How Do I Model Bind A List Of 'List' Using MVC.Net

Question

I am trying to create a form that will consist of a series of dropdown lists, all of which are loaded from a database. I will not know how many dropdown lists will be needed, or how many options each dropdown list will have at compile-time.

How can these fields be set-up to allow them to model-bind when posted?

There is a lot of other complexity in each of the below code elements, but I cannot get the model binding to work even when reduced down to a basic level.

The Models:

public class MyPageViewModel
{
    public List<MyDropDownListModel> ListOfDropDownLists { get; set; }
}

public class MyDropDownListModel
{
    public string Key { get; set; }
    public string Value { get; set; }
    public List<SelectListItem> Options { get; set; }
}

The Controller Get Action:

[AcceptVerbs(HttpVerbs.Get)]
[ActionName("MyAction")]
public ActionResult MyGetAction()
{
    var values_1 = new List<string> {"Val1", "Val2", "Val3"};
    var options_1 =
        values_1
            .ConvertAll(x => new SelectListItem{Text=x,Value=x});

    var myDropDownListModel_1 =
        new MyDropDownListModel { Key = "Key_1", Options = options_1 };


    var values_2 = new List<string> {"Val4", "Val5", "Val6"};
    var options_2 =
        values_2
            .ConvertAll(x => new SelectListItem{Text=x,Value=x})};

    var myDropDownListModel_2 =
        new MyDropDownListModel { Key = "Key_2", Options = options_2 };


    var model =
        new MyPageViewModel
        {
            ListOfDropDownLists = 
                new List<MyDropDownListModel>
                {
                    myDropDownListModel_1,
                    myDropDownListModel_2,
                }
        };

    return View(model);
}

The Controller Post Action:

[AcceptVerbs(HttpVerbs.Post)]
[ActionName("MyAction")]
public ActionResult MyPostAction(MyPageViewModel model)
{
    //Do something with posted model...
    //Except 'model.ListOfDropDownLists' is always null

    return View(model);
}

The View:

@model MyPageViewModel

@using (Html.BeginForm("MyPostAction"))
{
    foreach (var ddl in Model.ListOfDropDownLists)
    {
        @Html.DropDownListFor(x => ddl.Value, ddl.Options)
    }
    <button type="submit">Submit</button>
}

Edit: Corrected typos and copy-paste mistakes.

Solution:

The problem turned out to be the foreach-loop within the view. Changing it into a for-loop instead caused the post to populate as expected. The updated view is below:

@using (Html.BeginForm("MyPostAction"))
{
    for (int i = 0; i < Model.ListOfDropDownLists.Count; i++)
{
        @Html.HiddenFor(x => x.ListOfDropDownLists[i].Key)
        @Html.DropDownListFor(m => m.ListOfDropDownLists[i].Value, Model.ListOfDropDownLists[i].Options);
    }
    <button type="submit">Submit</button>
}

@user256103 No errors occur. When I am debugging at a breakpoint in th post action the model property 'MyPageViewModel.ListOfDropDownLists' is null. — Gifreakius, May 24 '14 at 14:26

score 2 · Accepted Answer · answered May 24 '14 at 06:46

Your view is only creating multiple select elements named dll.Value (and duplicate ID's) which has no relationship to your model. What you need is to create elements named ListOfDropDownLists[0].Value, ListOfDropDownLists[1].Value etc.

Change you loop in the view to this

for (int i = 0; i < Model.ListOfDropDownLists.Count; i++)
{     
    @Html.DropDownListFor(m => m.ListOfDropDownLists[i].Value, Model.ListOfDropDownLists[i].Options);
}

You posted code has multiple errors (e.g. your pass a model of type MyPageViewModel but the post action method expects type of MyModel). I assume these are just typo's.

This was right on the money. I also adde a hidden field to return the Key with the post. — Gifreakius, May 24 '14 at 14:31

Hitesh Modha · Answer 2 · 2014-05-24T05:35:57.133

I can give you my solution,It is working:

Method in base controller

//To bind Dropdown list 
    protected Dictionary<int, string> GenerateDictionaryForDropDown(DataTable dtSource, string keyColumnName, string valueColumnName)
    {
        return dtSource.AsEnumerable()
          .ToDictionary<DataRow, int, string>(row => row.Field<int>(keyColumnName),
                                    row => row.Field<string>(valueColumnName));
    }

Code in controller:

    DataTable dtList = new DataTable();

    dtList = location.GetDistrict();
    Dictionary<int, string> DistrictDictionary = GenerateDictionaryForDropDown(dtList, "Id", "DistrictName");
    model.DistrictList = DistrictDictionary;

Binding Data in view:

 @Html.DropDownListFor(model => model.DiscrictId, new SelectList(Model.DistrictList, "Key", "Value"), new { id = "ddlDist", @class = "form-control" })

Binding Other Dropdown from this(cascading): Other Dropdown:

@Html.DropDownListFor(model => model.TalukaId, new SelectList(Model.TalukaList, "Key", "Value"), new { id = "ddlTaluka", @class = "form-control" })

JQuery Code: $("#ddlDist").change(function () { var TalukaList = "Select" $('#ddlTaluka').html(TalukaList);

        $.ajax({
            type: "Post",
            dataType: 'json',
            url: 'GetTaluka',
            data: { "DistId": $('#ddlDist').val() },
            async: false,
            success: function (data) {
                $.each(data, function (index, optionData) {
                    TalukaList = TalukaList + "<option value='" + optionData.Key + "'>" + optionData.Value + "</option>";
                });
            },
            error: function (xhr, status, error) {
                //alert(error);
            }
        });
        $('#ddlTaluka').html(TalukaList);
    });

Controller Method Return JSON

public JsonResult GetTaluka(int DistId)
{
    LocationDH location = new LocationDH();
    DataTable dtTaluka = location.GetTaluka(DistId);
    Dictionary<int, string> DictionaryTaluka = GenerateDictionaryForDropDown(dtTaluka, "ID", "TalukaName");
    return Json(DictionaryTaluka.ToList(), JsonRequestBehavior.AllowGet);
}

I don't quite understand how this fits the question. The view code will result in only a single dropdown, correct? The model binding problem I am having occurs when multiple dropdowns are posted within the same form. — Gifreakius, May 24 '14 at 05:19

How Do I Model Bind A List Of 'List' Using MVC.Net

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