MVC Dropdown list from different entity

I have a view, that is of Inherits from System.W开发者_开发技巧eb.Mvc.ViewPage<P>.

Type P has a property of Type C, ie, P could be:

class P
{
   public string Name { get; set; }
   public C OtherData { get; set; }
}

The View in question is for creating a new P, and as such I want to create a DropDown list of all the available C's.

So the dropdown could be:

• C1
• C2
• C3
• C4

I've tried creating the List<SelectListItem> object for the C's in my controller and then pass them to the view using ViewData but this does not work, as when I try to submit I get the exception:

System.InvalidOperationException: There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key C.

So my question is, how do I create a list of C objects to display in my View that inherits from type P. The list of objects is pulled from a database and as such I cannot just simply hardcode the options into the view.

Do I need to create another Model type that marries the 2 types together?

The way I do this is as follows:

ViewModel:

public class ViewModel
{
    public string Name { get; set; }
    public List<SelectListItem> ListOfCs { get; set; }
}

My View has the following:

<%:Html.EditorFor(model => model, new { ListOfCs = Model.ListOfCs })%>

I use the default Object.ascx outlined in Brad Wilson's excellent series of blog posts on ASP.NET MVC 2 Templates

Then I add a dropdown template for that dropdown in the Views/Shared/EditorTemplates folder and call it something like CClassDropDown:

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<string>" %>

<%: Html.DropDownList(null, new List<SelectListItem>(ViewData["ListOfCs"] as List<SelectListItem>), "Select C...")%>

And then I add a partial metadata class for the P object:

[MetadataType(typeof(PMetaData))]
public partial class P
{
    public class PMetaData
    {
        [UIHint("CClassDropDown")] 
        [DataType(DataType.Text)]
        public object C { get; set; }

    }
}

to the metadata for that class property so it knows to use the template I specified.

I've only done this with Linq2Sql so YMMW.

In Linq2Sql the entity has both an foreign entity, and a foreign entity key.

So, it would have both C and C_id.

class P
{
public string Name { get; set; }
public C OtherData { get; set; }
public int OtherData_id { get; set; }
}

In that case its just a case of doing

MyViewData {
  SelectList CList = new SelectList( ctx.getCs(), "C_id", "C_name")
}  

and in the page doing

<%= Html.DropDownList("OtherData_id", Model.CList,"-") %>

In this way

TryModelUpdate(...) 

will correctly set the OtherData_id, which in turn will cause OtherData to be resolved when accessed.