How to bind dropdownlist in mvc 3.0

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ASP.NET

SQL-Server

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MVC

Dear Sir,
I am new in mvc and i want to bind dropdownlist in MVC 3.0. My Code is given below

tables

tbl_Modules
Module_Id,ModuleName,ModuleDescription

tbl_DocumentTypes
Document_Id,DocumentName,DocumentDescription,Module_Id

now i want to craete form to add document type and in form i want a dropdownlist with Module_Id as value,ModuleName as text.

public class DocumetRepository
    {

        InwardManagementEntities db = new InwardManagementEntities();
       
        public IQueryable<tbl_DocumentTypes> FindAllDocumentTypes()
        {
            return db.tbl_DocumentTypes;
        }
       
    }


public class DocumentTypeViewModel
    {
         ModuleRepository _modulerepository = new ModuleRepository();
        public tbl_DocumentTypes Document { get; private set; }
        public SelectList Modules { get; private set; }

        public DocumentTypeViewModel(tbl_DocumentTypes document)
        {
            Document = document;
            //var _modules = _modulerepository.FindAllModules().Select(d => new  {Module_Id= SqlFunctions.StringConvert((double?)d.Module_Id), Text = d.ModuleName });
             var _modules = _modulerepository.FindAllModules().Select(d => new SelectListItem() {Value= SqlFunctions.StringConvert((double?)d.Module_Id), Text = d.ModuleName });
             Modules = new SelectList(_modules, Document.Module_Id);
        }
    }

And in conroller

public ActionResult AddDocument()
       {
           tbl_DocumentTypes _document = new tbl_DocumentTypes();

           return View(new DocumentTypeViewModel(_document));
       }

and in view

<div class="editor-label">
           Module
        </div>
        <div class="editor-field">
            @Html.DropDownList("Document.Module_Id", Model.Modules.ToList())
            @Html.ValidationMessageFor(model => model.Document.Module_Id)
        </div>

but in dropdownlist i get System.Web.Mvc.SelectListItem . Please help me