Update LangVersion to C# 9.

mono was the bottleneck restricting our ability to use a newer C# version. mono 6.12 is currently available. Although poorly documented on their website, this supports C# 9. https://www.mono-project.com/docs/about-mono/versioning/#mono-source-versioning indicates mono 6.12 uses Roslyn 3.9.0. https://github.com/dotnet/roslyn/blob/main/docs/wiki/NuGet-packages.md#versioning indicates Roslyn 3.9.0 supports C# 9.

This unlocks C# 8 and C# 9 features previously unavailable to us.
- https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-80
- https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-9

A newer version of StyleCop is required to avoid rules tripping up on the new syntax. Currently only prerelease versions are available but their use is encouraged DotNetAnalyzers/StyleCopAnalyzers#3420 (comment)

Fix style rule violations on existing rules where the newer language version makes some existing casts redundant or allows use of the null coalescing assignment operator.

.editorconfig

@@ -63,7 +63,7 @@ dotnet_diagnostic.IDE0044.severity = warning
 
 # IDE0062 Make local function static
 #csharp_prefer_static_local_function = true
-dotnet_diagnostic.IDE0062.severity = silent # Requires C# 8
+dotnet_diagnostic.IDE0062.severity = silent # Requires C# 8 - TODO Consider enabling
 
 # IDE0064 Make struct fields writable
 # No options
@@ -137,15 +137,15 @@ dotnet_diagnostic.IDE0050.severity = silent
 # IDE0054/IDE0074 Use compound assignment/Use coalesce compound assignment
 #dotnet_style_prefer_compound_assignment = true
 dotnet_diagnostic.IDE0054.severity = warning
-dotnet_diagnostic.IDE0074.severity = silent # Requires C# 8
+dotnet_diagnostic.IDE0074.severity = silent # Requires C# 8 - TODO Consider enabling
 
 # IDE0056 Use index operator
 #csharp_style_prefer_index_operator = true
-dotnet_diagnostic.IDE0056.severity = silent # Requires C# 8
+dotnet_diagnostic.IDE0056.severity = silent # Requires C# 8 - TODO Consider enabling
 
 # IDE0057 Use range operator
 #csharp_style_prefer_range_operator = true
-dotnet_diagnostic.IDE0057.severity = silent # Requires C# 8
+dotnet_diagnostic.IDE0057.severity = silent # Requires C# 8 - TODO Consider enabling
 
 # IDE0070 Use 'System.HashCode.Combine'
 # No options
@@ -169,7 +169,7 @@ dotnet_diagnostic.IDE0082.severity = warning
 
 # IDE0090 Simplify 'new' expression
 #csharp_style_implicit_object_creation_when_type_is_apparent = true
-dotnet_diagnostic.IDE0090.severity = silent # Requires C# 9
+dotnet_diagnostic.IDE0090.severity = silent # Requires C# 9 - TODO Consider enabling
 
 # IDE0180 Use tuple to swap values
 #csharp_style_prefer_tuple_swap = true
@@ -204,7 +204,7 @@ dotnet_diagnostic.IDE0041.severity = warning
 
 # IDE0150 Prefer 'null' check over type check
 #csharp_style_prefer_null_check_over_type_check = true
-dotnet_diagnostic.IDE0150.severity = silent # Requires C# 9
+dotnet_diagnostic.IDE0150.severity = silent # Requires C# 9 - TODO Consider enabling
 
 # IDE1005 Use conditional delegate call
 csharp_style_conditional_delegate_call = true # true is the default, but the rule is not triggered if this is not specified.
@@ -272,11 +272,11 @@ dotnet_diagnostic.IDE0066.severity = silent
 
 # IDE0078 Use pattern matching
 #csharp_style_prefer_pattern_matching = true
-dotnet_diagnostic.IDE0078.severity = silent # Requires C# 9
+dotnet_diagnostic.IDE0078.severity = silent # Requires C# 9 - TODO Consider enabling
 
 # IDE0083 Use pattern matching ('not' operator)
 #csharp_style_prefer_not_pattern = true
-dotnet_diagnostic.IDE0083.severity = silent # Requires C# 9
+dotnet_diagnostic.IDE0083.severity = silent # Requires C# 9 - TODO Consider enabling
 
 # IDE0170 Simplify property pattern
 #csharp_style_prefer_extended_property_pattern = true
@@ -291,7 +291,7 @@ dotnet_diagnostic.IDE0011.severity = none
 
 # IDE0063 Use simple 'using' statement
 #csharp_prefer_simple_using_statement = true
-dotnet_diagnostic.IDE0063.severity = silent # Requires C# 8
+dotnet_diagnostic.IDE0063.severity = silent # Requires C# 8 - TODO Consider enabling
 
 ## 'using' directive preferences
 
@@ -375,7 +375,7 @@ dotnet_diagnostic.IDE0100.severity = warning
 
 # IDE0110 Remove unnecessary discard
 # No options
-dotnet_diagnostic.IDE0110.severity = silent # Requires C# 9
+dotnet_diagnostic.IDE0110.severity = silent # Requires C# 9 - TODO Consider enabling
 
