ASP.NET MVC DropDownListFor not selecting value from model

Question

I'm using ASP.NET MVC 3, and just ran into a 'gotcha' using the DropDownListFor HTML Helper.

I do this in my Controller:

ViewBag.ShippingTypes = this.SelectListDataRepository.GetShippingTypes();

And the GetShippingTypes method:

public SelectList GetShippingTypes()
{
    List<ShippingTypeDto> shippingTypes = this._orderService.GetShippingTypes();

    return new SelectList(shippingTypes, "Id", "Name");
}

The reason I put it in the ViewBag and not in the model (I have strongly typed models for each view), is that I have a collection of items that renders using an EditorTemplate, which also needs to access the ShippingTypes select list.

Otherwise I need to loop through the entire collection, and assign a ShippingTypes property then.

So far so good.

In my view, I do this:

@Html.DropDownListFor(m => m.RequiredShippingTypeId, ViewBag.ShippingTypes as SelectList)

(RequiredShippingTypeId is of type Int32)

What happens is, that the value of RequiredShippingTypeId is not selected in the drop down.

I came across this: http://web.archive.org/web/20090628135923/http://blog.benhartonline.com/post/2008/11/24/ASPNET-MVC-SelectList-selectedValue-Gotcha.aspx

He suggests that MVC will lookup the selected value from ViewData, when the select list is from ViewData. I'm not sure this is the case anymore, since the blog post is old and he's talking about MVC 1 beta.

A workaround that solves this issue is this:

@Html.DropDownListFor(m => m.RequiredShippingTypeId, new SelectList(ViewBag.ShippingTypes as IEnumerable<SelectListItem>, "Value", "Text", Model.RequiredShippingTypeId.ToString()))

I tried not to ToString on RequiredShippingTypeId at the end, which gives me the same behavior as before: No item selected.

I'm thinking this is a datatype issue. Ultimately, the HTML helper is comparing strings (in the Select List) with the Int32 (from the RequiredShippingTypeId).

But why does it not work when putting the SelectList in the ViewBag -- when it works perfectly when adding it to a model, and doing this inside the view:

@Html.DropDownListFor(m => m.Product.RequiredShippingTypeId, Model.ShippingTypes)

Answer 1

The reason why this doesn't work is because of a limitation of the DropDownListFor helper: it is able to infer the selected value using the lambda expression passed as first argument only if this lambda expression is a simple property access expression. For example this doesn't work with array indexer access expressions which is your case because of the editor template.

You basically have (excluding the editor template):

@Html.DropDownListFor(
    m => m.ShippingTypes[i].RequiredShippingTypeId, 
    ViewBag.ShippingTypes as IEnumerable<SelectListItem>
)

The following is not supported: m => m.ShippingTypes[i].RequiredShippingTypeId. It works only with simple property access expressions but not with indexed collection access.

The workaround you have found is the correct way to solve this problem, by explicitly passing the selected value when building the SelectList.