Displaying line numbers in code behind or aspx page would help to reference a particular line in the editor. Also it would help when discussing or reviewing the code with more than one developer, so that we can point out the line number quickly.
By default it’s in disabled mode and we need to enable if you want to display the line numbers. See the steps and screens shots how to enable.
1.Tools -> select Options…
2.Click Text Editor-> All Languages
3.On right hand side, under Display heading check the Line Numbers check box and click OK
Once above setting is done, you will get the line numbers as like below in the editor.
This line number will display for all type of files in visual studio editor and not only cs or aspx.