inheritance.cs

[ProtoContract(Name = "MSG1")]
[ProtoInclude(3, typeof(Message2))]
class Message1
{
    [ProtoMember(1, Name = "account", IsRequired = true)]
    public string Account { get; set; }
    [ProtoMember(2, Name = "symbol", IsRequired = true)]
    public string Symbol { get; set; }
}
[ProtoContract(Name = "MSG2")]
class Message2 : Message1
{
    [ProtoMember(2, Name = "id", IsRequired = true)]
    public int Id { get; set; }
}

proto2.proto

syntax = "proto2";

message MSG1 {
   required string account = 1;
   required string symbol = 2;
   oneof subtype {
      MSG2 MSG2 = 3;
   }
}
message MSG2 {
   required int32 id = 2;
}

proto3.proto

syntax = "proto3";

message MSG1 {
   string account = 1;
   string symbol = 2;
   oneof subtype {
      MSG2 MSG2 = 3;
   }
}
message MSG2 {
   int32 id = 2;
}