Set "Last-Modified" Header to "Date-Modified" of File in ASP.NET MVC?

I am trying to figure out how to optimize getting the "Date modified" to use in the last-modified header in ASP.NET MVC.

I don't want to check the date of each request as this seems loss of performance.

Basically, at the moment I am using "Release Date" for date-modified, but the problem here is that every time I do a release - all the last-modified headers update to this date.

How to set the last-modified header to the "last Date-modifie开发者_如何学JAVAd" of the file ?