Hi neuecc,

This issue/feature request is related to #152 #110 #107 , but is with a much easier approach to solve the same problem.

It is very important to reuse objects because no matter how memory efficient the Serialize/Deserialize is, if we have to throw away all the created objects it will still be very wasteful memory allocation.
This is the main reason why we need to deserialize into an existing object, so we can reuse an old message object from a pool.

Instead of modifying and extending the interface to support a new method DeserializeTo, I think a much easier approach is to allow us to define a custom object create function which replaces the objInfo.GetConstructorString() in the code generator. Such as adding a new attribute tag for a function that defines how to create the object and is picked up by the code generator.

[MessageConstructor]
public static MessageType Create(){
    return MessageTypePool.Pop();
}

As Example Protobuf-net's RuntimeTypeModel.SetFactory is very helpful for implementing object pools to be used with deserialization. If we can have something similar, which replaces the new object Constructor line in the current code generator it will be the most versatile and don't require any interface change. We can then simply handle Collections recycling using custom formatters or modifying the source formatter.
This of course does not allow for the MergeTo feature, but I believe Merging collections have a much rarer use case than message object pooling.