Add DynamicMessage support for C#

[jtattermusch]

Not a requirement as of now, but it would be nice to have one day in not too distant future.

[jskeet]

It would be nice to have specific requirements around this - scenarios we're trying to make simple.
A few questions that arise:

• Do we need reflection support? (I assume so.)
• Will these messages be mutable, just like the rest? (I assume so.)
• Do we need to be able to mix and match, e.g. set a field in a dynamic message to a value which is a strongly-typed message? (Could be tricky.)
• Do we need C# dynamic typing support?

[xfxyjwf]

@jskeet @anandolee Is this DynamicMessage implemented in C#?

[jskeet]

Not that I'm aware of.

[dluc]

The simpler, still very useful, support would be removing the need to compile .proto files to DLLs at build time, so that a .NET application could generate the serializer/deserializer at runtime (caching it in memory) from .proto files provided by users for example.

see for example https://stackoverflow.com/a/27505474/2793555

[jskeet]

@dluc: That's far from simple - the C# code has no facility for parsing .proto files at all. That's all in protoc. If any dynamic message support is introduced, it would almost certainly work with the descriptors, not the original .proto files.

(And generating the code at execution time is far from trivial too - again, all the code generation is in protoc, not in the Google.Protobuf library.)

[jskeet]

I'm actively working on this now. The initial support is likely to be relatively bare, but it'll be there. The main reason for me doing this is for the sake of writing protoc plugins in C#.