,13 +148,13 @@ namespace StardewModdingAPI.Toolkit.Framework.GameScanning
                         };
                         foreach (var pair in registryKeys)
                         {
-                            string path = this.GetLocalMachineRegistryValue(pair.Key, pair.Value);
+                            string? path = this.GetLocalMachineRegistryValue(pair.Key, pair.Value);
                             if (!string.IsNullOrWhiteSpace(path))
                                 yield return path;
                         }
 
                         // via Steam library path
-                        string steamPath = this.GetCurrentUserRegistryValue(@"Software\Valve\Steam", "SteamPath");
+                        string? steamPath = this.GetCurrentUserRegistryValue(@"Software\Valve\Steam", "SteamPath");
                         if (steamPath != null)
                             yield return Path.Combine(steamPath.Replace('/', '\\'), @"steamapps\common\Stardew Valley");
 #endif
@@ -188,7 +188,7 @@ namespace StardewModdingAPI.Toolkit.Framework.GameScanning
         private IEnumerable<string> GetCustomInstallPaths()
         {
             // get home path
-            string homePath = Environment.GetEnvironmentVariable(this.Platform == Platform.Windows ? "USERPROFILE" : "HOME");
+            string homePath = Environment.GetEnvironmentVariable(this.Platform == Platform.Windows ? "USERPROFILE" : "HOME")!;
             if (string.IsNullOrWhiteSpace(homePath))
                 yield break;
 
@@ -210,7 +210,7 @@ namespace StardewModdingAPI.Toolkit.Framework.GameScanning
             }
 
             // get install path
-            XElement element = root.XPathSelectElement("//*[local-name() = 'GamePath']"); // can't use '//GamePath' due to the default namespace
+            XElement? element = root.XPathSelectElement("//*[local-name() = 'GamePath']"); // can't use '//GamePath' due to the default namespace
             if (!string.IsNullOrWhiteSpace(element?.Value))
                 yield return element.Value.Trim();
         }
@@ -219,27 +219,27 @@ namespace StardewModdingAPI.Toolkit.Framework.GameScanning
         /// <summary>Get the value of a key in the Windows HKLM registry.</summary>
         /// <param name="key">The full path of the registry key relative to HKLM.</param>
         /// <param name="name">The name of the value.</param>
-        private string GetLocalMachineRegistryValue(string key, string name)
+        private string? GetLocalMachineRegistryValue(string key, string name)
         {
             RegistryKey localMachine = Environment.Is64BitOperatingSystem ? RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry64) : Registry.LocalMachine;
-            RegistryKey openKey = localMachine.OpenSubKey(key);
+            RegistryKey? openKey = localMachine.OpenSubKey(key);
             if (openKey == null)
                 return null;
             using (openKey)
-                return (string)openKey.GetValue(name);
+                return (string?)openKey.GetValue(name);
         }
 
         /// <summary>Get the value of a key in the Windows HKCU registry.</summary>
         /// <param name="key">The full path of the registry key relative to HKCU.</param>
         /// <param name="name">The name of the value.</param>
-        private string GetCurrentUserRegistryValue(string key, string name)
+        private string? GetCurrentUserRegistryValue(string key, string name)
         {
-            RegistryKey currentuser = Environment.Is64BitOperatingSystem ? RegistryKey.OpenBaseKey(RegistryHive.CurrentUser, RegistryView.Registry64) : Registry.CurrentUser;
-            RegistryKey openKey = currentuser.OpenSubKey(key);
+            RegistryKey currentUser = Environment.Is64BitOperatingSystem ? RegistryKey.OpenBaseKey(RegistryHive.CurrentUser, RegistryView.Registry64) : Registry.CurrentUser;
+            RegistryKey? openKey = currentUser.OpenSubKey(key);
             if (openKey == null)
                 return null;
             using (openKey)
-                return (string)openKey.GetValue(name);
+                return (string?)openKey.GetValue(name);
         }
 #endif
     }
diff --git a/src/SMAPI.Toolkit/Framework/LowLevelEnvironmentUtility.cs b/src/SMAPI.Toolkit/Framework/LowLevelEnvironmentUtility.cs
index 9998edec..6978567e 100644
--- a/src/SMAPI.Toolkit/Framework/LowLevelEnvironmentUtility.cs
+++ b/src/SMAPI.Toolkit/Framework/LowLevelEnvironmentUtility.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
@@ -59,11 +57,13 @@ namespace StardewModdingAPI.Toolkit.Framework
 #if SMAPI_FOR_WINDOWS
             try
             {
-                return new ManagementObjectSearcher("SELECT Caption FROM Win32_OperatingSystem")
+                string? result = new ManagementObjectSearcher("SELECT Caption FROM Win32_OperatingSystem")
                     .Get()
                     .Cast<ManagementObject>()
                     .Select(entry => entry.GetPropertyValue("Caption").ToString())
                     .FirstOrDefault();
+
+                return result ?? "Windows";
             }
             catch { }
 #endif
@@ -137,7 +137,7 @@ namespace StardewModdingAPI.Toolkit.Framework
                 buffer = Marshal.AllocHGlobal(8192);
                 if (LowLevelEnvironmentUtility.uname(buffer) == 0)
                 {
-                    string os = Marshal.PtrToStringAnsi(buffer);
+                    string? os = Marshal.PtrToStringAnsi(buffer);
                     return os == "Darwin";
                 }
                 return false;
diff --git a/src/SMAPI.Toolkit/Framework/ModScanning/ModFolder.cs b/src/SMAPI.Toolkit/Framework/ModScanning/ModFolder.cs
index 81d72c0b..da2a3c85 100644
--- a/src/SMAPI.Toolkit/Framework/ModScanning/ModFolder.cs
+++ b/src/SMAPI.Toolkit/Framework/ModScanning/ModFolder.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -24,13 +22,13 @@ namespace StardewModdingAPI.Toolkit.Framework.ModScanning
         public ModType Type { get; }
 
