ASP.NET MVC 2 - Html.DropDownListFor confusion with ViewModel

Question

I'm getting totally lost and confused on how to use the new strongly typed Html.DropDownListFor helper on ASP.NET MVC 2.0 R2

In the View I'm writting:

<%= Html.DropDownListFor(m => m.ParentCategory, new SelectList(Model.Categories, "CategoryId", "Name", Model.ParentCategory), "[ None ]")%>

<%= Html.ValidationMessageFor(m => m.ParentCategory)%>

and my Model object is thus:

public class CategoryForm : FormModelBase
{
    public CategoryForm()
    {
        Categories = new List<Category>();

        Categories.Add(new CategoryForm.Category() {
                           CategoryId = 1, 
                           Name = "CPUs" });
        Categories.Add(new CategoryForm.Category() { 
                           CategoryId = 2, 
                           Name = "Memory" });
        Categories.Add(new CategoryForm.Category() { 
                           CategoryId = 3, 
                           Name = "Hard drives" });
    }

    // ...other props, snip... //

    public Category ParentCategory { get; set; }

    public IList<Category> Categories { get; protected set; }

    public class Category
    {
        public int? CategoryId { get; set; }
        public string Name { get; set; }
    }
}

The problem is that when I select an item from the dropdown list, say the first item, I get the following ValidationMessageFor error "The value '1' is invalid."

So I change the View to...

<%= Html.DropDownListFor(m => m.ParentCategory.**CategoryId**, 
                              new SelectList .../ snip  ) %>

Now it works, kinda. The ParentCategory property in my ViewModel is set with the correct 'CategoryId' but the 'Name' is NULL. Am I better off just having a nullable int for ParentCategory property instead of a strongly typed 'Category' object?

Answer 1

I was also experiencing the same issue.

When I debug the Action and look at the ModelState.Values[1].Errors[0].Exception for example, I see the following:

{"The parameter conversion from type 'System.String' to type 'System.Collections.Generic.KeyValuePair`2[[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Int64, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]' failed because no type converter can convert between these types."} System.Exception {System.InvalidOperationException}

In my scenario, my SelectList is created from a Dictionary and i use this in my view:

<%=Html.DropDownListFor(x => x.MyDictionary, 
                             new SelectList(
                                 Model.MyDictionary, 
                                 "Value", 
                                 "Key")) %>

When I changed it to:

<%=Html.DropDownListFor(x => x.MyDictionary.Keys, // <-- changed to .Keys
                             new SelectList(
                                 Model.MyDictionary, 
                                 "Value", 
                                 "Key")) %>

It worked without issues.

Thank you.

Answer 2

<%= Html.DropDownListFor(m => m.ParentCategory, 
                              new SelectList(
                                  Model.Categories,
                                  "CategoryId", 
                                  "Name",
                                  Model.ParentCategory),
                             "[ None ]")%> 

Did you try to use Model.ParentCategory.CategoryId as a last parameter in SelectList and remove [None] parameter?

Answer 3

I would get rid of

public Category ParentCategory { get; set; }

and make a

public int? CategoryId { get; set; }

instead. You probably only need the Id anyways - you could always look up the actual object using the Id as key (using a linq/lambda on your list in your case).

The view will then have these two:

<%= Html.DropDownListFor(m => m.CategoryId, 
                              new SelectList(
                                  Model.Categories, 
                                  "CategoryId", 
                                  "Name", 
                                  Model.CategoryId), 
                              "[ None ]")%>

<%= Html.ValidationMessageFor(m => m.CategoryId)%>