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POASP.NET MVC 4 DropDownListFor error: Null Values
 

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  1. POASP.NET MVC 4 DropDownListFor error: Null Values
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    <p>I am a beginner programmer and having trouble with the @Html.DropDownListFor helper...</p> <p>I am using a General Repository and Unit of Work pattern based off of the tutorial here: <a href="http://www.asp.net/mvc/tutorials/getting-started-with-ef-using-mvc/implementing-the-repository-and-unit-of-work-patterns-in-an-asp-net-mvc-application" rel="nofollow noreferrer">http://www.asp.net/mvc/tutorials/getting-started-with-ef-using-mvc/implementing-the-repository-and-unit-of-work-patterns-in-an-asp-net-mvc-application</a></p> <p>Here is my code for the Repository:</p> <pre><code>public class GenericRepository&lt;TEntity&gt; where TEntity : class { internal UsersContext context; internal DbSet&lt;TEntity&gt; dbSet; public GenericRepository(UsersContext context) { this.context = context; this.dbSet = context.Set&lt;TEntity&gt;(); } public virtual IEnumerable&lt;TEntity&gt; Get( Expression&lt;Func&lt;TEntity, bool&gt;&gt; filter = null, Func&lt;IQueryable&lt;TEntity&gt;, IOrderedQueryable&lt;TEntity&gt;&gt; orderBy = null, string includeProperties = "") { IQueryable&lt;TEntity&gt; query = dbSet; if (filter != null) { query = query.Where(filter); } foreach (var includeProperty in includeProperties.Split (new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries)) { query = query.Include(includeProperty); } if (orderBy != null) { return orderBy(query).ToList(); } else { return query.ToList(); } } public virtual TEntity GetByID(object id) { return dbSet.Find(id); } public virtual void Insert(TEntity entity) { dbSet.Add(entity); } // Delete methods not shown public virtual void Update(TEntity entityToUpdate) { dbSet.Attach(entityToUpdate); context.Entry(entityToUpdate).State = EntityState.Modified; } } </code></pre> <p>Here is my code for my UnitOfWork class:</p> <pre><code>public class UnitOfWork : IDisposable { private UsersContext context = new UsersContext(); private GenericRepository&lt;UserProfile&gt; userProfileRepository; private GenericRepository&lt;Lead&gt; leadRepository; private GenericRepository&lt;UnitedStatesState&gt; unitedStatesStateRepository; public GenericRepository&lt;UserProfile&gt; UserProfileRepository { get { if (this.userProfileRepository == null) { this.userProfileRepository = new GenericRepository&lt;UserProfile(context); } return userProfileRepository; } } public GenericRepository&lt;Lead&gt; LeadRepository { get { if (this.leadRepository == null) { this.leadRepository = new GenericRepository&lt;Lead&gt;(context); } return leadRepository; } } public GenericRepository&lt;UnitedStatesState&gt; UnitedStatesStateRepository { get { if (this.unitedStatesStateRepository == null) { this.unitedStatesStateRepository = new GenericRepository&lt;UnitedStatesState&gt;(context); } return unitedStatesStateRepository; } } </code></pre> <p>I am trying to use strongly typed views and models in order to pass the selectlist data to the view without using ViewData/ViewBag. From what I understand, the best practice is to do something similar to what I saw here: <a href="https://stackoverflow.com/questions/4729440/validate-a-dropdownlist-in-asp-net-mvc">validate a dropdownlist in asp.net mvc</a></p> <p>I tried following that as closely as possible and this is what I came up with</p> <p>My View Model looks like this:</p> <pre><code>public class Lead { public int LeadID { get; set; } public int CompanyID { get; set; } [Required(ErrorMessage = "Please enter state")] [Display(Name = "State")] [MaxLength(2)] public string State { get; set; } [Display(Name = "Assigned To")] public string AssignedTo { get; set; } [Timestamp] public Byte[] Timestamp { get; set; } public virtual Company Company { get; set; } // IEnumerables for Dropdown Lists passed to views public IEnumerable&lt;UnitedStatesState&gt; UnitedStatesStates { get; set; } public IEnumerable&lt;UserProfile&gt; UserProfiles { get; set; } // Objects passed to views public Lead lead { get; set; } } </code></pre> <p>These IEnumerables for my dropdown lists are then populated in my controller from my database through my repository. The odd part is that I am using these dropdown lists in two different views, Create and Edit. When I use the dropdown lists in the Create view they work perfectly both on the GET and POST ActionResults. When I try and use the same dropdown lists for my Edit view they work for the GET ActionResult (the view loads and the dropdowns work) but when I try to POST them to my Edit ActionResult I get the following error:</p> <p>{"Value cannot be null.