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?

This is a good way to user DropDownListFor -- stackoverflow.com/questions/3386998/… — bcahill
– bcahill, Commented Jul 31, 2012 at 14:22

Scott Baker · Accepted Answer · 2011-04-25 20:14:45Z

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.

Scott Baker · Accepted Answer · 2011-04-25 20:13:38Z

9

<%= 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?

edited Apr 25, 2011 at 20:13

Scott Baker

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answered Apr 5, 2010 at 5:13

Edmon

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1 Comment

Rich Andrews Over a year ago

wont replacing the "[ None ]" with the CategoryId only change the default/initial value of the drop down list and not the value in the viewmodel on postback? Was previously a downvote but I took it back in a fit of uncertainty after realising there is probably a different overload. Can you clarify a bit?

Scott Baker · Accepted Answer · 2011-04-25 20:14:19Z

8

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)%>

edited Apr 25, 2011 at 20:14

Scott Baker

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answered Sep 7, 2010 at 22:11

Per Hornshøj-Schierbeck

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Comments

Gertjan · Accepted Answer · 2012-11-30 13:25:23Z

1

If the SelectList is null in the ViewModel I also received the above error

Make sure that the SelectList is set

answered Nov 30, 2012 at 13:25

Gertjan

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