DropDownListFor SelectList不能识别SelectedValue

Question

I am working with a simple example of a call to HtmlHelper.DropDownListFor() , and the passed SelectList object has a default value indicated in the constructor:

SelectList MySelectList = new SelectList(MyCollection, "ValueField", "TextField", DefaultIdValue);

And then this SelectList object is passed into my view model into a strongly-typed view.

If I break on the @Html.DropDownListFor() call, I can see that indeed the SelectList object passed into the DropDownListFor() method invocation has the proper SelectedValue property set to the correct value field value. And even through enumerating the select list items, I can see that the correct item has Selected set to true . Yet, when this renders this selected value is not actually initially selected, it's still just the first item in the drop down listbox.

What could I be doing wrong here? I've done all object verification, but this SelectedValue just doesn't seem to be "recognized" here. Thank you in advance for the assistance!

EDIT

Here is my model (it's a viewmodel that is passed into a strongly-typed view)

public class MyViewModel
{
    public IEnumerable<MyClass> MyClasses { get; set; }
    public SelectList MyClassList { get; set; }

    public int SelectedId { get; set; }
    public int PreSelectedId { get; set; }
}

Then I have logic to determine the PreSelectedId in the action prior to pushing the viewmodel out to the view:

        PreSelectedId = // code to get preselected id.  Debugged and working

        SelectList TempList = new SelectList(MyCollectionOfObj, "Id", "TextField", PreSelectedId);

        return View(
            new MyViewModel()
            {
                MyClassList = TempList,
                MyClasses = // retrieves some objects here,
                PreSelectedId = PreSelectedId
            });

The actual selected value visually is the first element. The actual selected value by breaking on the code and looking at the SelectList.SelectedValue is the correct one (the 5th item, for instance). Visual and implementation are showing two different things, confusingly.

Answer 1

It appears you are trying to bind to property SelectedId , as in

@Html.DropDownListFor(m => m.SelectedId, Model.ClassList)

Since the value of SelectedId is 0 (you don't seem to be setting it anywhere) the helper tries to find an option with a value of 0 , which it cant, so the first option is displayed (it has to show something) which is why you often use the overload of @Html.DropDownListFor() that adds the "--Please select--" label which has a null value.

Change you code to

return View(new MyViewModel()
{
  MyClassList = TempList,
  MyClasses = // retrieves some objects here,
  SelectedId = PreSelectedId // change this
});

and remove the 4th argument from the SelectList constructor (its not necessary since the purpose of the strongly typed helpers are to bind your model values)