Question

I’m having a moment. I can’t seem to get the selected items from a ListBox to be bound in a parameter to the action method that handles the post event.

Model is of type SystemRoleList:

public class SystemRoleList {
    public IEnumerable<SystemRole> List { get; set; }
}

SystemRole is defined as:

public class SystemRole {
    public Int32 Id { get; set; }

    public String Name { get; set; }
}

This code generates the ListBox:

<%: this.Html.ListBox("Roles", new SelectList(Model.List, "Id", "Name")) %>

The action method receiving the selected items is set up like this:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult UpdateSystemRoles(Int32[] roles) {
    // do something with the results....
}

The trouble is that roles is always null. I’ve tried using other data types – string[], ICollection<int>, and so on. I can see the values in the Request.Form collection if I do Request.Form["Roles[]"]. Typical values might be 1,3,4 if I selected those items from the ListBox.

How can I name either the ListBox or my parameter so that MVC will automatically bind the values?

Answer 1

Weird, the following works perfectly fine for me.

Model:

public class SystemRoleList
{
    public IEnumerable<SystemRole> List { get; set; }
}

public class SystemRole
{
    public int Id { get; set; }
    public string Name { get; set; }
}

Controller:

[HandleError]
public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new SystemRoleList
        {
            List = new[]
            {
                new SystemRole { Id = 1, Name = "role 1" },
                new SystemRole { Id = 2, Name = "role 2" },
                new SystemRole { Id = 3, Name = "role 3" },
            }
        };
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(int[] roles)
    {
        return Content("thanks for submitting");
    }
}

View:

<%@ Page 
    Language="C#" 
    MasterPageFile="~/Views/Shared/Site.Master" 
    Inherits="System.Web.Mvc.ViewPage<MvcApplication1.Controllers.SystemRoleList>" 
%>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">

    <% using (Html.BeginForm()) { %>
        <%= Html.ListBox("Roles", new SelectList(Model.List, "Id", "Name")) %>
        <button type="submit">OK</button>
    <% } %>

</asp:Content>

This being said I would use the strongly typed version of the ListBox helper, like this:

Model:

public class SystemRoleList
{
    public int[] Roles { get; set; }
    public IEnumerable<SystemRole> List { get; set; }
}

Controller:

[HandleError]
public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new SystemRoleList
        {
            List = new[]
            {
                new SystemRole { Id = 1, Name = "role 1" },
                new SystemRole { Id = 2, Name = "role 2" },
                new SystemRole { Id = 3, Name = "role 3" },
            }
        };
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(SystemRoleList model)
    {
        return Content("thanks for submitting");
    }
}

View:

<%@ Page 
    Language="C#" 
    MasterPageFile="~/Views/Shared/Site.Master" 
    Inherits="System.Web.Mvc.ViewPage<MvcApplication1.Controllers.SystemRoleList>" 
%>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">

    <% using (Html.BeginForm()) { %>
        <%= Html.ListBoxFor(x => x., new SelectList(Model.List, "Id", "Name")) %>
        <button type="submit">OK</button>
    <% } %>

</asp:Content>