It should bring up the option to automatically add the use statement or type the fully qualified name but sometimes neither appears.
In short, when it's not underlined, then there are no quick-fixes.
So the question is, why it's not underlined, but I guess it just takes the editor a second(s) to analyze the code. I agree it would be better to show such fixes asap.
The document/solution have been open for hours at that point. After a restart it still didn't show though.
This seems to happen if DataPart is simply a known class name that does not need a quick fix.
Mouse-hovering over the DataPart should reveal the information about the class.
There was no use ...\DataPart statement yet, so it wasn't a known class name and absolutely needed quick fixing by auto-importing the use statement. In most files it would do that, in some it just doesn't.