enable nullable annotations in the web project & related code (#837)

97 files changed, 1044 insertions, 768 deletions

diff --git a/build/common.targets b/build/common.targets
index c227190a..c04546d0 100644
--- a/build/common.targets
+++ b/build/common.targets
@@ -7,8 +7,8 @@
     <AssemblySearchPaths>$(AssemblySearchPaths);{GAC}</AssemblySearchPaths>
 
     <!--enable nullable annotations, except in .NET Standard 2.0 where they aren't supported-->
-    <Nullable Condition="'$(TargetFramework)' == 'net5.0'">enable</Nullable>
-    <NoWarn Condition="'$(TargetFramework)' != 'net5.0'">$(NoWarn);CS8632</NoWarn>
+    <Nullable Condition="'$(TargetFramework)' != 'netstandard2.0'">enable</Nullable>
+    <NoWarn Condition="'$(TargetFramework)' == 'netstandard2.0'">$(NoWarn);CS8632</NoWarn>
 
     <!--set platform-->
     <DefineConstants Condition="$(OS) == 'Windows_NT'">$(DefineConstants);SMAPI_FOR_WINDOWS</DefineConstants>
diff --git a/src/SMAPI.Toolkit.CoreInterfaces/ISemanticVersion.cs b/src/SMAPI.Toolkit.CoreInterfaces/ISemanticVersion.cs
index 7998272f..dc226b7c 100644
--- a/src/SMAPI.Toolkit.CoreInterfaces/ISemanticVersion.cs
+++ b/src/SMAPI.Toolkit.CoreInterfaces/ISemanticVersion.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace StardewModdingAPI
 {
@@ -28,6 +29,9 @@ namespace StardewModdingAPI
         ** Accessors
         *********/
         /// <summary>Whether this is a prerelease version.</summary>
+#if NET5_0_OR_GREATER
+        [MemberNotNullWhen(true, nameof(ISemanticVersion.PrereleaseTag))]
+#endif
         bool IsPrerelease();
 
         /// <summary>Get whether this version is older than the specified version.</summary>
diff --git a/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModEntryModel.cs b/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModEntryModel.cs
index d5ca2034..4fc4ea54 100644
--- a/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModEntryModel.cs
+++ b/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModEntryModel.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 
 namespace StardewModdingAPI.Toolkit.Framework.Clients.WebApi
@@ -11,15 +9,26 @@ namespace StardewModdingAPI.Toolkit.Framework.Clients.WebApi
         ** Accessors
         *********/
         /// <summary>The mod's unique ID (if known).</summary>
-        public string ID { get; set; }
+        public string ID { get; }
 
         /// <summary>The update version recommended by the web API based on its version update and mapping rules.</summary>
-        public ModEntryVersionModel SuggestedUpdate { get; set; }
+        public ModEntryVersionModel? SuggestedUpdate { get; set; }
 
         /// <summary>Optional extended data which isn't needed for update checks.</summary>
-        public ModExtendedMetadataModel Metadata { get; set; }
+        public ModExtendedMetadataModel? Metadata { get; set; }
 
         /// <summary>The errors that occurred while fetching update data.</summary>
         public string[] Errors { get; set; } = Array.Empty<string>();
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="id">The mod's unique ID (if known).</param>
+        public ModEntryModel(string id)
+        {
+            this.ID = id;
+        }
     }
 }
diff --git a/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModEntryVersionModel.cs b/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModEntryVersionModel.cs
index 9aac7fd3..a1e78986 100644
--- a/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModEntryVersionModel.cs
+++ b/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModEntryVersionModel.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using Newtonsoft.Json;
 using StardewModdingAPI.Toolkit.Serialization.Converters;
 
@@ -13,19 +11,16 @@ namespace StardewModdingAPI.Toolkit.Framework.Clients.WebApi
         *********/
         /// <summary>The version number.</summary>
         [JsonConverter(typeof(NonStandardSemanticVersionConverter))]
-        public ISemanticVersion Version { get; set; }
+        public ISemanticVersion Version { get; }
 
