Why won't Html.ListBoxFor() highlight current selected items? - asp.net

Question

I am trying to understand why my Html.ListBoxFor() is not highlighting current selected items when the view loads.
I have a database model:
public class Issue
{
[Key]
public int IssueId { get; set; }
public int Number { get; set; }
public string Title { get; set; }
public DateTime Date { get; set; }
public virtual ICollection<Creator> Creators { get; set; }
}
public class Creator
{
[Key]
public int CreatorId { get; set; }
public string FirstName { get; set; }
public string MiddleName { get; set; }
public string LastName { get; set; }
public virtual ICollection<Issue> Issues { get; set; }
}
public class Icbd : DbContext
{
public DbSet<Issue> Issues { get; set; }
public DbSet<Creator> Creators { get; set; }
}
I then have an editing model:
public class IssueEditModel
{
public Issue Issue { get; set; }
public IEnumerable<Creator> Creators { get; set; }
public IEnumerable<Creator> SelectedCreators { get {return Issue.Creators;} }
}
Then, in my controller I populate IssueEditModel:
public ActionResult EditIssue( int id = 0 )
{
IssueEditModel issueEdit = new IssueEditModel{
Creators = db.Creators.ToList(),
Issue = new Issue{ Creators = new List<Creator>()},
};
if (id > 0)
{
issueEdit.Issue = db.Issues.Include("Creators").Where(x => x.IssueId == id).Single();
}
return View(issueEdit);
}
This populates all objects correctly (as far as I can tell, anyway.) In my View, I am writing a listbox like this:
<%: Html.ListBoxFor(
x => x.SelectedCreators,
new SelectList(
Model.Creators,
"CreatorId",
"LastName"
)
)%>
This lists all the options correctly, but I cannot get the currently select items to highlight. I almost want to write my own Html Helper because this is such a simple operation, I don't understand why this is being so difficult.
Why wont the Html Helper highlight the current items?

Answer 1

You need a list of scalar types as first argument to the ListBoxFor helper which will map to the creator ids that you want preselected:
public class IssueEditModel
{
public IEnumerable<Creator> Creators { get; set; }
public IEnumerable<int> SelectedCreatorIds { get; set; }
}
and then:
IssueEditModel issueEdit = new IssueEditModel
{
Creators = db.Creators,
SelectedCreatorIds = db.Creators.Select(x => x.CreatorId)
};
and in the view:
<%: Html.ListBoxFor(
x => x.SelectedCreatorIds,
new SelectList(
Model.Creators,
"CreatorId",
"LastName"
)
) %>

Answer 2

The example submitted by Darin is ALMOST correct, but there is a slight error. Presently, his example will preselect ALL creators! However, the desired result is to only preselect the creators associated with a particular instance of Issue.
So this:
IssueEditModel issueEdit = new IssueEditModel
{
Creators = db.Creators,
SelectedCreatorIds = db.Creators.Select(x => x.CreatorId)
};
Should be this:
IssueEditModel issueEdit = new IssueEditModel
{
Creators = db.Creators,
SelectedCreatorIds = CurrentIssue.Creators.Select(x => x.CreatorId)
};
Where CurrentIssue is an instantiation of the Issue class (presumably previously populated from the datastore).