Fix IDE0251

.editorconfig

@@ -333,6 +333,10 @@ dotnet_diagnostic.IDE0064.severity = warning
 #csharp_style_prefer_readonly_struct = true
 dotnet_diagnostic.IDE0250.severity = warning
 
+# IDE0251 Member can be made 'readonly'
+#csharp_style_prefer_readonly_struct_member = true
+dotnet_diagnostic.IDE0251.severity = warning
+
 ## Null-checking preferences
 
 # IDE1005 Use conditional delegate call

OpenRA.Game/Exts.cs

@@ -600,7 +600,7 @@ namespace OpenRA
 			Current = default;
 		}
 
-		public LineSplitEnumerator GetEnumerator() => this;
+		public readonly LineSplitEnumerator GetEnumerator() => this;
 
 		public bool MoveNext()
 		{

OpenRA.Game/Map/CellRegion.cs

@@ -162,8 +162,8 @@ namespace OpenRA
 			}
 
 			public CPos Current { get; private set; }
-			object IEnumerator.Current => Current;
-			public void Dispose() { }
+			readonly object IEnumerator.Current => Current;
+			public readonly void Dispose() { }
 		}
 	}
 }

OpenRA.Game/Map/MapCoordsRegion.cs

@@ -53,8 +53,8 @@ namespace OpenRA
 			}
 
 			public MPos Current { get; private set; }
-			object IEnumerator.Current => Current;
-			public void Dispose() { }
+			readonly object IEnumerator.Current => Current;
+			public readonly void Dispose() { }
 		}
 
 		public MapCoordsRegion(MPos mapTopLeft, MPos mapBottomRight)

OpenRA.Game/Map/ProjectedCellRegion.cs

@@ -118,8 +118,8 @@ namespace OpenRA
 			}
 
 			public PPos Current { get; private set; }
-			object IEnumerator.Current => Current;
-			public void Dispose() { }
+			readonly object IEnumerator.Current => Current;
+			public readonly void Dispose() { }
 		}
 	}
 }

OpenRA.Game/Network/OrderManager.cs

@@ -70,7 +70,7 @@ namespace OpenRA.Network
 			public int Client;
 			public Order Order;
 
-			public override string ToString()
+			public override readonly string ToString()
 			{
 				return $"ClientId: {Client} {Order}";
 			}

OpenRA.Game/Network/SyncReport.cs

@@ -300,7 +300,7 @@ namespace OpenRA.Network
 
 			public object this[int index]
 			{
-				get
+				readonly get
 				{
 					if (item2OrSentinel == Sentinel)
 						return ((object[])item1OrArray)[index];

OpenRA.Game/Primitives/Rectangle.cs

@@ -58,35 +58,35 @@ namespace OpenRA.Primitives
 			Height = size.Height;
 		}
 
-		public int Left => X;
-		public int Right => X + Width;
-		public int Top => Y;
-		public int Bottom => Y + Height;
-		public bool IsEmpty => X == 0 && Y == 0 && Width == 0 && Height == 0;
-		public int2 Location => new(X, Y);
-		public Size Size => new(Width, Height);
+		public readonly int Left => X;
+		public readonly int Right => X + Width;
+		public readonly int Top => Y;
+		public readonly int Bottom => Y + Height;
+		public readonly bool IsEmpty => X == 0 && Y == 0 && Width == 0 && Height == 0;
+		public readonly int2 Location => new(X, Y);
+		public readonly Size Size => new(Width, Height);
 
-		public int2 TopLeft => Location;
-		public int2 TopRight => new(X + Width, Y);
-		public int2 BottomLeft => new(X, Y + Height);
-		public int2 BottomRight => new(X + Width, Y + Height);
+		public readonly int2 TopLeft => Location;
+		public readonly int2 TopRight => new(X + Width, Y);
+		public readonly int2 BottomLeft => new(X, Y + Height);
+		public readonly int2 BottomRight => new(X + Width, Y + Height);
 
