Why does the DropDownListFor lose the multiple selection after Submit but the ListBoxFor doesn’t?

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As the names of the methods imply, DropDownListFor() is for creating a <select> (to select 1 option) and ListBoxFor() is for creating a <select multiple> (to select multiple options). While both methods share a lot of common code, they do produce different results.

Adding the multiple="multiple" attribute changes the display, but it does not change the functionality of the code executed by these methods.

If you inspect the source code, you will note that all the overloads of DropDownListFor() ultimately call the private static MvcHtmlString DropDownListHelper() method, and similarly ListBoxFor() ultimately calls the private static MvcHtmlString ListBoxHelper() method.

Both these methods call the private static MvcHtmlString SelectInternal() method, but the difference is that DropDownListHelper() passes allowMultiple = false while the ListBoxHelper() passes allowMultiple = true.

Within the SelectInternal() method, the key line of code is

object defaultValue = (allowMultiple) ? htmlHelper.GetModelStateValue(fullName, typeof(string[])) : htmlHelper.GetModelStateValue(fullName, typeof(string));

The value of defaultValue is then used when building html for the <option> elements and is used to set the selected attribute(s).

In the case of ListBoxFor(), the value of defaultValue will be the array defined by your SelectedAssignees property. In the case of DropDownListFor() it returns null because the value of your property cannot be cast to string (its an array).

Because defaultValue is null, none of the <option> elements have the selected attribute set and you lose model binding.

As a side note, if you were to set the values of SelectedAssignees in the GET method before you pass the model to the view, you will see that none of them are selected when using DropDownListFor() for the same reasons described above.

Note also that the code for generating the SelectList should just be

vm.TasksFilterGroup.Assignees = new SelectList(employees, "Id", "FullName" });

There is no point setting the 3rd parameter when using either the DropDownListFor() or ListBoxFor() methods because its the value of the property your binding to (SelectedAssignees) that determines which options are selected (the 3rd parameter is ignored by the methods). If you want the options matching those Guid values to be selected, then in the GET method, use

vm.TasksFilterGroup.SelectedAssignees= new string[]{ "51b6f06a-e04d-4f98-88ef-cd0cfa8a2757", "51b6f06a-e04d-4f98-88ef-cd0cfa8a2769" };

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