Added readonly modifier (#8386)

Author: glen-84

.editorconfig

@@ -175,6 +175,9 @@ dotnet_diagnostic.IDE0005.severity = warning
 dotnet_diagnostic.IDE0028.severity = warning
 # Order modifiers.
 dotnet_diagnostic.IDE0036.severity = warning
+# Add readonly modifier.
+dotnet_diagnostic.IDE0044.severity = warning
+dotnet_diagnostic.RCS1169.severity = none
 # Remove unused private member.
 dotnet_diagnostic.IDE0051.severity = warning
 dotnet_diagnostic.RCS1213.severity = none

src/HotChocolate/Core/src/Features/PooledFeatureCollection.cs

@@ -15,7 +15,7 @@ public sealed class PooledFeatureCollection : IFeatureCollection
     private static readonly KeyComparer s_featureKeyComparer = new();
     private readonly Dictionary<Type, object> _features = [];
     private readonly List<KeyValuePair<Type, object>> _pooledFeatures = [];
-    private object _state;
+    private readonly object _state;
     private IFeatureCollection? _defaults;
     private volatile int _containerRevision;
 

src/HotChocolate/Core/test/StarWars/Data/CharacterRepository.cs

@@ -5,8 +5,8 @@ namespace HotChocolate.StarWars.Data;
 
 public class CharacterRepository
 {
-    private Dictionary<string, ICharacter> _characters;
-    private Dictionary<string, Starship> _starships;
+    private readonly Dictionary<string, ICharacter> _characters;
+    private readonly Dictionary<string, Starship> _starships;
 
     public CharacterRepository()
     {

src/HotChocolate/Core/test/Subscriptions.Postgres.Tests/PostgresChannelTests.cs

@@ -27,7 +27,7 @@ public PostgresChannelTests(PostgreSqlResource resource, ITestOutputHelper outpu
         };
     }
 
-    private PostgresSubscriptionOptions _options;
+    private readonly PostgresSubscriptionOptions _options;
 
     [Fact]
     public async Task Subscribe_Should_ReceiveMessage_When_MessageIsSent()

src/HotChocolate/Core/test/Types.Analyzers.Tests/TestHelper.cs

@@ -21,7 +21,7 @@ namespace HotChocolate.Types;
 
 internal static partial class TestHelper
 {
-    private static HashSet<string> s_ignoreCodes = ["CS8652", "CS8632", "CS5001", "CS8019"];
+    private static readonly HashSet<string> s_ignoreCodes = ["CS8652", "CS8632", "CS5001", "CS8019"];
 
     public static Snapshot GetGeneratedSourceSnapshot([StringSyntax("csharp")] string sourceText)
     {

src/HotChocolate/Core/test/Types.Tests/Types/Descriptors/ObjectTypeDescriptorTests.cs

@@ -245,7 +245,7 @@ public ValueTask InvokeAsync(IMiddlewareContext context)
 
     public class TestFieldMiddleware2
     {
-        private FieldDelegate _next;
+        private readonly FieldDelegate _next;
 
         public TestFieldMiddleware2(FieldDelegate next)
         {

src/HotChocolate/Spatial/test/Types.Tests/GeoJsonMultiPolygonSerializerTests.cs

@@ -40,7 +40,7 @@ public class GeoJsonMultiPolygonSerializerTests
                     new IntValueNode(15),
                     new IntValueNode(5)))));
 
-    private Geometry _geometry = new MultiPolygon(
+    private readonly Geometry _geometry = new MultiPolygon(
     [
         new Polygon(
                     new LinearRing(

src/StrawberryShake/CodeGeneration/src/CodeGeneration.CSharp/Builders/XmlCommentBuilder.cs

@@ -3,7 +3,7 @@ namespace StrawberryShake.CodeGeneration.CSharp.Builders;
 public class XmlCommentBuilder : ICodeBuilder
 {
     private string? _summary;
-    private List<string> _code = [];
+    private readonly List<string> _code = [];
 
     public XmlCommentBuilder SetSummary(string summary)
     {

src/StrawberryShake/CodeGeneration/src/CodeGeneration.CSharp/Generators/DependencyInjectionGenerator.cs

@@ -900,7 +900,7 @@ private static ICode CreateBaseCode(CSharpSyntaxGeneratorSettings settings)
                             .AddArgument(Sp)))));
     }
 
-    private static string s_clientServiceProvider =
+    private static readonly string s_clientServiceProvider =
         @"private sealed class ClientServiceProvider
                 : System.IServiceProvider
                 , System.IDisposable