         /// <summary>The mod manifest.</summary>
-        public Manifest Manifest { get; }
+        public Manifest? Manifest { get; }
 
         /// <summary>The error which occurred parsing the manifest, if any.</summary>
         public ModParseError ManifestParseError { get; set; }
 
         /// <summary>A human-readable message for the <see cref="ManifestParseError"/>, if any.</summary>
-        public string ManifestParseErrorText { get; set; }
+        public string? ManifestParseErrorText { get; set; }
 
 
         /*********
@@ -51,7 +49,7 @@ namespace StardewModdingAPI.Toolkit.Framework.ModScanning
         /// <param name="manifest">The mod manifest.</param>
         /// <param name="manifestParseError">The error which occurred parsing the manifest, if any.</param>
         /// <param name="manifestParseErrorText">A human-readable message for the <paramref name="manifestParseError"/>, if any.</param>
-        public ModFolder(DirectoryInfo root, DirectoryInfo directory, ModType type, Manifest manifest, ModParseError manifestParseError, string manifestParseErrorText)
+        public ModFolder(DirectoryInfo root, DirectoryInfo directory, ModType type, Manifest? manifest, ModParseError manifestParseError, string? manifestParseErrorText)
         {
             // save info
             this.Directory = directory;
@@ -61,9 +59,9 @@ namespace StardewModdingAPI.Toolkit.Framework.ModScanning
             this.ManifestParseErrorText = manifestParseErrorText;
 
             // set display name
-            this.DisplayName = manifest?.Name;
-            if (string.IsNullOrWhiteSpace(this.DisplayName))
-                this.DisplayName = PathUtilities.GetRelativePath(root.FullName, directory.FullName);
+            this.DisplayName = !string.IsNullOrWhiteSpace(manifest?.Name)
+                ? manifest.Name
+                : PathUtilities.GetRelativePath(root.FullName, directory.FullName);
         }
 
         /// <summary>Get the update keys for a mod.</summary>
diff --git a/src/SMAPI.Toolkit/Framework/ModScanning/ModScanner.cs b/src/SMAPI.Toolkit/Framework/ModScanning/ModScanner.cs
index 621f1e28..2af30092 100644
--- a/src/SMAPI.Toolkit/Framework/ModScanning/ModScanner.cs
+++ b/src/SMAPI.Toolkit/Framework/ModScanning/ModScanner.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -117,7 +115,7 @@ namespace StardewModdingAPI.Toolkit.Framework.ModScanning
         public ModFolder ReadFolder(DirectoryInfo root, DirectoryInfo searchFolder)
         {
             // find manifest.json
-            FileInfo manifestFile = this.FindManifest(searchFolder);
+            FileInfo? manifestFile = this.FindManifest(searchFolder);
 
             // set appropriate invalid-mod error
             if (manifestFile == null)
@@ -147,13 +145,13 @@ namespace StardewModdingAPI.Toolkit.Framework.ModScanning
             }
 
             // read mod info
-            Manifest manifest = null;
+            Manifest? manifest = null;
             ModParseError error = ModParseError.None;
-            string errorText = null;
+            string? errorText = null;
             {
                 try
                 {
-                    if (!this.JsonHelper.ReadJsonFileIfExists<Manifest>(manifestFile.FullName, out manifest) || manifest == null)
+                    if (!this.JsonHelper.ReadJsonFileIfExists<Manifest>(manifestFile.FullName, out manifest))
                     {
                         error = ModParseError.ManifestInvalid;
                         errorText = "its manifest is invalid.";
@@ -186,7 +184,7 @@ namespace StardewModdingAPI.Toolkit.Framework.ModScanning
             }
 
             // build result
-            return new ModFolder(root, manifestFile.Directory, type, manifest, error, errorText);
+            return new ModFolder(root, manifestFile.Directory!, type, manifest, error, errorText);
         }
 
