使用协议缓冲区模拟泛型

Question

I have a class that stores and manipulates some entities. Depending on the number of inputs, I may not be able to store entities in memory, so I'm trying to serialize my objects to be written on hard disk using protocol buffers. I'm using C# and protobuf-csharp-port . I'm aware of protobuf-net as an alternative port; so far I have been working with first option but I'm open for changes if it's required based on my needs.

The class to be serialized in it's simplified form is as follows:

class Entity<T> where T: IComparable<T>
{
    int id;
    T metaData;
}

So at compile time I have no clues about metaData . Googling I noticed that extensions are the right path to follow (as suggested on google's page and this question ); hence I'm defining the Entity.proto file for class Entity as following:

message Entity
{
    required int32 id = 1 [default = 0];
    extensions 2 to max; 
}

and I would like the user to provide his own .proto file for T without the need to access or re-compile Entity.proto . In this regard, my questions are:

1. Do I need to change Entity.proto ?
2. What should be the T.proto ?
3. How can I access T in my C# code ?

Answer 1

With that scheme, any extensions are going to be child values (not subclasses) of the non-generic Entity . That doesn't sound like generics, but ultimately storage (serialization) is often quite different to implementation ( Entity<T> etc). If you can manuallyap between them: fine. But it isn't something the library will provide, AFAIK.

For completeness, in protobuf-net terms: it is perfectly fine with Entity<T> - it essentially considers each ( Entity<Foo> , Entity<Bar> , etc) to be completely separate messages. Protobuf-net isn't hugely motivated by .proto schemas (although a code-gen tool is provided, for completeness) - it mainly uses runtime metadata.