diff --git a/OpenRA.Mods.Common/Effects/Contrail.cs b/OpenRA.Mods.Common/Effects/Contrail.cs
index 0a784df1a3..c5c54df60b 100644
--- a/OpenRA.Mods.Common/Effects/Contrail.cs
+++ b/OpenRA.Mods.Common/Effects/Contrail.cs
@@ -36,9 +36,11 @@ namespace OpenRA.Mods.Common.Effects
 
 	class Contrail : ITick, IRender
 	{
-		ContrailInfo info;
+		readonly ContrailInfo info;
+		readonly IBodyOrientation body;
+
+		// This is a mutable struct, so it can't be readonly.
 		ContrailRenderable trail;
-		IBodyOrientation body;
 
 		public Contrail(Actor self, ContrailInfo info)
 		{
diff --git a/OpenRA.Mods.Common/Traits/ExternalCapturableBar.cs b/OpenRA.Mods.Common/Traits/ExternalCapturableBar.cs
index 02651c10f9..2634392f15 100644
--- a/OpenRA.Mods.Common/Traits/ExternalCapturableBar.cs
+++ b/OpenRA.Mods.Common/Traits/ExternalCapturableBar.cs
@@ -21,20 +21,20 @@ namespace OpenRA.Mods.Common.Traits
 
 	class ExternalCapturableBar : ISelectionBar
 	{
-		ExternalCapturable cap;
+		readonly ExternalCapturable capturable;
 
 		public ExternalCapturableBar(Actor self)
 		{
-			this.cap = self.Trait<ExternalCapturable>();
+			capturable = self.Trait<ExternalCapturable>();
 		}
 
 		public float GetValue()
 		{
 			// only show when building is being captured
-			if (!cap.CaptureInProgress)
+			if (!capturable.CaptureInProgress)
 				return 0f;
 
-			return (float)cap.CaptureProgressTime / (cap.Info.CaptureCompleteTime * 25);
+			return (float)capturable.CaptureProgressTime / (capturable.Info.CaptureCompleteTime * 25);
 		}
 
 		public Color GetColor() { return Color.Orange; }
diff --git a/OpenRA.Mods.Common/Traits/Render/RenderBuildingTurreted.cs b/OpenRA.Mods.Common/Traits/Render/RenderBuildingTurreted.cs
index 337847128a..413ee3f180 100644
--- a/OpenRA.Mods.Common/Traits/Render/RenderBuildingTurreted.cs
+++ b/OpenRA.Mods.Common/Traits/Render/RenderBuildingTurreted.cs
@@ -36,7 +36,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	class RenderBuildingTurreted : RenderBuilding
 	{
-		Turreted t;
+		readonly Turreted turreted;
 
 		static Func<int> MakeTurretFacingFunc(Actor self)
 		{
@@ -48,14 +48,14 @@ namespace OpenRA.Mods.Common.Traits
 		public RenderBuildingTurreted(ActorInitializer init, RenderBuildingInfo info)
 			: base(init, info, MakeTurretFacingFunc(init.Self))
 		{
-			t = init.Self.TraitsImplementing<Turreted>().FirstOrDefault();
-			t.QuantizedFacings = DefaultAnimation.CurrentSequence.Facings;
+			turreted = init.Self.TraitsImplementing<Turreted>().FirstOrDefault();
+			turreted.QuantizedFacings = DefaultAnimation.CurrentSequence.Facings;
 		}
 
 		public override void DamageStateChanged(Actor self, AttackInfo e)
 		{
 			base.DamageStateChanged(self, e);
-			t.QuantizedFacings = DefaultAnimation.CurrentSequence.Facings;
+			turreted.QuantizedFacings = DefaultAnimation.CurrentSequence.Facings;
 		}
 	}
 }
diff --git a/OpenRA.Mods.Common/Traits/Render/RenderDetectionCircle.cs b/OpenRA.Mods.Common/Traits/Render/RenderDetectionCircle.cs
index 2c5e25b266..33f49144ef 100644
--- a/OpenRA.Mods.Common/Traits/Render/RenderDetectionCircle.cs
+++ b/OpenRA.Mods.Common/Traits/Render/RenderDetectionCircle.cs
@@ -24,7 +24,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	class RenderDetectionCircle : IPostRenderSelection
 	{
-		Actor self;
+		readonly Actor self;
 
