ASP.NET MVC 2 strongly typed partial view for foreign key list - what type gets passed in?

Question

I would like to pass the item list for a drop-down list into a strongly-typed partial view. The parent view uses a ViewModel class that contains the properties both for the value I want to set and the list of options the associated drop-down list should contain.

My ViewModel looks like:

public class EntityFormViewModel
{
    private MyDBEntity Entity { get; set; }

    // *** The list of options. ***
    public SelectList DataTypeSelectList { get; private set; }

    public int EntityId { get; set; }
    public byte FormatId { get; set; }

    // *** The property the options relate to. ***
    [UIHint("DataTypesDropDownST")]
    public string DataType { get; set; }
    // Other properties snipped...


    public EntityFormViewModel(EntityModel db, int Id)
    {
        Entity = db.MyDBEntity.First(e => e.EntityId == Id);
        DataTypeSelectList = new SelectList(db.SDDS_DataType.ToList(), "DataType", "Description", this);

        EntityId = Entity.EntityId;
        FormatId = Entity.FormatId;
        DataType = Entity.DataType;
        // Etc ...
    }

    public void Update(EntityModel db)
    {
        Entity.EntityId = EntityId;
        Entity.FormatId = FormatId;
        Entity.DataType = DataType;
        // Etc ...

        db.SaveChanges();
    }
};

My partial view in EditorTemplates/DataTypesDropDown.ascx looks like

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<MvcApp.Controllers.EntityFormViewModel>" %>

<!-- Doesn't work. -->
<%= Html.DropDownListFor(m => Model, Model.DataTypeSelectList) %>

<!-- Doesn't work either. -->
<!-- <% Html.DropDownListFor(m => Model, Model.DataTypeSelectList); %> -->

And my parent view looks like

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<MvcApp.Controllers.EntityFormViewModel>" %>

<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    Edit
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">

    <h2>Edit</h2>

    <% using (Html.BeginForm()) {%>
        <%: Html.ValidationSummary(true) %>

        <fieldset>
            <legend>Fields</legend>

            <%: Html.EditorFor(m => m) %>

            <p>
                <input type="submit" value="Save" />
            </p>
        </fieldset>

    <% } %>

    <div>
        <%: Html.ActionLink("Back to List", "Index") %>
    </div>

</asp:Content>

I can get it to work fine without strong typing by passing the list of items in the ViewData dictionary, thus:

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl" %>

<%: Html.DropDownList("", new SelectList(ViewData["DataTypes"] as IEnumerable, "DataType", "Description", Model)) %>

but I get the following error with strong typing:

The model item passed into the dictionary is of type 'System.String', but this dictionary requires a model item of type 'MvcApp.Controllers.EntityFormViewModel'.

It looks like the wrong type (a string) is being passed into the partial view, but what can I do to change the type I pass? Or is my approach completely wrong? If so, how should it be done? I imagine this is a very common scenario.

I know I should use a "repository" class for database access, but I want to get this working before building too much other stuff around it. Note my ViewModel is in the controllers namspace. I guess I should move it to the Model namespace, but I doubt that's the cause of my problem.

=================================================================================

UPDATE

I still have a problem, but at least I solved the compiler error. I changed the view model as follows

    // Make this private
    private string DataType { get; set; }

    // Add this which supplies all the list information.
    [UIHint("DataTypesDropDownST")]
    public DataTypeOptsAndVal DataTypeAndOpts
    {
        get { return new DataTypeOptsAndVal(DataType, DataTypeSelectList); } 
        set { DataType = value.DataType; } 
    }

The idea being derived from the fact the return value of the property controls the type passed into the partial view; the whole model is not passed, which was my earlier mistake. Therefore, I return a type that contains everything the partial view needs. The new partial view is

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<MvcApp.Controllers.DataTypeOptsAndVal>" %>

<%: Html.DropDownListFor(m => m.DataType, Model.DataTypeSelectList) %>

However, the form now renders with this field completely absent. I tried including static text in the partial view, but even this was not rendered.

Any help appreciated. Surely it isn't that hard to strongly type a list of options?

Answer 1

In your main view instead of:

<%: Html.EditorFor(m => m) %>

use:

<%: Html.EditorForModel() %>

It's equivalent but more readable.

As far as why the editor template is not rendered you used an UIHint attribute on your model. This means that it will look for ~/Views/Home/EditorTemplates/DataTypesDropDownST.ascx. Make sure this file is present at that location and contains the code you've shown in your update.