Navigate up to
firstName in Normal mode and type
qaIprivate readonly string <ESC>jq
This will record the macro (in register
a) of you adding "private readonly string " to the beginning of the line, then moving one line down. If you want to repeat this macro twice (thus repeating the command for the next two lines), in command mode type
2@a on the
lastName line, which will execute the macro twice more.
This method fulfills your requirement of "not manually visiting each line"; however it's a little heavyweight for your application.
If it's the typing of
private readonly string that you object to, a quick solution is to make your edit on the first line, then move down to each line you want to make the edit and use
. in Normal mode. This will repeat your last command (in this case adding the text to the line) with no fuss.