First implementation of C# nullable reference types enablement using the current (unreleased) edition.

PiperOrigin-RevId: 926073660

csharp/Google.Protobuf.Tools.nuspec

@@ -23,8 +23,8 @@
     <file src="protoc/linux_aarch64/protoc" target="tools/linux_aarch64/protoc"/>
     <file src="protoc/macosx_x64/protoc" target="tools/macosx_x64/protoc"/>
     <!--
-      - Include the protos for the well-known types in a directory where protoc will
-      - find them by default.
+      - Include the protos for the well-known types and language-specific editions features in a
+      - directory where protoc will find them by default.
       -->
     <file src="src/google/protobuf/any.proto" target="tools/include/google/protobuf"/>
     <file src="src/google/protobuf/api.proto" target="tools/include/google/protobuf"/>
@@ -37,9 +37,11 @@
     <file src="src/google/protobuf/timestamp.proto" target="tools/include/google/protobuf"/>
     <file src="src/google/protobuf/type.proto" target="tools/include/google/protobuf"/>
     <file src="src/google/protobuf/wrappers.proto" target="tools/include/google/protobuf"/>
+    <file src="csharp/google/protobuf/c_sharp_features.proto" target="tools/include/google/protobuf"/>
     <!--
       - Include the protos for the well-known types again in their old location,
-      - for backward compatibility.
+      - for backward compatibility. This does not include features files, as those have never been
+      - in tools/google/protobuf directory.
       -->
     <file src="src/google/protobuf/any.proto" target="tools/google/protobuf"/>
     <file src="src/google/protobuf/api.proto" target="tools/google/protobuf"/>

csharp/generate_protos.sh

@@ -67,6 +67,7 @@ $PROTOC -Isrc -I. -Ijava/core/src/main/resources/ \
     conformance/test_protos/test_messages_edition2023.proto \
     conformance/test_protos/test_messages_edition_unstable.proto \
     csharp/protos/map_unittest_proto3.proto \
+    csharp/protos/nrt.proto \
     csharp/protos/unittest_issues.proto \
     csharp/protos/unittest_custom_options_proto3.proto \
     csharp/protos/unittest_proto3.proto \

csharp/google/protobuf/c_sharp_features.proto

@@ -20,4 +20,17 @@ extend google.protobuf.FeatureSet {
 }
 
 message CSharpFeatures {
+  // Whether the generated files should have nullable reference type annotations.
+  // When enabled, the generated C# code includes `#nullable enable annotations`
+  // and annotates message-typed properties and certain parameters with `?`.
+  optional bool nullable_reference_types = 1 [
+    retention = RETENTION_RUNTIME,
+    targets = TARGET_TYPE_FIELD,
+    targets = TARGET_TYPE_MESSAGE,
+    targets = TARGET_TYPE_FILE,
+    feature_support = {
+      edition_introduced: EDITION_UNSTABLE,
+    },
+    edition_defaults = { edition: EDITION_LEGACY, value: "false" }
+  ];
 }

csharp/protos/nrt.proto

@@ -0,0 +1,55 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2026 Google Inc.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+edition = "UNSTABLE";
+
+package protobuf_editionstesting;
+
+import "csharp/protos/unittest_import_proto3.proto";
+import "csharp/protos/unittest.proto";
+
+import option "csharp/google/protobuf/c_sharp_features.proto";
+option features.(pb.csharp).nullable_reference_types = true;
+option csharp_namespace = "Google.Protobuf.TestProtos";
+
+// This file is used for testing NRT support in C#, both in terms of
+// manual inspection of the generated code, and checking that it compiles.
+// It exercises many aspects of protobuf, but does not attempt to be
+// comprehensive in checking (for example) all primitive numeric types.
+
+extend protobuf_unittest_proto2.TestAllExtensions {
+  int32 int32_ext = 1234567;
+  string string_ext = 1234568;
+  protobuf_unittest_proto2.TestAllExtensions message_ext = 1234569;
+}
+
+message NrtMessage {
+  protobuf_unittest_import.ImportMessage proto3_import_message = 1;
+  OtherNrtMessage other_message = 2;
+  string text = 3;
+  int32 integer = 4;
+  bytes blob = 5;
+  NrtEnum enum_field = 6;
+
+  message NestedMessage {
+    string text = 1;
+  }
+}
+
+message OtherNrtMessage {
+  string text = 1;
+}
+
+message ExtendableNrtMessage {
+  extensions 65536 to max;
+}
+
+enum NrtEnum {
+  NRT_ENUM_UNSPECIFIED = 0;
+  FOO = 1;
+  BAR = 2;
+}

csharp/src/Google.Protobuf.Test/RefStructCompatibilityTest.cs

@@ -10,6 +10,7 @@
 using NUnit.Framework;
 using System.Diagnostics;
 using System;
+using System.Linq;
 using System.Reflection;
 using System.IO;
 
@@ -43,11 +44,13 @@ namespace Google.Protobuf
             // "XYZ is obsolete: 'Types with embedded references are not supported in this version of your compiler.'"
             // We build the code with GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE to avoid the use of ref struct in the generated code.
             var compatibilityFlag = "-define:GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE";
-            var sources = "*.cs";  // the generated sources from the TestProtos project
+            var sources = Directory.GetFiles(testProtosProjectDir, "*.cs")
+                .Select(Path.GetFileName)
+                .Except(new[] { "Nrt.pb.cs" }); // We don't expect the NRT code to compile with the old compiler.
             // We suppress CS1691, which flags a warning for the generated line of
             // #pragma warning disable 1591, 0612, 3021, 8981
             // because CS8981 is unknown to this version of the compiler.
-            var args = $"-langversion:3 -nologo -nowarn:1691 -target:library {compatibilityFlag} -reference:netstandard.dll -reference:{testProtosOutputDir}\\Google.Protobuf.dll -out:{testProtosOutputDir}\\TestProtos.RefStructCompatibilityTest.OldCompiler.dll {sources}";
+            var args = $"-langversion:3 -nologo -nowarn:1691 -target:library {compatibilityFlag} -reference:netstandard.dll -reference:{testProtosOutputDir}\\Google.Protobuf.dll -out:{testProtosOutputDir}\\TestProtos.RefStructCompatibilityTest.OldCompiler.dll {string.Join(" ", sources)}";
             RunOldCsharpCompilerAndCheckSuccess(args, testProtosProjectDir);
         }