MVC post a list of complex objects

Using Razor you can implement the for loop using a dictionary as follows without making changes to your object:

@foreach (var x in Model.Questions.Select((value,i)=>new { i, value }))
{
     if (Model.Questions[x.i].QuestionType == "Single")
     {
          @Html.EditorFor(modelItem => (modelItem.Questions[x.i] as OpenDataPortal.ViewModels.SingleQuestionViewModel).AnswerText)
     }
   ...
}

The collection needs to be either a List or Array for this to work.

After much research I've found two solutions:

1. One is to write HTML that has hardcoded Id's and Names
2. Two is to convert your ICollection/IEnumerable to an Array or List (i.e IList something with an 'index'), and have an Array object in your BindingModel in your Controller POST Action.

Thanks to Phil Haack's (@haacked) 2008 blog post http://haacked.com/archive/2008/10/23/model-binding-to-a-list.aspx/ Which is still relevant to how the default ModelBinder works today for MVC. (NB: the links in Phil's article to sample porject and extension methods are broken)

HTML snippet that inspired me:

<form method="post" action="/Home/Create">
    <input type="hidden" name="products.Index" value="cold" />
    <input type="text" name="products[cold].Name" value="Beer" />
    <input type="text" name="products[cold].Price" value="7.32" />

    <input type="hidden" name="products.Index" value="123" />
    <input type="text" name="products[123].Name" value="Chips" />
    <input type="text" name="products[123].Price" value="2.23" />

    <input type="submit" />
</form>

Post array looks a bit like:

products.Index=cold&products[cold].Name=Beer&products[cold].Price=7.32&products.Index=123&products[123].Name=Chips&products[123].Price=2.23

Model:

public class CreditorViewModel
{
    public CreditorViewModel()
    {
        this.Claims = new HashSet<CreditorClaimViewModel>();
    }
    [Key]
    public int CreditorId { get; set; }
    public string Comments { get; set; }
    public ICollection<CreditorClaimViewModel> Claims { get; set; }
    public CreditorClaimViewModel[] ClaimsArray { 
        get { return Claims.ToArray(); }
    }
}

public class CreditorClaimViewModel
{
    [Key]
    public int CreditorClaimId { get; set; }
    public string CreditorClaimType { get; set; }
    [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:N2}")]
    public Decimal ClaimedTotalAmount { get; set; }
}

Controller GET:

public async Task<ActionResult> Edit(int id)
    {
        var testmodel = new CreditorViewModel
        {
            CreditorId = 1,
            Comments = "test",
            Claims = new HashSet<CreditorClaimViewModel>{
                new CreditorClaimViewModel{ CreditorClaimId=1, CreditorClaimType="1", ClaimedTotalAmount=0.00M},
                new CreditorClaimViewModel{ CreditorClaimId=2, CreditorClaimType="2", ClaimedTotalAmount=0.00M},
            }
        };
        return View(model);
    }

Edit.cshtml:

@Html.DisplayNameFor(m => m.Comments)
@Html.EditorFor(m => m.Comments)

<table class="table">
    <tr>
        <th>
            @Html.DisplayNameFor(m => Model.Claims.FirstOrDefault().CreditorClaimType)
        </th>
        <th>
            @Html.DisplayNameFor(m => Model.Claims.FirstOrDefault().ClaimedTotalAmount)
        </th>
    </tr>        
<!--Option One-->
@foreach (var item in Model.Claims)
{
    var fieldPrefix = string.Format("{0}[{1}].", "Claims", item.CreditorClaimId);
    <tr>
        <td>
            @Html.DisplayFor(m => item.CreditorClaimType)
        </td>
        <td>
        @Html.TextBox(fieldPrefix + "ClaimedTotalAmount", item.ClaimedTotalAmount.ToString("F"),
        new
        {
            @class = "text-box single-line",
            data_val = "true",
            data_val_number = "The field ClaimedTotalAmount must be a number.",
            data_val_required = "The ClaimedTotalAmount field is required."
        })
        @Html.Hidden(name: "Claims.index", value: item.CreditorClaimId, htmlAttributes: null)
        @Html.Hidden(name: fieldPrefix + "CreditorClaimId", value: item.CreditorClaimId, htmlAttributes: null)
        </td>
    </tr>
    }
</table>    
<!--Option Two-->
@for (var itemCnt = 0; itemCnt < Model.ClaimsArray.Count(); itemCnt++)
{
    <tr>
        <td></td>
        <td>
            @Html.TextBoxFor(m => Model.ClaimsArray[itemCnt].ClaimedTotalAmount)
            @Html.HiddenFor(m => Model.ClaimsArray[itemCnt].CreditorClaimId)
    </td></tr>
}

Form is processed in the Controller:

Post Model:

public class CreditorPostViewModel
{
    public int CreditorId { get; set; }
    public string Comments { get; set; }
    public ICollection<CreditorClaimPostViewModel> Claims { get; set; }
    public CreditorClaimPostViewModel[] ClaimsArray  { get; set; }
}

public class CreditorClaimPostViewModel
{
    public int CreditorClaimId { get; set; }
    public Decimal ClaimedTotalAmount { get; set; }
}

Controller:

[HttpPost]
    public ActionResult Edit(int id, CreditorPostViewModel creditorVm)
    {
        //...

Make sure you are rendering your view in order so that Model.Questions[i] renders in order.

For example, Model.Questions[0], Model.Questions[1], Model.Questions[2]. I noticed that if the order is not correct mvc model binder will only bind the first element.

Thanks for pointing me in the right direction with this post. I was struggling to get the syntax right for binding a non-sequential IDictionary<string, bool> object. Not sure this is 100% correct, but this Razor code worked for me:

<input type="hidden" name="MyDictionary.Index" value="ABC" />
<input type="hidden" name="MyDictionary[ABC].Key" value="ABC" />
@Html.CheckBox(name: "MyDictionary[ABC].Value", isChecked: Model.MyDictionary["ABC"], htmlAttributes: null)

If you need a checkbox, be sure to use Html.CheckBox instead of a standard HTML checkbox. The model will blow up if a value is not provided, and Html.CheckBox generates a hidden field to ensure a value is present when the checkbox is not checked.