 
@@ -249,7 +247,7 @@ namespace StardewModdingAPI.Toolkit.Framework.ModScanning
 
         /// <summary>Find the manifest for a mod folder.</summary>
         /// <param name="folder">The folder to search.</param>
-        private FileInfo FindManifest(DirectoryInfo folder)
+        private FileInfo? FindManifest(DirectoryInfo folder)
         {
             while (true)
             {
diff --git a/src/SMAPI.Toolkit/Framework/SemanticVersionReader.cs b/src/SMAPI.Toolkit/Framework/SemanticVersionReader.cs
index 57eea2f7..836b1134 100644
--- a/src/SMAPI.Toolkit/Framework/SemanticVersionReader.cs
+++ b/src/SMAPI.Toolkit/Framework/SemanticVersionReader.cs
@@ -1,4 +1,4 @@
-#nullable disable
+using System.Diagnostics.CodeAnalysis;
 
 namespace StardewModdingAPI.Toolkit.Framework
 {
@@ -18,7 +18,7 @@ namespace StardewModdingAPI.Toolkit.Framework
         /// <param name="prereleaseTag">An optional prerelease tag.</param>
         /// <param name="buildMetadata">Optional build metadata. This is ignored when determining version precedence.</param>
         /// <returns>Returns whether the version was successfully parsed.</returns>
-        public static bool TryParse(string versionStr, bool allowNonStandard, out int major, out int minor, out int patch, out int platformRelease, out string prereleaseTag, out string buildMetadata)
+        public static bool TryParse(string? versionStr, bool allowNonStandard, out int major, out int minor, out int patch, out int platformRelease, out string? prereleaseTag, out string? buildMetadata)
         {
             // init
             major = 0;
@@ -105,7 +105,7 @@ namespace StardewModdingAPI.Toolkit.Framework
         /// <param name="raw">The raw characters to parse.</param>
         /// <param name="index">The index of the next character to read.</param>
         /// <param name="tag">The parsed tag.</param>
-        private static bool TryParseTag(char[] raw, ref int index, out string tag)
+        private static bool TryParseTag(char[] raw, ref int index, [NotNullWhen(true)] out string? tag)
         {
             // read tag length
             int length = 0;
diff --git a/src/SMAPI.Toolkit/Framework/UpdateData/UpdateKey.cs b/src/SMAPI.Toolkit/Framework/UpdateData/UpdateKey.cs
index ec94ed51..d40d8f2b 100644
--- a/src/SMAPI.Toolkit/Framework/UpdateData/UpdateKey.cs
+++ b/src/SMAPI.Toolkit/Framework/UpdateData/UpdateKey.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace StardewModdingAPI.Toolkit.Framework.UpdateData
 {
@@ -17,10 +16,11 @@ namespace StardewModdingAPI.Toolkit.Framework.UpdateData
         public ModSiteKey Site { get; }
 
         /// <summary>The mod ID within the repository.</summary>
-        public string ID { get; }
+        [MemberNotNullWhen(true, nameof(LooksValid))]
+        public string? ID { get; }
 
         /// <summary>If specified, a substring in download names/descriptions to match.</summary>
-        public string Subkey { get; }
+        public string? Subkey { get; }
 
         /// <summary>Whether the update key seems to be valid.</summary>
         public bool LooksValid { get; }
@@ -34,9 +34,9 @@ namespace StardewModdingAPI.Toolkit.Framework.UpdateData
         /// <param name="site">The mod site containing the mod.</param>
         /// <param name="id">The mod ID within the site.</param>
         /// <param name="subkey">If specified, a substring in download names/descriptions to match.</param>
-        public UpdateKey(string rawText, ModSiteKey site, string id, string subkey)
+        public UpdateKey(string? rawText, ModSiteKey site, string? id, string? subkey)
         {
-            this.RawText = rawText?.Trim();
+            this.RawText = rawText?.Trim() ?? string.Empty;
             this.Site = site;
             this.ID = id?.Trim();
             this.Subkey = subkey?.Trim();
@@ -49,19 +49,19 @@ namespace StardewModdingAPI.Toolkit.Framework.UpdateData
         /// <param name="site">The mod site containing the mod.</param>
         /// <param name="id">The mod ID within the site.</param>
         /// <param name="subkey">If specified, a substring in download names/descriptions to match.</param>
-        public UpdateKey(ModSiteKey site, string id, string subkey)
+        public UpdateKey(ModSiteKey site, string? id, string? subkey)
             : this(UpdateKey.GetString(site, id, subkey), site, id, subkey) { }
 
         /// <summary>Parse a raw update key.</summary>
         /// <param name="raw">The raw update key to parse.</param>
-        public static UpdateKey Parse(string raw)
+        public static UpdateKey Parse(string? raw)
         {
             // extract site + ID
-            string rawSite;
-            string id;
+            string? rawSite;
+            string? id;
             {
-                string[] parts = raw?.Trim().Split(':');
-                if (parts == null || parts.Length != 2)
+                string[]? parts = raw?.Trim().Split(':');
+                if (parts?.Length != 2)
                     return new UpdateKey(raw, ModSiteKey.Unknown, null, null);
 
                 rawSite = parts[0].Trim();
@@ -71,7 +71,7 @@ namespace StardewModdingAPI.Toolkit.Framework.UpdateData
                 id = null;
 
