Question

How do you associate a MultiSelectList with a list of checkboxes?

eg. I pass something like this to the model

  model.Groups = new MultiSelectList(k.Groups, "Id", "Name", selectedGroups)

How should I render it? This doesn’t work

<% foreach (var item in Model.Groups.Items) { %>
  <input type="checkbox" name="groups" value="<%=item.Value%>" id="group<%=item.Value%>" checked="<%=item.Selected?"yes":"no"%>" />
  <label for="group<%=item.Value%>"><%=item.Text%></label>
<% } %>

Error CS1061: ‘object’ does not contain a definition for ‘Value’…

Is there a HTML Helper method that I can use?

(Then, unless it is straightforward, how should I then get the selected values back on the Controller when the form is submitted?)

Answer 1

I just tested to see how we can see if the selection was changed.

public class Group {
    public int ID { get; set; }
    public string Name { get; set; }
}

//And some data to play with
var allGroups = new List<Group>();
allGroups.Add(new Group { ID = 1, Name = "one" });
allGroups.Add(new Group { ID = 2, Name = "two" });
allGroups.Add(new Group { ID = 3, Name = "three" });

var selectedGroups = new List<Group>();
selectedGroups.Add(allGroups[0]);
selectedGroups.Add(allGroups[2]);

var m = new MultiSelectList(allGroups, "ID", "Name", 
    selectedGroups.Select(x => x.ID));

//passed that data to the view with ViewData
ViewData["list"] = m;

The checkbox elements :

<% foreach (var item in (MultiSelectList)ViewData["list"]) { %>
    <input type="checkbox" name="groups" value="<%=item.Value%>"
        id="group<%=item.Value%>"
        <%=item.Selected ? "checked=\"checked\"" : String.Empty%>/>
    <label for="group<%=item.Value%>"><%=item.Text%></label>
<% } %>   

Accepted an int array in the action :

[HttpPost]
public ActionResult SomeAction(int[] groups) {
    if (groups != null) {
        var postedSelection = allGroups.Where(x => groups.Contains(x.ID));
        if (!selectedGroups.SequenceEqual(postedSelection)) {
            //selection was changed
        }
        else {
            //selection is the same
        }
    }
    else {
        //no group ID was posted
    }
}

I hope that gives some idea.