Test proto and unit test for issue #9605

This doesn't test *all* possibilities (e.g. the field being a
wrapper, or a message field, etc) - but I'm fairly confident that I
found all the places referring to the case.

csharp/protos/unittest_issues.proto

@@ -156,3 +156,17 @@ message MixedRegularAndOptional {
   string regular_field = 1;
   optional string optional_field = 2;
 }
+
+message OneofWithNoneField {
+  oneof test {
+    string x = 1;
+    string none = 2;
+  }
+}
+
+message OneofWithNoneName {
+  oneof none {
+    string x = 1;
+    string y = 2;
+  }
+}

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

@@ -112,5 +112,21 @@ namespace Google.Protobuf
             // See https://github.com/protocolbuffers/protobuf/pull/7289
             cis.AssertNextTag(WireFormat.MakeTag(11, WireFormat.WireType.Varint));
         }
+
+        [Test]
+        public void NoneFieldInOneof()
+        {
+            var message = new OneofWithNoneField();
+            var emptyHashCode = message.GetHashCode();
+            Assert.AreEqual(OneofWithNoneField.TestOneofCase.None, message.TestCase);
+            message.None = "test";
+            Assert.AreEqual(OneofWithNoneField.TestOneofCase.None_, message.TestCase);
+            Assert.AreNotEqual(emptyHashCode, message.GetHashCode());
+
+            var bytes = message.ToByteArray();
+            var parsed = OneofWithNoneField.Parser.ParseFrom(bytes);
+            Assert.AreEqual(message, parsed);
+            Assert.AreEqual("test", parsed.None);
+        }
     }
 }