\r\nParameter name: items"} // This is the error as shown in Visual Studio 2012</p> <p>System.ArgumentNullException: Value cannot be null. Parameter name: items // This is the error shown in Google Chrome</p> <p>Below is my Lead Controller with the Edit and Create ActionResults:</p> <pre><code>public class LeadController : Controller { // create instance of Repository Unit of Work private UnitOfWork unitOfWork = new UnitOfWork(); public ActionResult Create() { // Get the current users profile UserProfile userProfile = UserProfile.GetCurrentUserProfile(); // Creates Dropdown Lists to pass to view var model = new Lead { UnitedStatesStates = unitOfWork.UnitedStatesStateRepository.Get(u =&gt; u.StateAbbreviation != null), UserProfiles = unitOfWork.UserProfileRepository.Get(u =&gt; u.CompanyID == userProfile.CompanyID) }; // Return View return View(model); } [HttpPost] public ActionResult Create(Lead model) { try { if (ModelState.IsValid) { // Call the current users profile UserProfile userProfile = UserProfile.GetCurrentUserProfile(); // Create a new lead and apply all attirbutes that were entered Lead lead = new Lead(); lead.CompanyID = userProfile.CompanyID; lead.State = model.State; lead.AssignedTo = model.AssignedTo; // Add the lead and save the changes. Redirect to Lead Index. unitOfWork.LeadRepository.Insert(lead); unitOfWork.Save(); return RedirectToAction("Index"); } } catch (DataException) { ModelState.AddModelError("", "Unable to save changes. Try again and if the problem persists, see your system administrator."); } // Return view if ModelState is not valid return View(); } public ActionResult Edit(int id = 0) { // Get Users Profile UserProfile userProfile = UserProfile.GetCurrentUserProfile(); // Check to see if Lead Exists if (unitOfWork.LeadRepository.GetByID(id) == null) { return HttpNotFound(); } // Creates Dropdown Lists and Gets current lead values to pass to view var model = new Lead { lead = unitOfWork.LeadRepository.GetByID(id), UnitedStatesStates = unitOfWork.UnitedStatesStateRepository.Get(u =&gt; u.StateAbbreviation != null), UserProfiles = unitOfWork.UserProfileRepository.Get(u =&gt; u.CompanyID == userProfile.CompanyID) }; return View(model); } [HttpPost] public ActionResult Edit(Lead lead) { try { // Update lead if model state is valid if (ModelState.IsValid) { unitOfWork.LeadRepository.Update(lead); unitOfWork.Save(); return RedirectToAction("Index"); } } // Catch any concurrency exceptions catch (DbUpdateConcurrencyException ex) { var entry = ex.Entries.Single(); var databaseValues = (Lead)entry.GetDatabaseValues().ToObject(); var clientValues = (Lead)entry.Entity; if (databaseValues.State != clientValues.State) ModelState.AddModelError("State", "Current value: " + databaseValues.State); if (databaseValues.AssignedTo != clientValues.AssignedTo ) ModelState.AddModelError("Assigned To ", "Current value: " + databaseValues.AssignedTo ); ModelState.AddModelError(string.Empty, "The record you attempted to edit " + "was modified by another user after you got the original value. The " + "edit operation was canceled and the current values in the database " + "have been displayed. If you still want to edit this record, click " + "the Save button again. Otherwise click the Back to List hyperlink."); lead.Timestamp = databaseValues.Timestamp; } catch (DataException) { //Log the error (add a variable name after Exception) ModelState.AddModelError(string.Empty, "Unable to save changes. Try again, and if the problem persists contact your system administrator."); } // Return View if Model State is not valid return View(lead); } </code></pre> <p>The POST Edit ActionResult includes code to catch concurrencies which I created following the tutorial shown here: <a href="http://www.asp.net/mvc/tutorials/getting-started-with-ef-using-mvc/handling-concurrency-with-the-entity-framework-in-an-asp-net-mvc-application" rel="nofollow noreferrer">http://www.asp.net/mvc/tutorials/getting-started-with-ef-using-mvc/handling-concurrency-with-the-entity-framework-in-an-asp-net-mvc-application</a></p> <p>Below is my view for Create (this works perfectly):</p> <pre><code>@model SolarToolbase.Models.Lead @using (Html.BeginForm()) { @Html.ValidationSummary(true) &lt;div&gt; &lt;div&gt; @Html.LabelFor(model =&gt; model.State) @Html.DropDownListFor(model =&gt; model.State, new SelectList(Model.UnitedStatesStates, "StateAbbreviation", "UnitedStatesStateName"),"Choose State")&lt;br /&gt; @Html.ValidationMessageFor(model =&gt; model.State) &lt;/div&gt; &lt;div&gt; @Html.LabelFor(model =&gt; model.AssignedTo) @Html.DropDownListFor(model =&gt; model.AssignedTo, new SelectList(Model.UserProfiles, "FullName", "FullName"),"Choose User")&lt;br /&gt; @Html.ValidationMessageFor(model =&gt; model.AssignedTo) &lt;/div&gt; &lt;p&gt; &lt;input type="submit" value="Create" /&gt; &lt;/p&gt; &lt;/div&gt; } </code></pre> <p>Below is my view for Edit(this throws the aforementioned errors when I hit the submit button. I inserted a comment below to show the line that the error is being thrown from):</p> <pre><code>@model SolarToolbase.Models.Lead @using (Html.BeginForm()) { @Html.ValidationSummary(true) @Html.HiddenFor(model =&gt; model.lead.LeadID) @Html.HiddenFor(model =&gt; model.lead.Timestamp) &lt;div&gt; &lt;div&gt; @Html.LabelFor(model =&gt; model.lead.State) @Html.DropDownListFor(model =&gt; model.lead.State, new SelectList(Model.UnitedStatesStates, "StateAbbreviation", "UnitedStatesStateName"))&lt;br /&gt; // Error thrown from this line @Html.ValidationMessageFor(model =&gt; model.lead.State) &lt;/div&gt; &lt;div&gt; @Html.LabelFor(model =&gt; model.lead.AssignedTo) @Html.DropDownListFor(model =&gt; model.lead.AssignedTo, new SelectList(Model.UserProfiles, "FullName", "FullName"))&lt;br /&gt; @Html.ValidationMessageFor(model =&gt; model.lead.AssignedTo) &lt;/div&gt; &lt;p&gt; &lt;input type="submit" value="Save" /&gt; &lt;/p&gt; &lt;/div&gt; } </code></pre> <p>I apologize in advance for posting so much code, I just honestly don't know where this error is coming from and I've beat my head against the wall trying to figure it out for about 4 hours now. Free virtual high fives and good karma for anyone that can help.</p> <p>Thanks!</p>
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