 		public RenderDetectionCircle(Actor self) { this.self = self; }
 
diff --git a/OpenRA.Mods.Common/Traits/Render/RenderRangeCircle.cs b/OpenRA.Mods.Common/Traits/Render/RenderRangeCircle.cs
index 387a98b926..bf205f39b8 100644
--- a/OpenRA.Mods.Common/Traits/Render/RenderRangeCircle.cs
+++ b/OpenRA.Mods.Common/Traits/Render/RenderRangeCircle.cs
@@ -56,8 +56,8 @@ namespace OpenRA.Mods.Common.Traits
 
 	class RenderRangeCircle : IPostRenderSelection
 	{
-		Actor self;
-		AttackBase attack;
+		readonly Actor self;
+		readonly AttackBase attack;
 
 		public RenderRangeCircle(Actor self)
 		{
diff --git a/OpenRA.Mods.Common/Traits/Render/RenderVoxels.cs b/OpenRA.Mods.Common/Traits/Render/RenderVoxels.cs
index 0895f1654f..42adab4235 100644
--- a/OpenRA.Mods.Common/Traits/Render/RenderVoxels.cs
+++ b/OpenRA.Mods.Common/Traits/Render/RenderVoxels.cs
@@ -73,11 +73,11 @@ namespace OpenRA.Mods.Common.Traits
 	public class RenderVoxels : IRender, INotifyOwnerChanged
 	{
 		readonly List<VoxelAnimation> components = new List<VoxelAnimation>();
-		Actor self;
-		RenderVoxelsInfo info;
-		IBodyOrientation body;
-		WRot camera;
-		WRot lightSource;
+		readonly Actor self;
+		readonly RenderVoxelsInfo info;
+		readonly IBodyOrientation body;
+		readonly WRot camera;
+		readonly WRot lightSource;
 
 		public RenderVoxels(Actor self, RenderVoxelsInfo info)
 		{
diff --git a/OpenRA.Mods.Common/Traits/Render/WithBuildingExplosion.cs b/OpenRA.Mods.Common/Traits/Render/WithBuildingExplosion.cs
index c5f90e386b..bd9e55d052 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithBuildingExplosion.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithBuildingExplosion.cs
@@ -35,8 +35,8 @@ namespace OpenRA.Mods.Common.Traits
 
 	class WithBuildingExplosion : INotifyKilled
 	{
-		WithBuildingExplosionInfo info;
-		BuildingInfo buildingInfo;
+		readonly WithBuildingExplosionInfo info;
+		readonly BuildingInfo buildingInfo;
 
 		public WithBuildingExplosion(Actor self, WithBuildingExplosionInfo info)
 		{
diff --git a/OpenRA.Mods.Common/Traits/Render/WithBuildingPlacedAnimation.cs b/OpenRA.Mods.Common/Traits/Render/WithBuildingPlacedAnimation.cs
index 783a3c6a4b..0e5b19bd93 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithBuildingPlacedAnimation.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithBuildingPlacedAnimation.cs
@@ -23,8 +23,8 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithBuildingPlacedAnimation : INotifyBuildingPlaced, INotifyBuildComplete
 	{
-		WithBuildingPlacedAnimationInfo info;
-		RenderSimple renderSimple;
+		readonly WithBuildingPlacedAnimationInfo info;
+		readonly RenderSimple renderSimple;
 		bool buildComplete;
 
 		public WithBuildingPlacedAnimation(Actor self, WithBuildingPlacedAnimationInfo info)
diff --git a/OpenRA.Mods.Common/Traits/Render/WithBuildingPlacedOverlay.cs b/OpenRA.Mods.Common/Traits/Render/WithBuildingPlacedOverlay.cs
index f886e826a7..87cf8a5ae6 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithBuildingPlacedOverlay.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithBuildingPlacedOverlay.cs
@@ -33,7 +33,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithBuildingPlacedOverlay : INotifyBuildComplete, INotifySold, INotifyDamageStateChanged, INotifyBuildingPlaced, INotifyTransform
 	{
-		Animation overlay;
+		readonly Animation overlay;
 		bool buildComplete;
 
