Create this method in global.asax.cs:
public static void RegisterGlobalFilters(GlobalFilterCollection filters)
{
// add filters here
filters.Add(new MyCustomFilter());
}
Then call it from the Application_Start
method:
protected void Application_Start()
{
AreaRegistration.RegisterAllAreas();
RegisterGlobalFilters(GlobalFilters.Filters);
RegisterRoutes(RouteTable.Routes);
}