Support custom options in C#

This consists of:
- Changing the codegen for the fixed set of options protos, to parse unknown fields instead of skipping them
- Add a new CustomOptions type in the C# support library
- Expose CustomOptions properties from the immutable proto wrappers in the support library

Only single-value options are currently supported, and fetching options values requires getting the type right
and knowing the field number. Both of these can be addressed at a later time.

Fixes #2143, at least as a first pass.

1 files changed, 5 insertions, 0 deletions

diff --git a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
index 94efea9e..9124beee 100644
--- a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
@@ -329,5 +329,10 @@ namespace Google.Protobuf.Reflection
         /// The file descriptor for <c>descriptor.proto</c>.
         /// </value>
         public static FileDescriptor DescriptorProtoFileDescriptor { get { return DescriptorReflection.Descriptor; } }
+
+        /// <summary>
+        /// The (possibly empty) set of custom options for this file.
+        /// </summary>
+        public CustomOptions CustomOptions => Proto.Options?.CustomOptions ?? CustomOptions.Empty;
     }
 }