Provide simple access to descriptor declarations in C#

This is primarily for access to comments, which would be expected to be available in a protoc plugin.

The implementation has two fiddly aspects:

- We use a Lazy<T> to avoid building the map before cross-linking. An alternative would be to crosslink at the end of the constructor, and remove the calls to CrossLink elsewhere. This would be generally better IMO, but deviate from the Java code.
- The casts to IReadOnlyList<DescriptorBase> are unfortunate. They'll always work, because these lists are always ReadOnlyCollection<T> for a descriptor type... but we can't use IList<DescriptorBase> as that's not covariant, and it's annoyingly fiddly to change the field to be of type ReadOnlyCollection<T>.

csharp/generate_protos.sh

@@ -43,6 +43,9 @@ $PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf \
 # Test protos
 $PROTOC -Isrc -Icsharp/protos \
     --csharp_out=csharp/src/Google.Protobuf.Test/TestProtos \
+    --descriptor_set_out=csharp/src/Google.Protobuf.Test/testprotos.pb \
+    --include_source_info \
+    --include_imports \
     csharp/protos/map_unittest_proto3.proto \
     csharp/protos/unittest_issues.proto \
     csharp/protos/unittest_custom_options_proto3.proto \

csharp/protos/unittest_proto3.proto

@@ -368,7 +368,9 @@ message FooResponse {}
 message FooClientMessage {}
 message FooServerMessage{}
 
+// This is a test service
 service TestService {
+  // This is a test method
   rpc Foo(FooRequest) returns (FooResponse);
   rpc Bar(BarRequest) returns (BarResponse);
 }
@@ -378,3 +380,31 @@ message BarRequest  {}
 message BarResponse {}
 
 message TestEmptyMessage {}
+
+// This is leading detached comment 1
+
+// This is leading detached comment 2
+
+// This is a leading comment
+message CommentMessage {
+  // Leading nested message comment
+  message NestedCommentMessage {
+    // Leading nested message field comment
+    string nested_text = 1;
+  }
+
+  // Leading nested enum comment
+  enum NestedCommentEnum {
+    // Zero value comment
+    ZERO_VALUE = 0;
+  }
+
+  // Leading field comment
+  string text = 1; // Trailing field comment
+}
+
+// Leading enum comment
+enum CommentEnum {
+  // Zero value comment
+  ZERO_VALUE = 0;
+}

csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj

@@ -27,4 +27,8 @@
     <TargetFrameworks>netcoreapp1.0</TargetFrameworks>
   </PropertyGroup>
 
+  <ItemGroup>
+    <EmbeddedResource Include="testprotos.pb" />
+  </ItemGroup>
+
 </Project>

