为下拉列表（MVC5）设置不同的默认值

Question

I have flights timetable - Schedule view with list of flights Where I just return View and have some filters and sorting.

Here is part of my view:

 @model IEnumerable<AirPortIS.Models.Flight>

    <div class="page-header">
        <h3>Flights</h3>
    </div>
    <div id="modDialog" class="modal fade">
        <div id="dialogContent" class="modal-dialog"></div>
    </div> 

    <table class="table-bordered table-condensed">
        <thead>
            <tr>
                <th>Flight №</th>
                <th>Departure</th>
                <th>Destination</th>
                <th>@Html.ActionLink("Day", "Schedule", new { sort = ViewBag.SortDay, company = ViewBag.FiltrC, destination = ViewBag.FiltrD })</th>
                <th>Departure Time</th>
                <th>Arrival Time</th>
                <th>Company</th>
                <th>@Html.ActionLink("Seats", "Schedule", new { sort = ViewBag.SortSeats, company = ViewBag.FiltrC, destination = ViewBag.FiltrD })</th>
                <th>@Html.ActionLink("Cost", "Schedule", new { sort = ViewBag.SortCost, company = ViewBag.FiltrC, destination = ViewBag.FiltrD })</th>
                <th>Book ticket</th>
            </tr>
        </thead>
        @foreach (var f in Model)
        {
            <tr>
                <td>@Html.ActionLink(f.FlightId.ToString(), "FlightDetails", new { id = f.FlightId }, new {@class = "flItem" } )</td>
                <td>@f.Departure</td>
                <td>@f.Destination</td>
                <td>@f.Day</td>
                <td>@f.DepartureTime</td>
                <td>@f.ArrivalTime</td>
                <td>@f.Company.Name</td>
                <td>@f.Seats</td>
                <td>@f.Cost</td>
                <td>@Html.ActionLink("Link for booking ticket")</td>
            </tr>
        }
    </table>

I need to do that by clicking on a button "Book ticket" user is getting a page where dropdownlist have a preset value of FlightId. For example we have a flight №1 and a link "Book ticket",so when user goes the booking ticket page he gets a droptdownlist with preselected value "1"

Here is my ticket Model

public class Tickets
{
    public int Id { get; set; }
    public int TicketId { get; set; }
    public Flight Flight { get; set; }
    public string Seat {get;set; }
    public string Passenger { get; set; }
    public int Flightid { get; set; }
    public string Status { get; set; }
} 

And part of TicketsController:

 public class TicketsController : Controller
{
    private readonly AirportContext _db = new AirportContext();
    [Authorize]
    public ActionResult Tickets()
    {
        var ticket = _db.Tickets.Include(t => t.Flight);
        return View(ticket);
    }
    [HttpGet]
    public ActionResult BookTicket()
    {
        IEnumerable<SelectListItem> statusList = new SelectList(new List<string> { "Book", "Buy" });
        IEnumerable<SelectListItem> flights = new SelectList(_db.Flights.ToList(), "FlightId", "FlightId");
        ViewData["flights"] = flights;
        ViewData["statusList"] = statusList;
        return View();
    }
    [HttpPost]
    public ActionResult BookTicket(Tickets ticket)
    {
        IEnumerable<SelectListItem> statusList = new SelectList(new List<string> { "Book", "Buy" });
        IEnumerable<SelectListItem> flights = new SelectList(_db.Flights.ToList(), "FlightId", "FlightId");
        ViewData["flights"] = flights;
        ViewData["statusList"] = statusList;
        foreach (var c in _db.Tickets.ToList())
        {
            if ((_db.Tickets.ToList().Exists(x => c.TicketId == ticket.TicketId)) || (ticket.TicketId <= 0))
            {
                ModelState.AddModelError("TicketId", "Wrong ticket id");
            }
            if ((_db.Tickets.ToList().Exists(x => c.Seat == ticket.Seat)) && (_db.Tickets.ToList().Exists(x => c.Flightid == ticket.Flightid))
                && (_db.Tickets.ToList().Exists(x => c.TicketId == ticket.TicketId)))
            {
                ModelState.AddModelError("Seat", "The seat is unavailable");
            }
            if (_db.Tickets.ToList().Exists(x => c.Passenger == ticket.Passenger))
            {
               ModelState.AddModelError("Passenger", "The ticket has already bought");
            }
        }
        if (ModelState.IsValid)
        {
            _db.Tickets.Add(ticket);
            _db.SaveChanges();
            return RedirectToAction("Tickets");
        }
        else return View(ticket);
    }

