diff --git a/OpenRA.Mods.Common/Traits/Air/Aircraft.cs b/OpenRA.Mods.Common/Traits/Air/Aircraft.cs
index f41a7954ba..94cc133711 100644
--- a/OpenRA.Mods.Common/Traits/Air/Aircraft.cs
+++ b/OpenRA.Mods.Common/Traits/Air/Aircraft.cs
@@ -283,7 +283,8 @@ namespace OpenRA.Mods.Common.Traits
 
 		IEnumerable<CPos> landingCells = Enumerable.Empty<CPos>();
 		bool requireForceMove;
-		int creationActivityDelay;
+
+		readonly int creationActivityDelay;
 
 		bool notify = true;
 
diff --git a/OpenRA.Mods.Common/Traits/Armament.cs b/OpenRA.Mods.Common/Traits/Armament.cs
index a4844b22e2..4a467e0c34 100644
--- a/OpenRA.Mods.Common/Traits/Armament.cs
+++ b/OpenRA.Mods.Common/Traits/Armament.cs
@@ -125,9 +125,9 @@ namespace OpenRA.Mods.Common.Traits
 
 		int ticksSinceLastShot;
 		int currentBarrel;
-		int barrelCount;
+		readonly int barrelCount;
 
-		List<(int Ticks, Action Func)> delayedActions = new List<(int, Action)>();
+		readonly List<(int Ticks, Action Func)> delayedActions = new List<(int, Action)>();
 
 		public WDist Recoil;
 		public int FireDelay { get; protected set; }
diff --git a/OpenRA.Mods.Common/Traits/Buildings/Building.cs b/OpenRA.Mods.Common/Traits/Buildings/Building.cs
index 5cf89569c8..158cf2b120 100644
--- a/OpenRA.Mods.Common/Traits/Buildings/Building.cs
+++ b/OpenRA.Mods.Common/Traits/Buildings/Building.cs
@@ -274,9 +274,9 @@ namespace OpenRA.Mods.Common.Traits
 		readonly Actor self;
 		readonly BuildingInfluence influence;
 
-		(CPos, SubCell)[] occupiedCells;
-		(CPos, SubCell)[] targetableCells;
-		CPos[] transitOnlyCells;
+		readonly (CPos, SubCell)[] occupiedCells;
+		readonly (CPos, SubCell)[] targetableCells;
+		readonly CPos[] transitOnlyCells;
 
 		public CPos TopLeft => topLeft;
 		public WPos CenterPosition { get; private set; }
diff --git a/OpenRA.Mods.Common/Traits/Palettes/IndexedPalette.cs b/OpenRA.Mods.Common/Traits/Palettes/IndexedPalette.cs
index 88100cc0f5..97977aaa66 100644
--- a/OpenRA.Mods.Common/Traits/Palettes/IndexedPalette.cs
+++ b/OpenRA.Mods.Common/Traits/Palettes/IndexedPalette.cs
@@ -70,8 +70,8 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class IndexedColorRemap : IPaletteRemap
 	{
-		Dictionary<int, int> replacements = new Dictionary<int, int>();
-		IPalette basePalette;
+		readonly Dictionary<int, int> replacements = new Dictionary<int, int>();
+		readonly IPalette basePalette;
 
 		public IndexedColorRemap(IPalette basePalette, int[] ramp, int[] remap)
 		{
diff --git a/OpenRA.Mods.Common/Traits/Player/ProvidesTechPrerequisite.cs b/OpenRA.Mods.Common/Traits/Player/ProvidesTechPrerequisite.cs
index 2d51fa78e3..423b126cfb 100644
--- a/OpenRA.Mods.Common/Traits/Player/ProvidesTechPrerequisite.cs
+++ b/OpenRA.Mods.Common/Traits/Player/ProvidesTechPrerequisite.cs
@@ -35,7 +35,7 @@ namespace OpenRA.Mods.Common.Traits
 	public class ProvidesTechPrerequisite : ITechTreePrerequisite
 	{
 		readonly ProvidesTechPrerequisiteInfo info;
-		bool enabled;
+		readonly bool enabled;
 
 		static readonly string[] NoPrerequisites = Array.Empty<string>();