how to resolve "cannot convert from int? to int" in c#, compile time error?

you can convert int? to int by using the Value property of Nullable int, you can consider following example let say your controller method taking nullable int as input parameter so you can use recordId as int in your inner method as shown.

   public ActionResult YourControllerMethod(int? Id)
        {
        //following line will set 0 if the Id is null otherwise it will set the value into recordId
        int recordId = Id == null ? 0 : Id.Value;
       // Here you can simply use the recordId in your inner method as int
        Records.GetbyId(recordId);
        }