ASP.NET MVC DropDownListFor not selecting value from model

asp.net-mvc-3

Solution

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`.

Problem

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) ```

Original source