Header not found when adding new messages (e.g. visualization_msgs)

[jhoare]

We were attempting to add visualization_msgs to be generated for C#, but noticed that the generated C# code is expecting Header to be a message within visualization_msgs and not the std_msgs/Header. When visual studio tries to compile the generated C# code, it will fail because no visualization_msgs/Header exsits.

We have found out that this appears to depend on the alphabetical order of the package that the messages are in. If we rename visualization_msgs to avisualization_msgs (or have it start with any letter before s) everything will generate and compile correctly, and if it comes after std_msgs it will experience the error described above.

It looks like we can get everything to build correctly if we modify the messages themselves to have std_msgs/Header instead of Header in the .msg definition. But this is not ideal since it requires us to maintain a different & parallel message definition, and I believe will also cause the md5 of the messages to not match.

[nuclearmistake]

Issue confirmed. If you use std_msgs/Header, the md5 sum will definitely match correctly (as in https://github.com/uml-robotics/ROS.NET/blob/master/common_msgs/trajectory_msgs/msg/JointTrajectory.msg).

If you're curious, non-literal fields' types' md5 sums are inserted in place of their type names during md5 calculation, allowing explicit or implicit package determination to work).

I hadn't noticed the alphabetical component, but that would definitely explain why sensor_msgs/Images can simply use "Header" with no package name, but trajectory_msgs cannot! Thanks!