             // extract subkey
-            string subkey = null;
+            string? subkey = null;
             if (id != null)
             {
                 string[] parts = id.Split('@');
@@ -111,7 +111,7 @@ namespace StardewModdingAPI.Toolkit.Framework.UpdateData
 
         /// <summary>Indicates whether the current object is equal to another object of the same type.</summary>
         /// <param name="other">An object to compare with this object.</param>
-        public bool Equals(UpdateKey other)
+        public bool Equals(UpdateKey? other)
         {
             if (!this.LooksValid)
             {
@@ -129,7 +129,7 @@ namespace StardewModdingAPI.Toolkit.Framework.UpdateData
 
         /// <summary>Determines whether the specified object is equal to the current object.</summary>
         /// <param name="obj">The object to compare with the current object.</param>
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             return obj is UpdateKey other && this.Equals(other);
         }
@@ -145,7 +145,7 @@ namespace StardewModdingAPI.Toolkit.Framework.UpdateData
         /// <param name="site">The mod site containing the mod.</param>
         /// <param name="id">The mod ID within the repository.</param>
         /// <param name="subkey">If specified, a substring in download names/descriptions to match.</param>
-        public static string GetString(ModSiteKey site, string id, string subkey = null)
+        public static string GetString(ModSiteKey site, string? id, string? subkey = null)
         {
             return $"{site}:{id}{subkey}".Trim();
         }
diff --git a/src/SMAPI.Toolkit/ModToolkit.cs b/src/SMAPI.Toolkit/ModToolkit.cs
index 9ae8cd1c..51f6fa24 100644
--- a/src/SMAPI.Toolkit/ModToolkit.cs
+++ b/src/SMAPI.Toolkit/ModToolkit.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -24,7 +22,7 @@ namespace StardewModdingAPI.Toolkit
         private readonly string UserAgent;
 
         /// <summary>Maps vendor keys (like <c>Nexus</c>) to their mod URL template (where <c>{0}</c> is the mod ID). This doesn't affect update checks, which defer to the remote web API.</summary>
-        private readonly IDictionary<ModSiteKey, string> VendorModUrls = new Dictionary<ModSiteKey, string>()
+        private readonly Dictionary<ModSiteKey, string> VendorModUrls = new()
         {
             [ModSiteKey.Chucklefish] = "https://community.playstarbound.com/resources/{0}",
             [ModSiteKey.GitHub] = "https://github.com/{0}/releases",
@@ -45,7 +43,7 @@ namespace StardewModdingAPI.Toolkit
         /// <summary>Construct an instance.</summary>
         public ModToolkit()
         {
-            ISemanticVersion version = new SemanticVersion(this.GetType().Assembly.GetName().Version);
+            ISemanticVersion version = new SemanticVersion(this.GetType().Assembly.GetName().Version!);
             this.UserAgent = $"SMAPI Mod Handler Toolkit/{version}";
         }
 
@@ -59,7 +57,7 @@ namespace StardewModdingAPI.Toolkit
         /// <summary>Extract mod metadata from the wiki compatibility list.</summary>
         public async Task<WikiModList> GetWikiCompatibilityListAsync()
         {
-            var client = new WikiClient(this.UserAgent);
+            WikiClient client = new(this.UserAgent);
             return await client.FetchModsAsync();
         }
 
@@ -89,13 +87,13 @@ namespace StardewModdingAPI.Toolkit
 
         /// <summary>Get an update URL for an update key (if valid).</summary>
         /// <param name="updateKey">The update key.</param>
-        public string GetUpdateUrl(string updateKey)
+        public string? GetUpdateUrl(string updateKey)
         {
             UpdateKey parsed = UpdateKey.Parse(updateKey);
             if (!parsed.LooksValid)
                 return null;
 
-            if (this.VendorModUrls.TryGetValue(parsed.Site, out string urlTemplate))
+            if (this.VendorModUrls.TryGetValue(parsed.Site, out string? urlTemplate))
                 return string.Format(urlTemplate, parsed.ID);
 
             return null;
diff --git a/src/SMAPI.Toolkit/SemanticVersion.cs b/src/SMAPI.Toolkit/SemanticVersion.cs
index ca9d15f5..cbdd7a85 100644
--- a/src/SMAPI.Toolkit/SemanticVersion.cs
+++ b/src/SMAPI.Toolkit/SemanticVersion.cs
@@ -91,7 +91,7 @@ namespace StardewModdingAPI.Toolkit
         {
             if (version == null)
                 throw new ArgumentNullException(nameof(version), "The input version string can't be null.");
-            if (!SemanticVersionReader.TryParse(version, allowNonStandard, out int major, out int minor, out int patch, out int platformRelease, out string prereleaseTag, out string buildMetadata) || (!allowNonStandard && platformRelease != 0))
+            if (!SemanticVersionReader.TryParse(version, allowNonStandard, out int major, out int minor, out int patch, out int platformRelease, out string? prereleaseTag, out string? buildMetadata) || (!allowNonStandard && platformRelease != 0))
                 throw new FormatException($"The input '{version}' isn't a valid semantic version.");
 
