ASP.NET MVC 2 - Html.DropDownListFor confusion wit

问题:

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?

回答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.

回答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?

回答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)%>

回答4:

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

Make sure that the SelectList is set