 		public WithBuildingPlacedOverlay(Actor self, WithBuildingPlacedOverlayInfo info)
diff --git a/OpenRA.Mods.Common/Traits/Render/WithHarvestOverlay.cs b/OpenRA.Mods.Common/Traits/Render/WithHarvestOverlay.cs
index ce6ed88fa1..295eadc1bb 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithHarvestOverlay.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithHarvestOverlay.cs
@@ -30,8 +30,8 @@ namespace OpenRA.Mods.Common.Traits
 
 	class WithHarvestOverlay : INotifyHarvesterAction
 	{
-		WithHarvestOverlayInfo info;
-		Animation anim;
+		readonly WithHarvestOverlayInfo info;
+		readonly Animation anim;
 		bool visible;
 
 		public WithHarvestOverlay(Actor self, WithHarvestOverlayInfo info)
diff --git a/OpenRA.Mods.Common/Traits/Render/WithIdleOverlay.cs b/OpenRA.Mods.Common/Traits/Render/WithIdleOverlay.cs
index 5a8014f3a3..e1e148a937 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithIdleOverlay.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithIdleOverlay.cs
@@ -59,7 +59,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithIdleOverlay : UpgradableTrait<WithIdleOverlayInfo>, INotifyDamageStateChanged, INotifyBuildComplete, INotifySold, INotifyTransform
 	{
-		Animation overlay;
+		readonly Animation overlay;
 		bool buildComplete;
 
 		public WithIdleOverlay(Actor self, WithIdleOverlayInfo info)
diff --git a/OpenRA.Mods.Common/Traits/Render/WithProductionOverlay.cs b/OpenRA.Mods.Common/Traits/Render/WithProductionOverlay.cs
index 2bf8988f92..da73cf03dd 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithProductionOverlay.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithProductionOverlay.cs
@@ -36,7 +36,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithProductionOverlay : INotifyDamageStateChanged, ITick, INotifyBuildComplete, INotifySold
 	{
-		Animation overlay;
+		readonly Animation overlay;
 		ProductionQueue queue;
 		bool buildComplete;
 
diff --git a/OpenRA.Mods.Common/Traits/Render/WithRepairAnimation.cs b/OpenRA.Mods.Common/Traits/Render/WithRepairAnimation.cs
index 7e4eecd030..d42abf1a1a 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithRepairAnimation.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithRepairAnimation.cs
@@ -27,7 +27,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithRepairAnimation : INotifyRepair
 	{
-		WithRepairAnimationInfo info;
+		readonly WithRepairAnimationInfo info;
 
 		public WithRepairAnimation(Actor self, WithRepairAnimationInfo info)
 		{
diff --git a/OpenRA.Mods.Common/Traits/Render/WithRepairOverlay.cs b/OpenRA.Mods.Common/Traits/Render/WithRepairOverlay.cs
index bb3d8b9b9b..fccd3e285f 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithRepairOverlay.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithRepairOverlay.cs
@@ -37,7 +37,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithRepairOverlay : INotifyDamageStateChanged, INotifyBuildComplete, INotifySold, INotifyRepair
 	{
-		Animation overlay;
+		readonly Animation overlay;
 		bool buildComplete;
 
 		public WithRepairOverlay(Actor self, WithRepairOverlayInfo info)
diff --git a/OpenRA.Mods.Common/Traits/Render/WithRotor.cs b/OpenRA.Mods.Common/Traits/Render/WithRotor.cs
index 8db1df98a1..6a7f98693c 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithRotor.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithRotor.cs
@@ -45,9 +45,9 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithRotor : ITick
 	{
-		WithRotorInfo info;
-		Animation rotorAnim;
-		IMove movement;
+		readonly WithRotorInfo info;
+		readonly Animation rotorAnim;
+		readonly IMove movement;
 
