diff --git a/osu.Game/Online/API/Requests/GetMessagesRequest.cs b/osu.Game/Online/API/Requests/GetMessagesRequest.cs
index d600f40716..68de194bae 100644
--- a/osu.Game/Online/API/Requests/GetMessagesRequest.cs
+++ b/osu.Game/Online/API/Requests/GetMessagesRequest.cs
@@ -11,7 +11,7 @@ namespace osu.Game.Online.API.Requests
     public class GetMessagesRequest : APIRequest<List<Message>>
     {
         private readonly List<Channel> channels;
-        private long? since;
+        private readonly long? since;
 
         public GetMessagesRequest(List<Channel> channels, long? sinceId)
         {
diff --git a/osu.Game/Online/API/Requests/GetUserRequest.cs b/osu.Game/Online/API/Requests/GetUserRequest.cs
index 9026d10334..607e8e5127 100644
--- a/osu.Game/Online/API/Requests/GetUserRequest.cs
+++ b/osu.Game/Online/API/Requests/GetUserRequest.cs
@@ -7,7 +7,7 @@ namespace osu.Game.Online.API.Requests
 {
     public class GetUserRequest : APIRequest<User>
     {
-        private long? userId;
+        private readonly long? userId;
 
         public GetUserRequest(long? userId = null)
         {
diff --git a/osu.Game/Screens/Edit/Screens/Compose/Layers/MaskContainer.cs b/osu.Game/Screens/Edit/Screens/Compose/Layers/MaskContainer.cs
index 6d75b8dc15..993594f1d2 100644
--- a/osu.Game/Screens/Edit/Screens/Compose/Layers/MaskContainer.cs
+++ b/osu.Game/Screens/Edit/Screens/Compose/Layers/MaskContainer.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using JetBrains.Annotations;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Input;
@@ -43,6 +44,8 @@ namespace osu.Game.Screens.Edit.Screens.Compose.Layers
 
         public override void Add(HitObjectMask drawable)
         {
+            if (drawable == null) throw new ArgumentNullException(nameof(drawable));
+
             base.Add(drawable);
 
             drawable.Selected += onMaskSelected;
@@ -51,8 +54,10 @@ namespace osu.Game.Screens.Edit.Screens.Compose.Layers
             drawable.DragRequested += onDragRequested;
         }
 
-        public override bool Remove(HitObjectMask drawable)
+        public override bool Remove([NotNull] HitObjectMask drawable)
         {
+            if (drawable == null) throw new ArgumentNullException(nameof(drawable));
+
             var result = base.Remove(drawable);
 
             if (result)
diff --git a/osu.Game/Screens/Ranking/ResultsPageScore.cs b/osu.Game/Screens/Ranking/ResultsPageScore.cs
index 9d92439a4b..42d8af07b9 100644
--- a/osu.Game/Screens/Ranking/ResultsPageScore.cs
+++ b/osu.Game/Screens/Ranking/ResultsPageScore.cs
@@ -223,11 +223,11 @@ namespace osu.Game.Screens.Ranking
 
         private class DateTimeDisplay : Container
         {
-            private DateTime datetime;
+            private readonly DateTime date;
 
-            public DateTimeDisplay(DateTime datetime)
+            public DateTimeDisplay(DateTime date)
             {
-                this.datetime = datetime;
+                this.date = date;
 
                 AutoSizeAxes = Axes.Y;
 
@@ -251,7 +251,7 @@ namespace osu.Game.Screens.Ranking
                     {
                         Origin = Anchor.CentreLeft,
                         Anchor = Anchor.CentreLeft,
-                        Text = datetime.ToShortDateString(),
+                        Text = date.ToShortDateString(),
                         Padding = new MarginPadding { Horizontal = 10, Vertical = 5 },
                         Colour = Color4.White,
                     },
@@ -259,7 +259,7 @@ namespace osu.Game.Screens.Ranking
                     {
                         Origin = Anchor.CentreRight,
                         Anchor = Anchor.CentreRight,
-                        Text = datetime.ToShortTimeString(),
+                        Text = date.ToShortTimeString(),
                         Padding = new MarginPadding { Horizontal = 10, Vertical = 5 },
                         Colour = Color4.White,
                     }