             this.MajorVersion = major;
diff --git a/src/SMAPI.Toolkit/SemanticVersionComparer.cs b/src/SMAPI.Toolkit/SemanticVersionComparer.cs
index a0472f62..85c974bd 100644
--- a/src/SMAPI.Toolkit/SemanticVersionComparer.cs
+++ b/src/SMAPI.Toolkit/SemanticVersionComparer.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 
 namespace StardewModdingAPI.Toolkit
@@ -18,7 +16,7 @@ namespace StardewModdingAPI.Toolkit
         ** Public methods
         *********/
         /// <inheritdoc />
-        public int Compare(ISemanticVersion x, ISemanticVersion y)
+        public int Compare(ISemanticVersion? x, ISemanticVersion? y)
         {
             if (object.ReferenceEquals(x, y))
                 return 0;
diff --git a/src/SMAPI.Toolkit/Serialization/Converters/ManifestContentPackForConverter.cs b/src/SMAPI.Toolkit/Serialization/Converters/ManifestContentPackForConverter.cs
index d2dc0d22..faaeedea 100644
--- a/src/SMAPI.Toolkit/Serialization/Converters/ManifestContentPackForConverter.cs
+++ b/src/SMAPI.Toolkit/Serialization/Converters/ManifestContentPackForConverter.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Newtonsoft.Json;
 using StardewModdingAPI.Toolkit.Serialization.Models;
@@ -35,7 +33,7 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
         /// <param name="objectType">The object type.</param>
         /// <param name="existingValue">The object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             return serializer.Deserialize<ManifestContentPackFor>(reader);
         }
@@ -44,7 +42,7 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
         /// <param name="writer">The JSON writer.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             throw new InvalidOperationException("This converter does not write JSON.");
         }
diff --git a/src/SMAPI.Toolkit/Serialization/Converters/ManifestDependencyArrayConverter.cs b/src/SMAPI.Toolkit/Serialization/Converters/ManifestDependencyArrayConverter.cs
index 2c3c2ee7..c499a2c6 100644
--- a/src/SMAPI.Toolkit/Serialization/Converters/ManifestDependencyArrayConverter.cs
+++ b/src/SMAPI.Toolkit/Serialization/Converters/ManifestDependencyArrayConverter.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using Newtonsoft.Json;
@@ -37,13 +35,13 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
         /// <param name="objectType">The object type.</param>
         /// <param name="existingValue">The object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             List<ManifestDependency> result = new List<ManifestDependency>();
             foreach (JObject obj in JArray.Load(reader).Children<JObject>())
             {
-                string uniqueID = obj.ValueIgnoreCase<string>(nameof(ManifestDependency.UniqueID));
-                string minVersion = obj.ValueIgnoreCase<string>(nameof(ManifestDependency.MinimumVersion));
+                string uniqueID = obj.ValueIgnoreCase<string>(nameof(ManifestDependency.UniqueID))!; // will be validated separately if null
+                string? minVersion = obj.ValueIgnoreCase<string>(nameof(ManifestDependency.MinimumVersion));
                 bool required = obj.ValueIgnoreCase<bool?>(nameof(ManifestDependency.IsRequired)) ?? true;
                 result.Add(new ManifestDependency(uniqueID, minVersion, required));
             }
@@ -54,7 +52,7 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
         /// <param name="writer">The JSON writer.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             throw new InvalidOperationException("This converter does not write JSON.");
         }
diff --git a/src/SMAPI.Toolkit/Serialization/Converters/SemanticVersionConverter.cs b/src/SMAPI.Toolkit/Serialization/Converters/SemanticVersionConverter.cs
index 9205cebe..c32c3185 100644
--- a/src/SMAPI.Toolkit/Serialization/Converters/SemanticVersionConverter.cs
+++ b/src/SMAPI.Toolkit/Serialization/Converters/SemanticVersionConverter.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
@@ -41,15 +39,17 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
         /// <param name="objectType">The object type.</param>
         /// <param name="existingValue">The object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             string path = reader.Path;
             switch (reader.TokenType)
             {
                 case JsonToken.StartObject:
                     return this.ReadObject(JObject.Load(reader));
+
                 case JsonToken.String:
                     return this.ReadString(JToken.Load(reader).Value<string>(), path);
+
                 default:
                     throw new SParseException($"Can't parse {nameof(ISemanticVersion)} from {reader.TokenType} node (path: {reader.Path}).");
             }
@@ -59,7 +59,7 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
         /// <param name="writer">The JSON writer.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             writer.WriteValue(value?.ToString());
         }
@@ -75,7 +75,7 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
             int major = obj.ValueIgnoreCase<int>(nameof(ISemanticVersion.MajorVersion));
             int minor = obj.ValueIgnoreCase<int>(nameof(ISemanticVersion.MinorVersion));
             int patch = obj.ValueIgnoreCase<int>(nameof(ISemanticVersion.PatchVersion));
-            string prereleaseTag = obj.ValueIgnoreCase<string>(nameof(ISemanticVersion.PrereleaseTag));
+            string? prereleaseTag = obj.ValueIgnoreCase<string>(nameof(ISemanticVersion.PrereleaseTag));
 
