HttpPatch action filter attribute confirm that specified method should be available only via HTTP PATCH request. HttpPatch action filter is new in MVC 4. HTTP PATCH verb is relatively new and it is used to modify an existing HTTP resource. HTTP PUT allow us to create or replace existing HTTP resource but we cant modify existing resource with HTTP PUT. This is where HTTP PATCH comes in picture.
Attribute Usage: Only method
public ActionResult About()
Alike other action filter, We can’t access such method with other than HTTP PATCH verb. If we try to access it with other HTTP verb then it will fail and return HTTP 404 not found error.
Check out ASP.NET MVC: Action filter series post to read about other available action filters.