feat: add initially-empty C# features proto

PiperOrigin-RevId: 890289024

BUILD.bazel

@@ -621,6 +621,16 @@ alias(
     visibility = ["//visibility:public"],
 )
 
+################################################################################
+# C# support
+################################################################################
+
+alias(
+    name = "csharp_features_proto",
+    actual = "//csharp:csharp_features_proto",  # proto_library
+    visibility = ["//visibility:public"],
+)
+
 ################################################################################
 # Test protos
 ################################################################################

cmake/install.cmake

@@ -92,6 +92,7 @@ set(protobuf_HEADERS
   ${cpp_features_proto_proto_srcs}
   ${descriptor_proto_proto_srcs}
   ${plugin_proto_proto_srcs}
+  ${c_sharp_features_proto_proto_srcs}
   ${java_features_proto_proto_srcs}
   ${go_features_proto_proto_srcs}
 )
@@ -109,6 +110,7 @@ endif ()
 set(protobuf_STRIP_PREFIXES
   "/src"
   "/java/core/src/main/resources"
+  "/csharp"
   "/go"
   "/"
 )

cmake/installed_include_golden.txt

@@ -9,6 +9,7 @@ google/protobuf/arena_allocation_policy.h
 google/protobuf/arena_cleanup.h
 google/protobuf/arenastring.h
 google/protobuf/arenaz_sampler.h
+google/protobuf/c_sharp_features.proto
 google/protobuf/compiler/code_generator.h
 google/protobuf/compiler/code_generator_lite.h
 google/protobuf/compiler/command_line_interface.h

csharp/BUILD.bazel

@@ -3,11 +3,28 @@
 # See also code generation logic under /src/google/protobuf/compiler/csharp.
 
 load("@rules_shell//shell:sh_binary.bzl", "sh_binary")
+load("//bazel:proto_library.bzl", "proto_library")
 load("//build_defs:internal_shell.bzl", "inline_sh_test")
 load("//conformance:defs.bzl", "conformance_test")
 load("//editions:defaults.bzl", "compile_edition_defaults", "embed_edition_defaults")
 load("//upb/cmake:build_defs.bzl", "staleness_test")
 
+################################################################################
+# C# features
+################################################################################
+filegroup(
+    name = "c_sharp_features_proto_srcs",
+    srcs = ["google/protobuf/c_sharp_features.proto"],
+    visibility = ["//:__subpackages__"],
+)
+
+proto_library(
+    name = "c_sharp_features_proto",
+    srcs = ["google/protobuf/c_sharp_features.proto"],
+    visibility = ["//:__subpackages__"],
+    deps = ["//:descriptor_proto"],
+)
+
 ################################################################################
 # Tests
 ################################################################################
@@ -44,6 +61,7 @@ filegroup(
             "src/*/obj/**/*",
         ],
     ) + [
+        "google/protobuf/c_sharp_features.proto",
         "src/Directory.Build.props",
         "src/Google.Protobuf.Test/testprotos.pb",
         "src/Google.Protobuf.sln",

csharp/generate_protos.sh

@@ -46,6 +46,12 @@ $PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf \
     src/google/protobuf/wrappers.proto \
     src/google/protobuf/compiler/plugin.proto
 
+# C# features
+$PROTOC -Icsharp -Isrc --csharp_out=csharp/src/Google.Protobuf \
+    --csharp_opt=base_namespace=Google.Protobuf \
+    --csharp_opt=file_extension=.pb.cs \
+    csharp/google/protobuf/c_sharp_features.proto
+
 # Test protos
 # Note that this deliberately does *not* include old_extensions1.proto
 # and old_extensions2.proto, which are generated with an older version

csharp/google/protobuf/c_sharp_features.proto

@@ -0,0 +1,23 @@
+// 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
+
+syntax = "proto2";
+
+package pb;
+
+option csharp_namespace = "Google.Protobuf.Reflection";
+option java_multiple_files = true;
+option java_outer_classname = "CSharpFeaturesOuterClass";
+
+import "google/protobuf/descriptor.proto";
+
+extend google.protobuf.FeatureSet {
+  optional CSharpFeatures csharp = 1004;
+}
+
+message CSharpFeatures {
+}

pkg/BUILD.bazel

@@ -16,6 +16,7 @@ pkg_files(
     name = "wkt_protos_files",
     srcs = [
         "//:well_known_type_protos",
+        "//csharp:c_sharp_features_proto_srcs",
         "//go:go_features_proto_srcs",
         "//java/core:java_features_proto_srcs",
         "//src/google/protobuf:cpp_features_proto_srcs",
@@ -121,6 +122,7 @@ gen_file_lists(
         "//src/google/protobuf:cpp_features_proto": "cpp_features_proto",
         "//src/google/protobuf:descriptor_proto": "descriptor_proto",
         "//src/google/protobuf/compiler:plugin_proto": "plugin_proto",
+        "//csharp:c_sharp_features_proto": "c_sharp_features_proto",
         "//java/core:java_features_proto": "java_features_proto,src/google/protobuf/compiler/java/",
         "//go:go_features_proto": "go_features_proto,go/",