             return new SemanticVersion(major, minor, patch, prereleaseTag: prereleaseTag);
         }
@@ -83,11 +83,11 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
         /// <summary>Read a JSON string.</summary>
         /// <param name="str">The JSON string value.</param>
         /// <param name="path">The path to the current JSON node.</param>
-        private ISemanticVersion ReadString(string str, string path)
+        private ISemanticVersion? ReadString(string str, string path)
         {
             if (string.IsNullOrWhiteSpace(str))
                 return null;
-            if (!SemanticVersion.TryParse(str, allowNonStandard: this.AllowNonStandard, out ISemanticVersion version))
+            if (!SemanticVersion.TryParse(str, allowNonStandard: this.AllowNonStandard, out ISemanticVersion? version))
                 throw new SParseException($"Can't parse semantic version from invalid value '{str}', should be formatted like 1.2, 1.2.30, or 1.2.30-beta (path: {path}).");
             return version;
         }
diff --git a/src/SMAPI.Toolkit/Serialization/Converters/SimpleReadOnlyConverter.cs b/src/SMAPI.Toolkit/Serialization/Converters/SimpleReadOnlyConverter.cs
index c923350c..1c59f5e7 100644
--- a/src/SMAPI.Toolkit/Serialization/Converters/SimpleReadOnlyConverter.cs
+++ b/src/SMAPI.Toolkit/Serialization/Converters/SimpleReadOnlyConverter.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
@@ -27,21 +25,12 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
             return objectType == typeof(T) || Nullable.GetUnderlyingType(objectType) == typeof(T);
         }
 
-        /// <summary>Writes the JSON representation of the object.</summary>
-        /// <param name="writer">The JSON writer.</param>
-        /// <param name="value">The value.</param>
-        /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
-        {
-            throw new InvalidOperationException("This converter does not write JSON.");
-        }
-
         /// <summary>Reads the JSON representation of the object.</summary>
         /// <param name="reader">The JSON reader.</param>
         /// <param name="objectType">The object type.</param>
         /// <param name="existingValue">The object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             string path = reader.Path;
             switch (reader.TokenType)
@@ -60,6 +49,15 @@ namespace StardewModdingAPI.Toolkit.Serialization.Converters
             }
         }
 
+        /// <summary>Writes the JSON representation of the object.</summary>
+        /// <param name="writer">The JSON writer.</param>
+        /// <param name="value">The value.</param>
+        /// <param name="serializer">The calling serializer.</param>
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
+        {
+            throw new InvalidOperationException("This converter does not write JSON.");
+        }
+
 
         /*********
         ** Protected methods
diff --git a/src/SMAPI.Toolkit/Serialization/InternalExtensions.cs b/src/SMAPI.Toolkit/Serialization/InternalExtensions.cs
index 1fce5f9d..78297035 100644
--- a/src/SMAPI.Toolkit/Serialization/InternalExtensions.cs
+++ b/src/SMAPI.Toolkit/Serialization/InternalExtensions.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Newtonsoft.Json.Linq;
 
@@ -12,12 +10,12 @@ namespace StardewModdingAPI.Toolkit.Serialization
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="obj">The JSON object to search.</param>
         /// <param name="fieldName">The field name.</param>
-        public static T ValueIgnoreCase<T>(this JObject obj, string fieldName)
+        public static T? ValueIgnoreCase<T>(this JObject obj, string fieldName)
         {
-            JToken token = obj.GetValue(fieldName, StringComparison.OrdinalIgnoreCase);
+            JToken? token = obj.GetValue(fieldName, StringComparison.OrdinalIgnoreCase);
             return token != null
                 ? token.Value<T>()
-                : default(T);
+                : default;
         }
     }
 }
diff --git a/src/SMAPI.Toolkit/Serialization/JsonHelper.cs b/src/SMAPI.Toolkit/Serialization/JsonHelper.cs
index 9700e712..7d1804e5 100644
--- a/src/SMAPI.Toolkit/Serialization/JsonHelper.cs
+++ b/src/SMAPI.Toolkit/Serialization/JsonHelper.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Converters;
@@ -38,7 +37,7 @@ namespace StardewModdingAPI.Toolkit.Serialization
         /// <returns>Returns false if the file doesn't exist, else true.</returns>
         /// <exception cref="ArgumentException">The given <paramref name="fullPath"/> is empty or invalid.</exception>
         /// <exception cref="JsonReaderException">The file contains invalid JSON.</exception>
-        public bool ReadJsonFileIfExists<TModel>(string fullPath, out TModel result)
+        public bool ReadJsonFileIfExists<TModel>(string fullPath, [NotNullWhen(true)] out TModel? result)
         {
             // validate
             if (string.IsNullOrWhiteSpace(fullPath))
@@ -52,7 +51,7 @@ namespace StardewModdingAPI.Toolkit.Serialization
             }
             catch (Exception ex) when (ex is DirectoryNotFoundException or FileNotFoundException)
             {
-                result = default(TModel);
+                result = default;
                 return false;
             }
 
