diff --git a/.editorconfig b/.editorconfig
index 2d9d943985..e594195d29 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -216,6 +216,9 @@ dotnet_diagnostic.IDE0120.severity = warning
 # Use tuple to swap values.
 dotnet_diagnostic.IDE0180.severity = warning
 
+# Make struct 'readonly'.
+dotnet_diagnostic.IDE0250.severity = warning
+
 # Naming rule violation.
 dotnet_diagnostic.IDE1006.severity = warning
 
diff --git a/OpenRA.Game/Network/SyncReport.cs b/OpenRA.Game/Network/SyncReport.cs
index 7eb2097a7e..eb7be5f125 100644
--- a/OpenRA.Game/Network/SyncReport.cs
+++ b/OpenRA.Game/Network/SyncReport.cs
@@ -188,7 +188,7 @@ namespace OpenRA.Network
 			public (string[] Names, Values Values) NamesValues;
 		}
 
-		struct TypeInfo
+		readonly struct TypeInfo
 		{
 			static readonly ParameterExpression SyncParam = Expression.Parameter(typeof(ISync), "sync");
 			static readonly ConstantExpression NullString = Expression.Constant(null, typeof(string));
diff --git a/OpenRA.Mods.Common/Traits/World/ResourceLayer.cs b/OpenRA.Mods.Common/Traits/World/ResourceLayer.cs
index 7c6eb8f793..498eedee5f 100644
--- a/OpenRA.Mods.Common/Traits/World/ResourceLayer.cs
+++ b/OpenRA.Mods.Common/Traits/World/ResourceLayer.cs
@@ -17,7 +17,7 @@ using OpenRA.Traits;
 
 namespace OpenRA.Mods.Common.Traits
 {
-	public struct ResourceLayerContents
+	public readonly struct ResourceLayerContents
 	{
 		public static readonly ResourceLayerContents Empty = default;
 		public readonly string Type;