The average programmer uses the general Debug mode by using the “Start Debugging” option from the Debug menu, or the F5 shortcut key. But what if you want to debug an application that is already running? If you faced this problem, then this tip should lighten up your day.

Choose Debug | Attach to Process….

Select the aspnet_wp.exe from the list of Available Processes and click on Attach

Attach To Process...

Alternatively you can also use ALT + D , P to select the option.