Html.ListBox() and MultiselectList

Question

I've a little problem with an Html.ListBox.

I am developing a personal blog in ASP.NET MVC 1.0 and I created an adminpanel where I can add and edit a post! During this two operations, I can add also tags.

I think of use an Html.ListBox() helper to list all tags, and so I can select multiple tags to add in a post! The problem isn't during the add mode, but in the edit mode, where I have to pre-select post's tags.

I read that I have to use a MultiSelectList and so in its constructor pass, tags' list and tag's list(pre-selected value).

But I don't know how to use this class.

I post, some code:

This is my models method that get all list tags in selectlist

public IEnumerable<SelectListItem> GetTagsListBox()
    {
            return   from t in db.Tags
                     orderby t.IDTag descending
                     select new SelectListItem {
                         Text = t.TagName,
                         Value = t.IDTag.ToString(),
                     };
    }

So in Edit (Get and Post), Add(Get and Post) I use a ViewData to pass this list in Html.ListBox().

ViewData["Tags"] = tagdb.GetTagsListBox();

And in my view

<%=Html.ListBox("Tags",ViewData["Tags"] as SelectList) %>

So with this code it's ok in Add Mode.

But in Edit Mode I need to pre-select those values.

So Now, of course I have to create a method that get all tagsbypostid.

and then in ViewData what Must I to pass?

Any suggest?

Answer 1

You could do the following I think:

public IEnumerable<SelectListItem> GetTagsListBoxWithPostTagsSelected(int postID)
{
    // Assuming you need to create this function and that Tag.IDTag is an int
    var postTags = GetAllTagsByPostID(postID);

    return from t in db.Tags           
           orderby t.IDTag descending           
           select new SelectListItem {           
               Text = t.TagName,           
               Value = t.IDTag.ToString(),
               Selected = postTags.Exists(pt => pt.IDTag == t.IDTag)
           };           
}           

This should return you the proper list with the values from the post selected.

You will need to make the GetAllTagsByPostID(postID) and have a new method that takes in a PostID to make sure the tags get selected properly.

No change would be required to:

<%=Html.ListBox("Tags", ViewData["Tags"] as SelectList) %>

The ViewData["Tags"] should now contain the required information to select your ListBox items.