In a couple of circumstances, you’ll see a red squiggle (like a syntax error) at the end of a line. This squiggle occurs when you’re renaming something, or when you’re referencing a class that exist in a namespace that hasn’t been used. If you press ALT + SHIFT + F10, you’ll get a handy menu that allows you to rename all of the references or add a "using" to that namespace. Since this is the first option, and both are really handy, you can fly using this keyboard combo in visual studio by using ALT + SHIFT + F10 + ENTER (to select the first item and add the using or rename other mentions of a property).
Hope this helps,
James Brundage [MSFT]