Did you used any custom activity like MsgReader while using the process with VB. please check if you missed to add these kind of custom activities while running the process using C#.
i think you have to import the name space MsgReader.Mime to resolve the issue.
What is the error you are facing now. And also please refer the below screenshot you have to create msg in MsgRead.Mime.Message type to resolve the error.
ok. i have tried in VB it is working as expected. Not sure what is the problem with C#. one last suggestion from my end please make sure that you have latest version of uipath.system package.