复杂模型在操作中返回null？

Question

A strongly typed view has a <form> and on submitting <form> I am expecting it should return updated value in model but it is not the case. I couldn't figure out what it is happening.

View Mode:

public class HomeLoanViewModel
{
    public Lead_Options leadOption { get; set; }       
    public Lead HomeLoanLead {get;set;}

    public HomeLoanViewModel()
    {
        this.leadOption= new Lead_Options();
        this.HomeLoanLead= new Lead();
    }
}

and Lead class inside ViewModel is as below:

public partial class Lead
{
    public string Lead_id { get; set; }      
    public string Income { get; set; }

    [NotMapped]
    public List<SelectListItem> Income_Binder
    {
        get
        {
            return new List<SelectListItem>()
            {
                new SelectListItem(){ Value="", Text="Please select...", Selected=true },
                new SelectListItem(){ Value="$0 - $30,000", Text="$0 - $30,000" },
                new SelectListItem(){ Value="$30,001 - $40,000", Text="$30,001 - $40,000" },
                new SelectListItem(){ Value="$40,001 - $50,000", Text="$40,001 - $50,000"},
                new SelectListItem(){ Value="$50,001 - $60,000", Text="$50,001 - $60,000"},
                new SelectListItem(){ Value="$60,001 - $70,000", Text="$60,001 - $70,000"},
                new SelectListItem(){ Value="$70,001 - $80,000", Text="$70,001 - $80,000"},
                new SelectListItem(){ Value="$80,001 - $90,000", Text="$80,001 - $90,000"},
                new SelectListItem(){ Value="$90,001 - $100,000", Text="$90,001 - $100,000"},
                new SelectListItem(){ Value="$100,001 - $150,000", Text="$100,001 - $150,000"},
                new SelectListItem(){ Value="$150,000+", Text="$150,000+"}
            };
        }
    }
}

Controller & Action:

public class HomeLoanController : ParentController
{
    [HttpGet]
    public ActionResult Index()
    {
        return View();
    }

    [HttpPost]
    public ActionResult Submit(FormCollection form)
    {
        Session["loan_type"] = form.GetValue("Type").AttemptedValue;
        Session["m_I_need_to"] = form.GetValue("LoanPurpose").AttemptedValue;
        Session["m_The_property_is_a"] = form.GetValue("LoanFor").AttemptedValue;
        Session["m_My_time_frame_to_buy_or_refinance_is"] = form.GetValue("LoanTimeFrame").AttemptedValue;

       // return View("LeadContact", new HomeLoanViewModel());
       return RedirectToAction("LeadContact");
    }

    [HttpGet]
    public ActionResult LeadContact(HomeLoanViewModel model)
    {
        return View(model);
    }
}

View:

@using HMC.Common.Utilities;
@model HMC.Common.ViewModel.HomeLoanViewModel
@{
    ViewBag.Title = "LeadContact";
    Layout = "~/Views/_Base.cshtml";
}

<form class="wrapper minheight homeloan-form border-top" id="homeloan-form" method="post" action="" novalidate="novalidate">
<label>Total Annual Income</label>
@Html.DropDownListFor(m => m.HomeLoanLead.Income, Model.HomeLoanLead.Income_Binder, new { required = "", aria_required = "true", name = "interestratetype" })
@Html.HiddenFor(m => m.HomeLoanLead.Income)
<div class="formnav row">
     <button class="btn btn-large">Show Top Home Loans <i class="fa fa-chevron-right"></i></button>
</div>
</form>

EDIT: From previous page/ view request come to Submit Action of controller, then it redirect to LeadContact. This is view where I m passing model and post back want to receive updated values.

Answer 1

This type of issue is common when using ViewModels or strongly typed views with models. Here's why... When a post back happens the model binder eventually gets called prior to the controller getting called for the post back. If the controller is accepting a class (model or viewmodel) as an input parameter, then MVC will first create an instance of the class using the null constructor. Then the model binder will take the HTML Query string (name value pairs) and attempt to fill in the model/view model properties based on the name value pairs. If the binder cannot find the proper Name matches it simply doesn't do anything (which is proper behavior), as a result the symptoms of this are "Hey I missing data that I know I posted from the view".

The best way to debug this is 1) Set a breakpoint on the controller's action method. 2) Go to the client side and press F12 to view the network post. 3) Start the post back and notice the name value pairs sent from the browser. Upon breaking at the controller entry go do Debug/Windows/Locals and look at the model portion of the controller parameters. You will see the same thing there as in the network side (proof they all made it back). Now look at the typed value, if anything is not there that should be there, it is simply because the post back and the model (Name values) did not match.

90% of the time the root cause is the first parameter of the DisplayFor, HiddenFor or other "For" elements is not pointing to the correct thing to be returned. This is why I like to use ViewModels..

Given:

public class HomeLoanViewModel
{
  public Lead_Options leadOption { get; set; }       
  public Lead HomeLoanLead {get;set;}
  public String SelectedValue {get;set;}

  public HomeLoanViewModel()
  {
    this.leadOption= new Lead_Options();
    this.HomeLoanLead= new Lead();
  }
  public void Post(){
     //do all your post logic here
     if(SelectedValue > XYZ) //do this
  }
}

I would create the controller signature like this:

 [HttpPost]
public ActionResult Submit(HomeLoanViewModel vm)
{
     if (ModelState.IsValid){
               vm.Post();
               return View(vm);
     }
  return View(vm);
}

I would create the view like this:

@using HMC.Common.Utilities;
@model HMC.Common.ViewModel.HomeLoanViewModel
@{
  ViewBag.Title = "LeadContact";
  Layout = "~/Views/_Base.cshtml";
}
@Using Html.BeginForm(){

    <label>Total Annual Income</label>
     @Html.DropDownListFor(m => m.HomeLoanLead.SelectedValue, Model.HomeLoanLead.Income_Binder, new { required = "", aria_required = "true"})

     <input type="submit/>

}

<div class="formnav row">
 <button class="btn btn-large">Show Top Home Loans <i class="fa fa-chevron-right"></i></button>
</div>

Finally one other pointer, no sense in the SelectListItems Text and Value being the same, you could just as easily have set the value to in Int such as [1,2,3,4,5] for each of the choices. Look at the value as the identifier of the text field, this works especially well with databases that have ID for looking up field values.