<p>In MVC there is no viewstate (webforms) that goes back to the server and tells the model what you had in your drop down list. So on a post where the result might be a return trip with errors, your drop down list would be empty because it did not persist through the HttpPost life cycle.</p>

      <p>You could either re-populate the drop down list in the controller or, as this example shows, write an html helper that produces html that binds back to the model so you don&#39;t have to remember to repopulate data.</p>

      <p>This also is an example of binding Lists to a model simply by incrementing the html names.</p>
      <p>Here is where I got my info..</p>
      <p><a href="http://haacked.com/archive/2008/10/23/model-binding-to-a-list.aspx" target="_blank">http://haacked.com/archive/2008/10/23/model-binding-to-a-list.aspx</a></p>
      <p><a href="http://www.hanselman.com/blog/ASPNETWireFormatForModelBindingToArraysListsCollectionsDictionaries.aspx" target="_blank">http://www.hanselman.com/blog/ASPNETWireFormatForModelBindingToArraysListsCollectionsDictionaries.aspx</a></p>

      <p>&nbsp;</p>

      <p><strong>The Controller..</strong></p>
      <script type="syntaxhighlighter" class="brush: csharp"><![CDATA[
[HttpGet]
        [SecureAction(AreaName = AdminAreaName, ControllerName = _controllerName, ActionName = "AddEdit")]
        public ActionResult AddEdit(Guid? blogEntryGuid)
        {
            var model = new BlogEntryAddEditModel();
            if (blogEntryGuid == null)
            {
                model.CrudType = CrudType.Add;
            }
            else
            {
                model =
                    Mapper.Map<BlogEntry, BlogEntryAddEditModel>(BlogEntryRepository.BlogEntrySelect((Guid)blogEntryGuid));
                model.CrudType = CrudType.Edit;
            }
            model.BlogEntryTypes =
                BlogEntryTypeRepository.BlogEntryTypeFind()
                                       .Select(b
                                               => new SelectListItem
                                                   {
                                                       Selected = false,
                                                       Text = b.Name,
                                                       Value = b.BlogEntryTypeGuid.ToString()
                                                   })
                                       .ToList();
            return View(model);
        }

        [HttpPost]
        [SecureAction(AreaName = AdminAreaName, ControllerName = _controllerName, ActionName = "AddEdit")]
        public ActionResult AddEdit(BlogEntryAddEditModel model)
        {
            ActionResult result = View(model);
            if (ModelState.IsValid)
            {
                try
                {
                    switch (model.CrudType)
                    {
                        case CrudType.None:
                            throw new ArgumentOutOfRangeException();
                        case CrudType.Add:
                            BlogEntryRepository.BlogEntryInsert(blogEntryGuid: SequentialGuid.NewSequentialGuid(),
                                                                blogEntryTypeGuid: model.BlogEntryTypeGuid,
                                                                title: model.Title, description: model.Description,
                                                                publishDate: (DateTime)model.PublishDate,
                                                                content: model.Content,
                                                                createdBy: base.CurrentUserName(),
                                                                keywords: model.Keywords, created: DateTime.Now,
                                                                updated: DateTime.Now);
                            result = RedirectToAction("Index");
                            break;
                        case CrudType.Edit:
                            BlogEntryRepository.BlogEntryUpdate(blogEntryGuid: model.BlogEntryGuid, title: model.Title,
                                                                blogEntryTypeGuid: model.BlogEntryTypeGuid,
                                                                description: model.Description, content: model.Content,
                                                                publishDate: (DateTime)model.PublishDate,
                                                                created: model.Created,
                                                                updated: DateTime.Now, createdBy: model.CreatedBy,
                                                                keywords: model.Keywords);
                            result = RedirectToAction("Index");
                            break;
                        default:
                            throw new ArgumentOutOfRangeException();
                    }
                }
                catch (Exception ex)
                {
                    base.HandleError(ex);
                    ModelState.AddModelError("",
                                             ex.Message + " " +
                                             (ex.InnerException != null ? ex.InnerException.Message : ""));
                }
            }

            return result;
        }
]]></script>

      <p><strong>The Html Helper</strong></p>
      <script type="syntaxhighlighter" class="brush: csharp"><![CDATA[
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web;
using System.Web.Mvc;
using System.Web.Mvc.Html;