@@ -60,7 +59,7 @@ namespace StardewModdingAPI.Toolkit.Serialization
             try
             {
                 result = this.Deserialize<TModel>(json);
-                return true;
+                return result != null;
             }
             catch (Exception ex)
             {
@@ -90,7 +89,7 @@ namespace StardewModdingAPI.Toolkit.Serialization
                 throw new ArgumentException("The file path is empty or invalid.", nameof(fullPath));
 
             // create directory if needed
-            string dir = Path.GetDirectoryName(fullPath);
+            string dir = Path.GetDirectoryName(fullPath)!;
             if (dir == null)
                 throw new ArgumentException("The file path is invalid.", nameof(fullPath));
             if (!Directory.Exists(dir))
@@ -108,7 +107,8 @@ namespace StardewModdingAPI.Toolkit.Serialization
         {
             try
             {
-                return JsonConvert.DeserializeObject<TModel>(json, this.JsonSettings);
+                return JsonConvert.DeserializeObject<TModel>(json, this.JsonSettings)
+                    ?? throw new InvalidOperationException($"Couldn't deserialize model type '{typeof(TModel)}' from empty or null JSON.");
             }
             catch (JsonReaderException)
             {
@@ -117,7 +117,8 @@ namespace StardewModdingAPI.Toolkit.Serialization
                 {
                     try
                     {
-                        return JsonConvert.DeserializeObject<TModel>(json.Replace('“', '"').Replace('”', '"'), this.JsonSettings);
+                        return JsonConvert.DeserializeObject<TModel>(json.Replace('“', '"').Replace('”', '"'), this.JsonSettings)
+                            ?? throw new InvalidOperationException($"Couldn't deserialize model type '{typeof(TModel)}' from empty or null JSON.");
                     }
                     catch { /* rethrow original error */ }
                 }
diff --git a/src/SMAPI.Toolkit/Serialization/Models/ManifestDependency.cs b/src/SMAPI.Toolkit/Serialization/Models/ManifestDependency.cs
index 64725818..5d1b73b1 100644
--- a/src/SMAPI.Toolkit/Serialization/Models/ManifestDependency.cs
+++ b/src/SMAPI.Toolkit/Serialization/Models/ManifestDependency.cs
@@ -26,7 +26,7 @@ namespace StardewModdingAPI.Toolkit.Serialization.Models
         /// <param name="uniqueID">The unique mod ID to require.</param>
         /// <param name="minimumVersion">The minimum required version (if any).</param>
         /// <param name="required">Whether the dependency must be installed to use the mod.</param>
-        public ManifestDependency(string uniqueID, string minimumVersion, bool required = true)
+        public ManifestDependency(string uniqueID, string? minimumVersion, bool required = true)
             : this(
                 uniqueID: uniqueID,
                 minimumVersion: !string.IsNullOrWhiteSpace(minimumVersion)
diff --git a/src/SMAPI.Toolkit/Serialization/SParseException.cs b/src/SMAPI.Toolkit/Serialization/SParseException.cs
index 1581e027..c2b3f68e 100644
--- a/src/SMAPI.Toolkit/Serialization/SParseException.cs
+++ b/src/SMAPI.Toolkit/Serialization/SParseException.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 
 namespace StardewModdingAPI.Toolkit.Serialization
@@ -13,7 +11,7 @@ namespace StardewModdingAPI.Toolkit.Serialization
         /// <summary>Construct an instance.</summary>
         /// <param name="message">The error message.</param>
         /// <param name="ex">The underlying exception, if any.</param>
-        public SParseException(string message, Exception ex = null)
+        public SParseException(string message, Exception? ex = null)
             : base(message, ex) { }
     }
 }
diff --git a/src/SMAPI.Toolkit/Utilities/EnvironmentUtility.cs b/src/SMAPI.Toolkit/Utilities/EnvironmentUtility.cs
index f14678be..1791c5b3 100644
--- a/src/SMAPI.Toolkit/Utilities/EnvironmentUtility.cs
+++ b/src/SMAPI.Toolkit/Utilities/EnvironmentUtility.cs
@@ -1,7 +1,4 @@
-#nullable disable
-
 using System;
-using System.Diagnostics.CodeAnalysis;
 using StardewModdingAPI.Toolkit.Framework;
 
 namespace StardewModdingAPI.Toolkit.Utilities
@@ -36,7 +33,6 @@ namespace StardewModdingAPI.Toolkit.Utilities
 