And my BookTikcet View:

@model AirPortIS.Models.Tickets

@{ 
ViewBag.Title = "Book ticket";
}
<h2>Book ticket:</h2>

<form class="form-inline" method="post">
    <div>
        @Html.ValidationSummary()
    </div>
    <div class="form-group col-md-2">
        Ticket №<br/>
        @Html.EditorFor(model => Model.TicketId)
    </div>
    <div class="form-group col-md-1">
       Flight №<br />
        @Html.DropDownListFor(model => Model.Flightid, ViewData["flights"] as IEnumerable<SelectListItem>)
    </div>
    <div class="form-group col-md-2">
        Место<br />
        @Html.EditorFor(model => Model.Seat)
    </div>
    <div class="form-group col-md-2">
        Passenger Name<br />
        @Html.EditorFor(model => Model.Passenger)
    </div>
    <div class="form-group col-md-2">
        Status<br />
        @Html.DropDownListFor(model => Model.Status, ViewData["statusList"] as IEnumerable<SelectListItem>)
    </div>
        <div>
            <input class="btn-success" type="submit" value="Book Ticket"/>
        </div>
</form>


<div>
<form method="get" action="Tickets">
    <button class="btn-danger" type="submit">Cancel</button>
</form>
</div>

I have no idea how to do it,so this whole code above just a standart code for creating a new ticket. How I should modify code or add something to have this (For example we have a flight №1 and a link "Book ticket",so when user goes the booking ticket page he gets a droptdownlist with preselected value "1",for flight №2 on a page dropdownlist has a preselected value "2" for FlightId. Hope that my question is clear,sorry if something is wrong written or not quite clear.

Answer 1

You need to pass the value of FlightId as a route (or query string) value to the BookTicket method. You link should be

@Html.ActionLink("Book ticket", "BookTicket", new { id = f.FlightId })

and modify the method to

[HttpGet]
public ActionResult BookTicket(int ID)
{
    ... // set you SelectLists as above
    // Initialize your model and set the Flightid property
    var model = new Tickets()
    {
        Flightid = ID
    };
    return View(model); // return the model to the view
}

Your dropdownlist will now have the option identified by Flightid selected when you first generate the view.

Note. I recommend you use a view model rather than your Tickets data model which will contain properties IEnumerable<SelectListItem> Flights and IEnumerable<SelectListItem> StatusList rather than using ViewData so that your view are strongly typed using

@Html.DropDownListFor(m => m.Flightid, Model.Flights)

You should also consider refactoring the code to populate the SelectLists into a private method so that you do not repeat code, for example

private void ConfigureViewModel(TicketVM model)
{
    model.Flights = new SelectList(...);
    model.StatusList = new SelectList(...);
}

Note also that it is a waste of resources to be calling your database to get the SelectList 's in the POST method if ModelState is valid. Your code should be

if (!ModelState.IsValid)
{
     ConfigureViewModel(model); // only necessary if you need to return the view
     return View(model);
}
// save and redirect

Side note: It's unclear why you actually need a dropdownlist for Flightid in the BookTicket view. The user has already selected the flight so why are you giving the option to change it? It might be more appropriate to just render the Flightid as a hidden or readonly input so its submitted back to the POST method.