csharp/src/Google.Protobuf.Test/Reflection/DescriptorDeclarationTest.cs

@@ -0,0 +1,151 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2018 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using Google.Protobuf.Reflection;
+using NUnit.Framework;
+using System.Linq;
+using System.Reflection;
+
+namespace Google.Protobuf.Test.Reflection
+{
+    // In reality this isn't a test for DescriptorDeclaration so much as the way they're loaded.
+    public class DescriptorDeclarationTest
+    {
+        static readonly FileDescriptor unitTestProto3Descriptor = LoadProtos();
+
+        [Test]
+        public void ServiceComments()
+        {
+            var service = unitTestProto3Descriptor.FindTypeByName<ServiceDescriptor>("TestService");
+            Assert.NotNull(service.Declaration);
+            Assert.AreEqual(" This is a test service\n", service.Declaration.LeadingComments);
+        }
+
+        [Test]
+        public void MethodComments()
+        {
+            var service = unitTestProto3Descriptor.FindTypeByName<ServiceDescriptor>("TestService");
+            var method = service.FindMethodByName("Foo");
+            Assert.NotNull(method.Declaration);
+            Assert.AreEqual(" This is a test method\n", method.Declaration.LeadingComments);
+        }
+
+        [Test]
+        public void MessageComments()
+        {
+            var message = unitTestProto3Descriptor.FindTypeByName<MessageDescriptor>("CommentMessage");
+            Assert.NotNull(message.Declaration);
+            Assert.AreEqual(" This is a leading comment\n", message.Declaration.LeadingComments);
+            Assert.AreEqual(new[] { " This is leading detached comment 1\n", " This is leading detached comment 2\n" },
+                message.Declaration.LeadingDetachedComments);
+        }
+
+        [Test]
+        public void EnumComments()
+        {
+            var descriptor = unitTestProto3Descriptor.FindTypeByName<EnumDescriptor>("CommentEnum");
+            Assert.NotNull(descriptor.Declaration);
+            Assert.AreEqual(" Leading enum comment\n", descriptor.Declaration.LeadingComments);
+        }
+
+        [Test]
+        public void NestedMessageComments()
+        {
+            var outer = unitTestProto3Descriptor.FindTypeByName<MessageDescriptor>("CommentMessage");
+            var nested = outer.FindDescriptor<MessageDescriptor>("NestedCommentMessage");
+            Assert.NotNull(nested.Declaration);
+            Assert.AreEqual(" Leading nested message comment\n", nested.Declaration.LeadingComments);
+        }
+
+        [Test]
+        public void NestedEnumComments()
+        {
+            var outer = unitTestProto3Descriptor.FindTypeByName<MessageDescriptor>("CommentMessage");
+            var nested = outer.FindDescriptor<EnumDescriptor>("NestedCommentEnum");
+            Assert.NotNull(nested.Declaration);
+            Assert.AreEqual(" Leading nested enum comment\n", nested.Declaration.LeadingComments);
+        }
+
+        [Test]
+        public void FieldComments()
+        {
+            var message = unitTestProto3Descriptor.FindTypeByName<MessageDescriptor>("CommentMessage");
+            var field = message.FindFieldByName("text");
+            Assert.NotNull(field.Declaration);
+            Assert.AreEqual(" Leading field comment\n", field.Declaration.LeadingComments);
+            Assert.AreEqual(" Trailing field comment\n", field.Declaration.TrailingComments);
+        }
+
+        [Test]
+        public void NestedMessageFieldComments()
+        {
+            var outer = unitTestProto3Descriptor.FindTypeByName<MessageDescriptor>("CommentMessage");
+            var nested = outer.FindDescriptor<MessageDescriptor>("NestedCommentMessage");
+            var field = nested.FindFieldByName("nested_text");
+            Assert.NotNull(field.Declaration);
+            Assert.AreEqual(" Leading nested message field comment\n", field.Declaration.LeadingComments);
+        }
+
+        [Test]
+        public void EnumValueComments()
+        {
+            var enumDescriptor = unitTestProto3Descriptor.FindTypeByName<EnumDescriptor>("CommentEnum");
+            var value = enumDescriptor.FindValueByName("ZERO_VALUE");
+            Assert.NotNull(value.Declaration);
+            Assert.AreEqual(" Zero value comment\n", value.Declaration.LeadingComments);
+        }
+
+        [Test]
+        public void NestedEnumValueComments()
+        {
+            var outer = unitTestProto3Descriptor.FindTypeByName<MessageDescriptor>("CommentMessage");
+            var nested = outer.FindDescriptor<EnumDescriptor>("NestedCommentEnum");
+            var value = nested.FindValueByName("ZERO_VALUE");
+            Assert.NotNull(value.Declaration);
+            Assert.AreEqual(" Zero value comment\n", value.Declaration.LeadingComments);
+        }
+
+        private static FileDescriptor LoadProtos()
+        {
+            var type = typeof(DescriptorDeclarationTest);
+            // TODO: Make this simpler :)
+            FileDescriptorSet descriptorSet;
+            using (var stream = type.GetTypeInfo().Assembly.GetManifestResourceStream($"Google.Protobuf.Test.testprotos.pb"))
+            {
+                descriptorSet = FileDescriptorSet.Parser.ParseFrom(stream);
+            }
+            var byteStrings = descriptorSet.File.Select(f => f.ToByteString()).ToList();
+            var descriptors = FileDescriptor.BuildFromByteStrings(byteStrings);
+            return descriptors.Single(d => d.Name == "unittest_proto3.proto");
+        }
+    }
+}