         /// <summary>The mod page URL.</summary>
-        public string Url { get; set; }
+        public string Url { get; }
 
 
         /*********
         ** Public methods
         *********/
         /// <summary>Construct an instance.</summary>
-        public ModEntryVersionModel() { }
-
-        /// <summary>Construct an instance.</summary>
         /// <param name="version">The version number.</param>
         /// <param name="url">The mod page URL.</param>
         public ModEntryVersionModel(ISemanticVersion version, string url)
diff --git a/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModExtendedMetadataModel.cs b/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModExtendedMetadataModel.cs
index eb54ec78..272a2063 100644
--- a/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModExtendedMetadataModel.cs
+++ b/src/SMAPI.Toolkit/Framework/Clients/WebApi/ModExtendedMetadataModel.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -23,7 +21,7 @@ namespace StardewModdingAPI.Toolkit.Framework.Clients.WebApi
         public string[] ID { get; set; } = Array.Empty<string>();
 
         /// <summary>The mod's display name.</summary>
-        public string Name { get; set; }
+        public string? Name { get; set; }
 
         /// <summary>The mod ID on Nexus.</summary>
         public int? NexusID { get; set; }
@@ -35,31 +33,31 @@ namespace StardewModdingAPI.Toolkit.Framework.Clients.WebApi
         public int? CurseForgeID { get; set; }
 
         /// <summary>The mod key in the CurseForge mod repo (used in mod page URLs).</summary>
-        public string CurseForgeKey { get; set; }
+        public string? CurseForgeKey { get; set; }
 
         /// <summary>The mod ID in the ModDrop mod repo.</summary>
         public int? ModDropID { get; set; }
 
         /// <summary>The GitHub repository in the form 'owner/repo'.</summary>
-        public string GitHubRepo { get; set; }
+        public string? GitHubRepo { get; set; }
 
         /// <summary>The URL to a non-GitHub source repo.</summary>
-        public string CustomSourceUrl { get; set; }
+        public string? CustomSourceUrl { get; set; }
 
         /// <summary>The custom mod page URL (if applicable).</summary>
-        public string CustomUrl { get; set; }
+        public string? CustomUrl { get; set; }
 
         /// <summary>The main version.</summary>
-        public ModEntryVersionModel Main { get; set; }
+        public ModEntryVersionModel? Main { get; set; }
 
         /// <summary>The latest optional version, if newer than <see cref="Main"/>.</summary>
-        public ModEntryVersionModel Optional { get; set; }
+        public ModEntryVersionModel? Optional { get; set; }
 
         /// <summary>The latest unofficial version, if newer than <see cref="Main"/> and <see cref="Optional"/>.</summary>
-        public ModEntryVersionModel Unofficial { get; set; }
+        public ModEntryVersionModel? Unofficial { get; set; }
 
         /// <summary>The latest unofficial version for the current Stardew Valley or SMAPI beta, if any.</summary>
-        public ModEntryVersionModel UnofficialForBeta { get; set; }
+        public ModEntryVersionModel? UnofficialForBeta { get; set; }
 
         /****
         ** Stable compatibility
@@ -69,10 +67,10 @@ namespace StardewModdingAPI.Toolkit.Framework.Clients.WebApi
         public WikiCompatibilityStatus? CompatibilityStatus { get; set; }
 
         /// <summary>The human-readable summary of the compatibility status or workaround, without HTML formatting.</summary>
-        public string CompatibilitySummary { get; set; }
+        public string? CompatibilitySummary { get; set; }
 
         /// <summary>The game or SMAPI version which broke this mod, if applicable.</summary>
-        public string BrokeIn { get; set; }
+        public string? BrokeIn { get; set; }
 
         /****
         ** Beta compatibility
@@ -82,22 +80,22 @@ namespace StardewModdingAPI.Toolkit.Framework.Clients.WebApi
         public WikiCompatibilityStatus? BetaCompatibilityStatus { get; set; }
 
         /// <summary>The human-readable summary of the compatibility status or workaround for the Stardew Valley beta (if any), without HTML formatting.</summary>
-        public string BetaCompatibilitySummary { get; set; }
+        publi