ViewBag property value in DropDownListFor instead of Model property value

Question

We found strange behaviour in DropDownListFor (ASP.NET MVC3 release). It selects ViewBag property value instead of Model property value in dropdown.

Model:

public class Country {
    public string Name { get; set; }
}
public class User {
    public Country Country { get; set; }
}

Controller Index action:

ViewBag.CountryList = new List<Country> {  /* Dropdown collection */
   new Country() { Name = "Danmark" }, 
   new Country() { Name = "Russia" } }; 

var user = new User();
user.Country = new Country(){Name = "Russia"}; /* User value */
ViewBag.Country = new Country() { Name = "Danmark" };  /* It affects user */
return View(user); 

View:

@Html.EditorFor(user => user.Country.Name)      
@Html.DropDownListFor(user => user.Country.Name,
    new SelectList(ViewBag.CountryList, "Name", "Name", Model.Country), "...")

It will show text box with "Russia" value and dropdown with "Danmark" value selected instead of "Russia".

I didn't find any documentation about this behaviour. Is this behaviour normal? And why is it normal? Because it is very hard to control ViewBag and Model properties names.

This sample MVC3 project sources

Answer 1

I'm not so sure why this decision was made, but it was happened because MVC framework tried to use the ViewData-supplied value before using the parameter-supplied value. That's why ViewBag.Country override parameter-supplied value Model.Country.

That was how it was written in MVC framework in the private method SelectInternal.

object defaultValue = (allowMultiple) ? htmlHelper.GetModelStateValue(fullName, typeof(string[])) : htmlHelper.GetModelStateValue(fullName, typeof(string));

// If we haven't already used ViewData to get the entire list of items then we need to
// use the ViewData-supplied value before using the parameter-supplied value.
if (!usedViewData) {
    if (defaultValue == null) {
        defaultValue = htmlHelper.ViewData.Eval(fullName);
    }
}

if (defaultValue != null) {
    IEnumerable defaultValues = (allowMultiple) ? defaultValue as IEnumerable : new[] { defaultValue };
    IEnumerable<string> values = from object value in defaultValues select Convert.ToString(value, CultureInfo.CurrentCulture);
    HashSet<string> selectedValues = new HashSet<string>(values, StringComparer.OrdinalIgnoreCase);
    List<SelectListItem> newSelectList = new List<SelectListItem>();

    foreach (SelectListItem item in selectList) {
        item.Selected = (item.Value != null) ? selectedValues.Contains(item.Value) : selectedValues.Contains(item.Text);
        newSelectList.Add(item);
    }
    selectList = newSelectList;
}

This code defaultValue = htmlHelper.ViewData.Eval(fullName); tried to get the value from ViewData and if it can get the value, it will override the supplied parameter selectList with new list.

Hope it can help. Thanks.

side-node: ViewBag is just a dynamic wrapper class of ViewData.

Answer 2

The following line from your action method is what is confusing the code:

ViewBag.Country = new Country() { Name = "Danmark" };  /* It affects user */

That's because the html helpers look into a few different places to pick up values for the generated controls. In this case ViewData["Country"] is clashing with ModelState["Country"] Rename that property to something else and everything should work.