Add DiscardUnknownFields support for C#

By default, unknown fields are preserved when parsing. To discard
them, use a parser configured to do so:

var parser = MyMessage.Parser.WithDiscardUnknownFields(true);

1 files changed, 7 insertions, 1 deletions

diff --git a/csharp/src/Google.Protobuf/UnknownFieldSet.cs b/csharp/src/Google.Protobuf/UnknownFieldSet.cs
index b43f4774..6404c3c0 100644
--- a/csharp/src/Google.Protobuf/UnknownFieldSet.cs
+++ b/csharp/src/Google.Protobuf/UnknownFieldSet.cs
@@ -230,7 +230,8 @@ namespace Google.Protobuf
         /// <summary>
         /// Create a new UnknownFieldSet if unknownFields is null.
         /// Parse a single field from <paramref name="input"/> and merge it
-        /// into unknownFields.
+        /// into unknownFields. If <paramref name="input"/> is configured to discard unknown fields,
+        /// <paramref name="unknownFields"/> will be returned as-is and the field will be skipped.
         /// </summary>
         /// <param name="unknownFields">The UnknownFieldSet which need to be merged</param>
         /// <param name="input">The coded input stream containing the field</param>
@@ -238,6 +239,11 @@ namespace Google.Protobuf
         public static UnknownFieldSet MergeFieldFrom(UnknownFieldSet unknownFields,
                                                      CodedInputStream input)
         {
+            if (input.DiscardUnknownFields)
+            {
+                input.SkipLastField();
+                return unknownFields;
+            }
             if (unknownFields == null)
             {
                 unknownFields = new UnknownFieldSet();