Thanks for putting this together! Note that we are not so happy about the name "FieldMaskUtil" in Java and have a plan to rename it to "FieldMasks". I would recommend against using the name "FieldMaksUtil" in C# just because Java did.

@xfxyjwf Thanks for the feedback. Would you prefer to include the FieldMaskTree in FieldMaskPartial (separate partial class), or maybe putting it in the root like JsonFormatter and other helper classes? I think it may be better to only have the stuff directly in FieldMasks and dropping the FieldMaskUtil class completely.

Can we remove FieldMaskUtil. I noticed FieldMaskPartial has most APIs except IsValid. FieldMaskPartial and FieldMaskUtil are kind of duplicated

I plan to do so, was just waiting on a response to my question. Would you prefer having the FieldMaskTree in a partial or as separate class in root like JsonFormatter?

@anandolee @xfxyjwf Still waiting for feedback on your preference how to separate FieldMaskTree. Either adding it to FieldMaskPartial which would blow it up a bit, or putting it into root like done with JsonFormatter but cluttering up the space there while it's only used by FieldMaskTree and completely internal?

Both are valid ways to proceed with their own up- and downsides. I would prefer the root JsonFormatter-like aproach.

I'd prefer to make FieldMaskTree in a separated file instead of blow FieldMaskPartial up. And yes FieldMaskTree should be internal.

Thanks for the feedback. Just applied the changes.

As a short summary:
I prefer Contains and checking the Count over ToString to avoid making assumptions on sorting and have it future proof. Also added assertions to proof specific members are not part of the created path list.

Due to the removal of FieldMaskTree.GetFieldPaths, FieldMaskTreeTest is now having asumptions based on FieldMask and FieldMaskTree.ToFieldMask.

Thanks for the change. It looks good to me, just wait the tests green

Oh for the newly added FieldMaskTreeTest.cs, you need to update Makefile.am to pass distcheck test:
https://github.com/protocolbuffers/protobuf/blob/master/Makefile.am

@anandolee I hope the distcheck works now :)

csharp/src/Google.Protobuf/FieldMaskTree.cs is also newly added and need to be in Makefile.am

Ok, commited. Is it only giving one error a time? This should be all of the newly added files. If there is anything else to check, just let me know :) (Currently I'm in the same timezone as you which should speed stuff up a bit).