 ### Miscellaneous Rules
 ### https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/miscellaneous-rules
@@ -598,6 +598,11 @@ dotnet_diagnostic.SA1633.severity = none # FileMustHaveHeader
 dotnet_diagnostic.SA1642.severity = none # ConstructorSummaryDocumentationShouldBeginWithStandardText
 dotnet_diagnostic.SA1649.severity = none # FileNameMustMatchTypeName
 
+# Requires C# 8/9 - TODO Consider enabling
+dotnet_diagnostic.SA1141.severity = none # UseTupleSyntax
+dotnet_diagnostic.SA1316.severity = none # TupleElementNamesShouldUseCorrectCasing
+dotnet_diagnostic.SA1414.severity = none # TupleTypesInSignaturesShouldHaveElementNames
+
 #### Code Quality Rules
 #### https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/
 

Directory.Build.props

@@ -5,7 +5,7 @@
     <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <Optimize>true</Optimize>
-    <LangVersion>7.3</LangVersion>
+    <LangVersion>9</LangVersion>
     <DebugSymbols>true</DebugSymbols>
     <EngineRootPath Condition="'$(EngineRootPath)' == ''">..</EngineRootPath>
     <OutputPath>$(EngineRootPath)/bin</OutputPath>
@@ -51,7 +51,6 @@
 
   <!-- StyleCop -->
   <ItemGroup>
-    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.2" PrivateAssets="All" />
-    <PackageReference Include="StyleCop.Analyzers" Version="1.1.118" PrivateAssets="All" />
+    <PackageReference Include="StyleCop.Analyzers" Version="1.2.0-beta.435" PrivateAssets="All" />
   </ItemGroup>
 </Project>

OpenRA.Game/Exts.cs

@@ -149,7 +149,7 @@ public static T RandomOrDefault<T>(this IEnumerable<T> ts, MersenneTwister r)
 		static T Random<T>(IEnumerable<T> ts, MersenneTwister r, bool throws)
 		{
 			var xs = ts as ICollection<T>;
-			xs = xs ?? ts.ToList();
+			xs ??= ts.ToList();
 			if (xs.Count == 0)
 			{
 				if (throws)
@@ -396,8 +396,8 @@ public static Dictionary<TKey, TElement> ToDictionaryWithConflictLog<TSource, TK
 			string debugName, Func<TKey, string> logKey = null, Func<TElement, string> logValue = null)
 		{
 			// Fall back on ToString() if null functions are provided:
-			logKey = logKey ?? (s => s.ToString());
-			logValue = logValue ?? (s => s.ToString());
+			logKey ??= s => s.ToString();
+			logValue ??= s => s.ToString();
 
 			// Try to build a dictionary and log all duplicates found (if any):
 			var dupKeys = new Dictionary<TKey, List<string>>();

OpenRA.Mods.Common/Activities/Move/Move.cs

@@ -116,7 +116,7 @@ protected override void OnFirstRun(Actor self)
 			if (evaluateNearestMovableCell && destination.HasValue)
 			{
 				var movableDestination = mobile.NearestMoveableCell(destination.Value);
-				destination = mobile.CanEnterCell(movableDestination, check: BlockedByActor.Immovable) ? movableDestination : (CPos?)null;
+				destination = mobile.CanEnterCell(movableDestination, check: BlockedByActor.Immovable) ? movableDestination : null;
 			}
 
 			// TODO: Change this to BlockedByActor.Stationary after improving the local avoidance behaviour

OpenRA.Mods.Common/Graphics/DefaultSpriteSequence.cs

@@ -492,7 +492,7 @@ public virtual void ResolveSprites(SpriteCache cache)
 				});
 			}).ToArray();
 
-			length = length ?? allSprites.Length - start;
+			length ??= allSprites.Length - start;
 
 			if (alpha != null)
 			{

OpenRA.Mods.Common/Pathfinder/PathSearch.cs

@@ -68,7 +68,7 @@ public static PathSearch ToTargetCell(
 			else
 				graph = new MapPathGraph(LayerPoolForWorld(world), locomotor, self, world, check, customCost, ignoreActor, laneBias, inReverse);
 
-			heuristic = heuristic ?? DefaultCostEstimator(locomotor, target);
+			heuristic ??= DefaultCostEstimator(locomotor, target);
 			var search = new PathSearch(graph, heuristic, heuristicWeightPercentage, loc => loc == target, recorder);
 
 			AddInitialCells(world, locomotor, froms, customCost, search);

