The CS0029 error is caused by the compiler which creates incorrect xaml entries for I/O arguments. You can fix it by editing the xaml to remove the incorrect lines. I have found that if you change certain activities (not directly related to arguments) in the workflow, like rebinding UI elements, the xaml containing these activities is recompiled with the erroneous lines reinserted requiring another xaml edit and recompile. Maybe someone can help find a more solid solution to this issue.