ASP.NET MVC 5 Edit and Create Separate - asp.net-mvc

Question

public ActionResult NewLine(int index, ExpenseLine newLine)
{
newLine.Indis = index;
List<ExpenseLine> expenses;
if (Session["ExpenseLines"] == null)
{
expenses = new List<ExpenseLine>();
}
else
{
expenses = (List<ExpenseLine>)Session["ExpenseLines"];
var OldExpense = expenses.Where(x => x.IndexNo == index).FirstOrDefault();
newLine.Key = OldExpense.Key;
}
var model = SetVariables(new Expense() { ExpenseLines = new List<ExpenseLine> { newLine } });
newLine.IndexNo = expenses.Count + 1;
var exp = expenses.Where(x => x.Indis == newLine.Indis).ToList();
foreach (var item in exp)
{
expenses.Remove(item);
}
expenses.Add(newLine);
Session["ExpenseLines"] = expenses;
return PartialView("~/Views/Expense/EditorTemplates/ExpenseLine.cshtml", model.ExpenseLines.First());
}
I have a expense modal in cshtml.
There is a button. The part where I press the button both adds a row and edits the modal if it is not empty. When I press save, it goes to the above action and the row I just added deletes it because it takes the key of the old row. I don't do it like that. The distinguishing feature is only key.index and others do not affect the action. If I'm adding a new line, I don't want it to enter the "else" part. How can I do it ?