OpenRA.Mods.Common/Scripting/Global/ReinforcementsGlobal.cs

@@ -79,7 +79,7 @@ public Actor[] Reinforce(Player owner, string[] actorTypes, CPos[] entryPath, in
 			for (var i = 0; i < actorTypes.Length; i++)
 			{
 				var af = actionFunc != null ? (LuaFunction)actionFunc.CopyReference() : null;
-				var actor = CreateActor(owner, actorTypes[i], false, entryPath[0], entryPath.Length > 1 ? entryPath[1] : (CPos?)null);
+				var actor = CreateActor(owner, actorTypes[i], false, entryPath[0], entryPath.Length > 1 ? entryPath[1] : null);
 				actors.Add(actor);
 
 				var actionDelay = i * interval;
@@ -118,7 +118,7 @@ public Actor[] Reinforce(Player owner, string[] actorTypes, CPos[] entryPath, in
 		public LuaTable ReinforceWithTransport(Player owner, string actorType, string[] cargoTypes, CPos[] entryPath, CPos[] exitPath = null,
 			LuaFunction actionFunc = null, LuaFunction exitFunc = null, int dropRange = 3)
 		{
-			var transport = CreateActor(owner, actorType, true, entryPath[0], entryPath.Length > 1 ? entryPath[1] : (CPos?)null);
+			var transport = CreateActor(owner, actorType, true, entryPath[0], entryPath.Length > 1 ? entryPath[1] : null);
 			var cargo = transport.TraitOrDefault<Cargo>();
 
 			var passengers = new List<Actor>();

OpenRA.Mods.Common/Scripting/Properties/RepairableBuildingProperties.cs

@@ -29,7 +29,7 @@ public RepairableBuildingProperties(ScriptContext context, Actor self)
 		[Desc("Start repairs on this building. `repairer` can be an allied player.")]
 		public void StartBuildingRepairs(Player repairer = null)
 		{
-			repairer = repairer ?? Self.Owner;
+			repairer ??= Self.Owner;
 
 			if (!rb.Repairers.Contains(repairer))
 				rb.RepairBuilding(Self, repairer);
@@ -38,7 +38,7 @@ public void StartBuildingRepairs(Player repairer = null)
 		[Desc("Stop repairs on this building. `repairer` can be an allied player.")]
 		public void StopBuildingRepairs(Player repairer = null)
 		{
-			repairer = repairer ?? Self.Owner;
+			repairer ??= Self.Owner;
 
 			if (rb.RepairActive && rb.Repairers.Contains(repairer))
 				rb.RepairBuilding(Self, repairer);

OpenRA.Mods.Common/Traits/Cargo.cs

@@ -331,7 +331,7 @@ public string VoicePhraseForOrder(Actor self, Order order)
 
 		public Actor Unload(Actor self, Actor passenger = null)
 		{
-			passenger = passenger ?? cargo.Last();
+			passenger ??= cargo.Last();
 			if (!cargo.Remove(passenger))
 				throw new ArgumentException("Attempted to unload an actor that is not a passenger.");
 

OpenRA.Mods.Common/Traits/Turreted.cs

@@ -71,7 +71,7 @@ public static Func<WAngle> WorldFacingFromInit(IActorInitializer init, TraitInfo
 			if (turretFacingInit != null)
 			{
 				var facing = turretFacingInit.Value;
-				return bodyFacing != null ? (Func<WAngle>)(() => bodyFacing() + facing) : () => facing;
+				return bodyFacing != null ? () => bodyFacing() + facing : () => facing;
 			}
 
 			var dynamicFacingInit = init.GetOrDefault<DynamicTurretFacingInit>(info);

OpenRA.Mods.Common/UpdateRules/Rules/20221203/ExplicitSequenceFilenames.cs

@@ -149,7 +149,7 @@ public override IEnumerable<string> UpdateSequenceNode(ModData modData, MiniYaml
 						continue;
 
 					resolvedSequenceNode.Value.Nodes = MiniYaml.Merge(new[] { resolvedDefaultsNode.Value.Nodes, resolvedSequenceNode.Value.Nodes });
-					resolvedSequenceNode.Value.Value = resolvedSequenceNode.Value.Value ?? resolvedDefaultsNode.Value.Value;
+					resolvedSequenceNode.Value.Value ??= resolvedDefaultsNode.Value.Value;
 				}
 			}
 

OpenRA.Mods.Common/Widgets/Logic/Ingame/MenuButtonsChromeLogic.cs

@@ -101,7 +101,7 @@ void OpenMenuPanel(MenuButtonWidget button, WidgetArgs widgetArgs = null)
 			if (button.DisableWorldSounds)
 				Game.Sound.DisableWorldSounds = true;
 
-			widgetArgs = widgetArgs ?? new WidgetArgs();
+			widgetArgs ??= new WidgetArgs();
 			widgetArgs.Add("onExit", () =>
 			{
 				if (button.HideIngameUI)

OpenRA.Mods.Common/Widgets/Logic/Lobby/LobbyLogic.cs

@@ -234,7 +234,7 @@ internal LobbyLogic(Widget widget, ModData modData, WorldRenderer worldRenderer,
 					{
 						{ "initialMap", modData.MapCache.PickLastModifiedMap(MapVisibility.Lobby) ?? map.Uid },
 						{ "initialTab", MapClassification.System },
-						{ "onExit", Game.IsHost ? (Action)UpdateSelectedMap : modData.MapCache.UpdateMaps },
+						{ "onExit", Game.IsHost ? UpdateSelectedMap : modData.MapCache.UpdateMaps },
 						{ "onSelect", Game.IsHost ? onSelect : null },
 						{ "filter", MapVisibility.Lobby },
 					});