How do I make repeatable properties 'optional' in protobuf for c#

[Manh Do]

I am working a project where we need to determine if a protobuf list
have is null or not in the protocol buffer.

The problem is the generated c# automatically instantiates the list.

IE. Protocol:
message B {
repeated A MyList = 0;
}

Generated C#:

private readonly global::System.Collections.Generic.List<A> _MyList =
new global::System.Collections.Generic.List<A>();
[global::ProtoBuf.ProtoMember(0, Name=@"MyList", DataFormat =
global::ProtoBuf.DataFormat.Default)]
public global::System.Collections.Generic.List<A> MyList
{
get { return _MyList ; }
}

The problem with this is I cannot determine if the list is null. I am
using an empty list as a way to remove all objects.

Is there a way where I can make the generator not automatically
instantiate the list? Better yet, how do I check for default values in
protocol buffer C#? I know you can check for
'default(typeof(MyType));'.

Thanks for the help!

-Manh

[Kenton Varda]

Protocol Buffers has no concept of "null".  Lists can be empty, but not "null".

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[Marc Gravell]

Kenton is of course quite correct. There is no *native* distinction between a null list and an empty list in the raw protobuf data stream. There *are* some ways of working around that, to distinguish between the two, but not when using the code-generation from .proto.

Marc

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Regards,

Marc

[Manh Do]

What are your suggestions? One option that I was thinking is to add a
boolean field in the message that determines if the list is empty or
not. What do you think?

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[Marc Gravell]

That was exactly my thought. Something *broadly* like (untested):

    [ProtoMember(1)]

    public List<Foo> Foos {get;set;}

    [ProtoMember(2), DefaultValue(false)]

    private bool FoosHasValue {

        get { return Foos != null; }

        set { Foos = value ? ( Foos ?? new List<Foo>() ) : null; }

    }

Note that this may have some impact if you are concatenating messages, and is slightly hacky - Kenton will be tutting, I expect ;p

Marc

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Marc

[Manish Agrawal]

I am trying to use this list structure to save a collection of protobufs in a database as a Byte[]. I created my PBFs, and then created another buffer with an internal List of PBFs as follows. The problem is that during serialization, I get an unexpected sub-type error. My questions are:

1. What is the recommended way to save a collection of PBFs in a database cell?

2. How can I serialize a PBF which has a field which is a collection of other PBFs?

[ProtoContract]

    class Buffers

    {

        [ProtoMember(2)]

        public int user_id { get; set; }

...

    }

[ProtoContract]

    class Albums : Buffers

    {

        [ProtoMember(4)]

        public int album_id { get; set; }

...   

  }

   [ProtoContract]

    class BuffersList

    {

        [ProtoMember(1)]

        public List<Buffers> Items { get; set; }

        public BuffersList()

        {

            Items = new List<Buffers>();

        }

...

    }

Now when I try:

Serializer.NonGeneric.SerializeWithLengthPrefix(<MemoryStream>, <BuffersList>, PrefixStyle.Base128, field);

the above line generates the error message: System.InvalidOperationException: Unexpected sub-type: Albums

Thanks

Manish

On Tuesday, June 22, 2010 4:31:21 PM UTC-4, Marc Gravell wrote:

That was exactly my thought. Something *broadly* like (untested):

    [ProtoMember(1)]

    public List<Foo> Foos {get;set;}

    [ProtoMember(2), DefaultValue(false)]

    private bool FoosHasValue {

        get { return Foos != null; }

        set { Foos = value ? ( Foos ?? new List<Foo>() ) : null; }

    }

Note that this may have some impact if you are concatenating messages, and is slightly hacky - Kenton will be tutting, I expect ;p

Marc