namespace Ch.WebUi.HtmlExtentions
{
    public static class ChadExtentions
    {
        /// <summary>
        /// Creates Html Markup that is formatted for successful automatic model binding using hidden fields for persistent data
        /// </summary>
        /// <param name="htmlHelper"></param>
        /// <param name="list"></param>
        /// <param name="collectionName"></param>
        /// <param name="controlName"></param>
        /// <param name="selectedValue"></param>
        /// <returns></returns>
        public static MvcHtmlString ChDropDownList(this HtmlHelper htmlHelper, List<SelectListItem> list, string collectionName, string controlName, string selectedValue)
        {
            var sb = new StringBuilder();

            if (list != null)
            {
                //Sort the list
                list = (from l in list
                        orderby l.Text ascending
                        select l).ToList();

                int i = 0;

                foreach (var l in list)
                {
                    string collectionNameIndex = String.Format("{0}[{1}]", collectionName, i);

                    var hiddenName = new TagBuilder("input");
                    hiddenName.Attributes.Add(new KeyValuePair<string, string>("type", "hidden"));
                    hiddenName.Attributes.Add(new KeyValuePair<string, string>("name", String.Format("{0}.{1}", collectionNameIndex, "Text")));
                    hiddenName.Attributes.Add(new KeyValuePair<string, string>("value", l.Text));

                    var hiddenValue = new TagBuilder("input");
                    hiddenValue.Attributes.Add(new KeyValuePair<string, string>("type", "hidden"));
                    hiddenValue.Attributes.Add(new KeyValuePair<string, string>("name", String.Format("{0}.{1}", collectionNameIndex, "Value")));
                    hiddenValue.Attributes.Add(new KeyValuePair<string, string>("value", l.Value));

                    var hiddenIsSelected = new TagBuilder("input");
                    hiddenIsSelected.Attributes.Add(new KeyValuePair<string, string>("type", "hidden"));
                    hiddenIsSelected.Attributes.Add(new KeyValuePair<string, string>("name", String.Format("{0}.{1}", collectionNameIndex, "Selected")));
                    hiddenIsSelected.Attributes.Add(new KeyValuePair<string, string>("value", (l.Value == selectedValue ? "true" : "false")));

                    sb.Append(hiddenName);
                    sb.Append(hiddenValue);
                    sb.Append(hiddenIsSelected);

                    i++;
                }

                var dropDown = htmlHelper.DropDownList(controlName, list);

                sb.Append(dropDown);
            }

            return MvcHtmlString.Create(sb.ToString());
        }
    }
}
]]></script>

      <p><strong>The View...</strong></p>
      <script type="syntaxhighlighter" class="brush: csharp"><![CDATA[
            @using Ch.WebUi.HtmlExtentions
            @Html.LabelFor(m => m.BlogEntryTypeGuid)
            @Html.ValidationMessageFor(m => m.BlogEntryTypeGuid)<br />
            @Html.ChDropDownList(Model.BlogEntryTypes, "BlogEntryTypes", "BlogEntryTypeGuid", Model.BlogEntryTypeGuid.ToString())
]]></script>

      <p><strong>Resulting Html...</strong></p>
      <script type="syntaxhighlighter" class="brush: html"><![CDATA[
<div>
      <label for="BlogEntryTypeGuid">BlogEntryTypeGuid</label>
      <span id="BlogEntryTypeGuid_validationMessage" class="field-validation-valid"></span>
      <br>
      <input type="hidden" value="Developer" name="BlogEntryTypes[0].Text">
      <input type="hidden" value="390d5b6b-668c-4e51-9d4c-4f21cfbf0369" name="BlogEntryTypes[0].Value">
      <input type="hidden" value="true" name="BlogEntryTypes[0].Selected">
      <input type="hidden" value="Electronics" name="BlogEntryTypes[1].Text">
      <input type="hidden" value="5ebb60d0-e168-42b3-bc12-e8d8239a0298" name="BlogEntryTypes[1].Value">
      <input type="hidden" value="false" name="BlogEntryTypes[1].Selected">
      <input type="hidden" value="Music" name="BlogEntryTypes[2].Text">
      <input type="hidden" value="96a86a75-87ac-4895-a962-44c6bda0baae" name="BlogEntryTypes[2].Value">
      <input type="hidden" value="false" name="BlogEntryTypes[2].Selected">

      <select name="BlogEntryTypeGuid" id="BlogEntryTypeGuid">
      <option value="390d5b6b-668c-4e51-9d4c-4f21cfbf0369" selected="selected">Developer</option>
      <option value="5ebb60d0-e168-42b3-bc12-e8d8239a0298">Electronics</option>
      <option value="96a86a75-87ac-4895-a962-44c6bda0baae">Music</option>
      </select>
      </div>
]]></script>

An Html Helper to create a Drop Down List in MVC that binds back to the model

Allan Chadwick

MVC Drop Down Control and Model Binding to Lists