         /// <summary>Get the human-readable OS name and version.</summary>
         /// <param name="platform">The current platform.</param>
-        [SuppressMessage("ReSharper", "EmptyGeneralCatchClause", Justification = "Error suppressed deliberately to fallback to default behaviour.")]
         public static string GetFriendlyPlatformName(Platform platform)
         {
             return LowLevelEnvironmentUtility.GetFriendlyPlatformName(platform.ToString());
diff --git a/src/SMAPI.Toolkit/Utilities/FileUtilities.cs b/src/SMAPI.Toolkit/Utilities/FileUtilities.cs
index ba2d0f47..a6bf5929 100644
--- a/src/SMAPI.Toolkit/Utilities/FileUtilities.cs
+++ b/src/SMAPI.Toolkit/Utilities/FileUtilities.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.IO;
 using System.Security.Cryptography;
diff --git a/src/SMAPI.Toolkit/Utilities/PathUtilities.cs b/src/SMAPI.Toolkit/Utilities/PathUtilities.cs
index d035d4cd..9b7a78a0 100644
--- a/src/SMAPI.Toolkit/Utilities/PathUtilities.cs
+++ b/src/SMAPI.Toolkit/Utilities/PathUtilities.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.IO;
 using System.Linq;
@@ -38,8 +37,11 @@ namespace StardewModdingAPI.Toolkit.Utilities
         /// <param name="path">The path to split.</param>
         /// <param name="limit">The number of segments to match. Any additional segments will be merged into the last returned part.</param>
         [Pure]
-        public static string[] GetSegments(string path, int? limit = null)
+        public static string[] GetSegments(string? path, int? limit = null)
         {
+            if (path == null)
+                return Array.Empty<string>();
+
             return limit.HasValue
                 ? path.Split(PathUtilities.PossiblePathSeparators, limit.Value, StringSplitOptions.RemoveEmptyEntries)
                 : path.Split(PathUtilities.PossiblePathSeparators, StringSplitOptions.RemoveEmptyEntries);
@@ -47,8 +49,14 @@ namespace StardewModdingAPI.Toolkit.Utilities
 
         /// <summary>Normalize an asset name to match how MonoGame's content APIs would normalize and cache it.</summary>
         /// <param name="assetName">The asset name to normalize.</param>
-        public static string NormalizeAssetName(string assetName)
+        [Pure]
+        [return: NotNullIfNotNull("assetName")]
+        public static string? NormalizeAssetName(string? assetName)
         {
+            assetName = assetName?.Trim();
+            if (string.IsNullOrEmpty(assetName))
+                return assetName;
+
             return string.Join(PathUtilities.PreferredAssetSeparator.ToString(), PathUtilities.GetSegments(assetName)); // based on MonoGame's ContentManager.Load<T> logic
         }
 
@@ -56,7 +64,8 @@ namespace StardewModdingAPI.Toolkit.Utilities
         /// <param name="path">The file path to normalize.</param>
         /// <remarks>This should only be used for file paths. For asset names, use <see cref="NormalizeAssetName"/> instead.</remarks>
         [Pure]
-        public static string NormalizePath(string path)
+        [return: NotNullIfNotNull("path")]
+        public static string? NormalizePath(string? path)
         {
             path = path?.Trim();
             if (string.IsNullOrEmpty(path))
@@ -80,7 +89,7 @@ namespace StardewModdingAPI.Toolkit.Utilities
             }
 
             // keep trailing separator
-            if ((!hasRoot || segments.Any()) && PathUtilities.PossiblePathSeparators.Contains(path[path.Length - 1]))
+            if ((!hasRoot || segments.Any()) && PathUtilities.PossiblePathSeparators.Contains(path[^1]))
                 newPath += PathUtilities.PreferredPathSeparator;
 
             return newPath;
@@ -98,7 +107,7 @@ namespace StardewModdingAPI.Toolkit.Utilities
         /// <summary>Get whether a path is relative and doesn't try to climb out of its containing folder (e.g. doesn't contain <c>../</c>).</summary>
         /// <param name="path">The path to check.</param>
         [Pure]
-        public static bool IsSafeRelativePath(string path)
+        public static bool IsSafeRelativePath(string? path)
         {
             if (string.IsNullOrWhiteSpace(path))
                 return true;
@@ -111,9 +120,11 @@ namespace StardewModdingAPI.Toolkit.Utilities
         /// <summary>Get whether a string is a valid 'slug', containing only basic characters that are safe in all contexts (e.g. filenames, URLs, etc).</summary>
         /// <param name="str">The string to check.</param>
         [Pure]
-        public static bool IsSlug(string str)
+        public static bool IsSlug(string? str)
         {
-            return !Regex.IsMatch(str, "[^a-z0-9_.-]", RegexOptions.IgnoreCase);
+            return
+                string.IsNullOrWhiteSpace(str)
+                || !Regex.IsMatch(str, "[^a-z0-9_.-]", RegexOptions.IgnoreCase);
         }
     }
 }