-		public bool Contains(int x, int y)
+		public readonly bool Contains(int x, int y)
 		{
 			return x >= Left && x < Right && y >= Top && y < Bottom;
 		}
 
-		public bool Contains(int2 pt)
+		public readonly bool Contains(int2 pt)
 		{
 			return Contains(pt.X, pt.Y);
 		}
 
-		public bool Equals(Rectangle other)
+		public readonly bool Equals(Rectangle other)
 		{
 			return this == other;
 		}
 
-		public override bool Equals(object obj)
+		public override readonly bool Equals(object obj)
 		{
 			if (obj is not Rectangle)
 				return false;
@@ -94,17 +94,17 @@ namespace OpenRA.Primitives
 			return this == (Rectangle)obj;
 		}
 
-		public override int GetHashCode()
+		public override readonly int GetHashCode()
 		{
 			return Height + Width ^ X + Y;
 		}
 
-		public bool IntersectsWith(Rectangle rect)
+		public readonly bool IntersectsWith(Rectangle rect)
 		{
 			return Left < rect.Right && Right > rect.Left && Top < rect.Bottom && Bottom > rect.Top;
 		}
 
-		bool IntersectsWithInclusive(Rectangle r)
+		readonly bool IntersectsWithInclusive(Rectangle r)
 		{
 			return Left <= r.Right && Right >= r.Left && Top <= r.Bottom && Bottom >= r.Top;
 		}
@@ -117,14 +117,14 @@ namespace OpenRA.Primitives
 			return FromLTRB(Math.Max(a.Left, b.Left), Math.Max(a.Top, b.Top), Math.Min(a.Right, b.Right), Math.Min(a.Bottom, b.Bottom));
 		}
 
-		public bool Contains(Rectangle rect)
+		public readonly bool Contains(Rectangle rect)
 		{
 			return rect == Intersect(this, rect);
 		}
 
 		public static Rectangle operator *(int a, Rectangle b) { return new Rectangle(a * b.X, a * b.Y, a * b.Width, a * b.Height); }
 
-		public override string ToString()
+		public override readonly string ToString()
 		{
 			return $"{X},{Y},{Width},{Height}";
 		}

OpenRA.Game/TraitDictionary.cs

@@ -210,9 +210,9 @@ namespace OpenRA
 
 				public void Reset() { index = actors.BinarySearchMany(actor) - 1; }
 				public bool MoveNext() { return ++index < actors.Count && actors[index].ActorID == actor; }
-				public T Current => traits[index];
-				object System.Collections.IEnumerator.Current => Current;
-				public void Dispose() { }
+				public readonly T Current => traits[index];
+				readonly object System.Collections.IEnumerator.Current => Current;
+				public readonly void Dispose() { }
 			}
 
 			public IEnumerable<TraitPair<T>> All()
@@ -276,9 +276,9 @@ namespace OpenRA
 
 				public void Reset() { index = -1; }
 				public bool MoveNext() { return ++index < actors.Count; }
-				public TraitPair<T> Current => new(actors[index], traits[index]);
-				object System.Collections.IEnumerator.Current => Current;
-				public void Dispose() { }
+				public readonly TraitPair<T> Current => new(actors[index], traits[index]);
+				readonly object System.Collections.IEnumerator.Current => Current;
+				public readonly void Dispose() { }
 			}
 
 			public void RemoveActor(uint actor)

OpenRA.Mods.Common/Traits/World/ActorMap.cs

@@ -239,8 +239,8 @@ namespace OpenRA.Mods.Common.Traits
 			public void Reset() { throw new NotSupportedException(); }
 			public Actor Current { get; private set; }
 
-			object IEnumerator.Current => Current;
-			public void Dispose() { }
+			readonly object IEnumerator.Current => Current;
+			public readonly void Dispose() { }
 			public bool MoveNext()
 			{
 				while (node != null)