 		public WithRotor(Actor self, WithRotorInfo info)
 		{
diff --git a/OpenRA.Mods.Common/Traits/Render/WithTurret.cs b/OpenRA.Mods.Common/Traits/Render/WithTurret.cs
index 8ff1251a6e..2259e1aa3e 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithTurret.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithTurret.cs
@@ -58,11 +58,11 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithTurret : UpgradableTrait<WithTurretInfo>, ITick, INotifyDamageStateChanged
 	{
-		RenderSprites rs;
-		IBodyOrientation body;
-		AttackBase ab;
-		Turreted t;
-		Armament[] arms;
+		readonly RenderSprites rs;
+		readonly IBodyOrientation body;
+		readonly AttackBase ab;
+		readonly Turreted t;
+		readonly Armament[] arms;
 		public readonly Animation DefaultAnimation;
 
 		public WithTurret(Actor self, WithTurretInfo info)
diff --git a/OpenRA.Mods.Common/Traits/Render/WithVoxelBarrel.cs b/OpenRA.Mods.Common/Traits/Render/WithVoxelBarrel.cs
index 621af1dce1..4aaf785543 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithVoxelBarrel.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithVoxelBarrel.cs
@@ -48,11 +48,11 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithVoxelBarrel
 	{
-		WithVoxelBarrelInfo info;
-		Actor self;
-		Armament armament;
-		Turreted turreted;
-		IBodyOrientation body;
+		readonly WithVoxelBarrelInfo info;
+		readonly Actor self;
+		readonly Armament armament;
+		readonly Turreted turreted;
+		readonly IBodyOrientation body;
 
 		public WithVoxelBarrel(Actor self, WithVoxelBarrelInfo info)
 		{
diff --git a/OpenRA.Mods.Common/Traits/Render/WithVoxelBody.cs b/OpenRA.Mods.Common/Traits/Render/WithVoxelBody.cs
index ea840465b4..0f44be7fae 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithVoxelBody.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithVoxelBody.cs
@@ -39,7 +39,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithVoxelBody : IAutoSelectionSize
 	{
-		int2 size;
+		readonly int2 size;
 
 		public WithVoxelBody(Actor self, WithVoxelBodyInfo info)
 		{
diff --git a/OpenRA.Mods.Common/Traits/SmokeTrailWhenDamaged.cs b/OpenRA.Mods.Common/Traits/SmokeTrailWhenDamaged.cs
index a62020d088..26ca8f5d0b 100644
--- a/OpenRA.Mods.Common/Traits/SmokeTrailWhenDamaged.cs
+++ b/OpenRA.Mods.Common/Traits/SmokeTrailWhenDamaged.cs
@@ -28,8 +28,8 @@ namespace OpenRA.Mods.Common.Traits
 
 	class SmokeTrailWhenDamaged : ITick
 	{
-		IBodyOrientation body;
-		SmokeTrailWhenDamagedInfo info;
+		readonly SmokeTrailWhenDamagedInfo info;
+		readonly IBodyOrientation body;
 		int ticks;
 
 		public SmokeTrailWhenDamaged(Actor self, SmokeTrailWhenDamagedInfo info)
diff --git a/OpenRA.Mods.Common/Traits/SupportPowers/NukePower.cs b/OpenRA.Mods.Common/Traits/SupportPowers/NukePower.cs
index ac3548ffdd..cf4d141ab7 100644
--- a/OpenRA.Mods.Common/Traits/SupportPowers/NukePower.cs
+++ b/OpenRA.Mods.Common/Traits/SupportPowers/NukePower.cs
@@ -53,7 +53,7 @@ namespace OpenRA.Mods.Common.Traits
 	class NukePower : SupportPower
 	{
 		readonly NukePowerInfo info;
-		IBodyOrientation body;
+		readonly IBodyOrientation body;
 
 		public NukePower(Actor self, NukePowerInfo info)
 			: base(self, info)