Null Object refrence in the acess of the view in A

问题:

I am developing an ASP .Net MVC 3 application using C# and SQL Server 2005.

I am using also Entity Framework and Code First Method.

I have a view Index which contains some links to othres views.

Among those views, there is 'Update.aspx' which i created (not generated automaticalu by Entity Framework).

Its object is to edit some values in a table 'Gamme' in my base.

The problem is that : when I click on the link of this view (Update), this error appears :

Object reference not set to an instance of an object.

This a part of the code of the view Index which contain the link for the view Upadate :

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.master" Inherits="System.Web.Mvc.ViewPage<MvcApplication2.Models.FlowViewModel>" %>
<%@ Import Namespace="Helpers" %>

<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    Index
</asp:Content>

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


<fieldset>
<legend>Gestion des Gammes</legend>
<table>
    <tr>
        <th>
            ID Gamme
        </th>
        <th>
            ID Poste
        </th>
        <th>
           Nombre de Passage
        </th>
        <th>
            Position
        </th>
        <th>
            Poste Précédent
        </th>
         <th>
            Poste Suivant
        </th>
        <th>Opérations</th>
    </tr>

<% foreach (var item in Model.GaItems) { %>
    <tr>
         <td>
            <%: Html.DisplayFor(modelItem => item.ID_Gamme) %>
        </td>
        <td>
            <%: Html.DisplayFor(modelItem => item.ID_Poste) %>
        </td>
        <td>
            <%: Html.DisplayFor(modelItem => item.Nbr_Passage) %>
        </td>
        <td>
            <%: Html.DisplayFor(modelItem => item.Position) %>
        </td>
        <td>
            <%: Html.DisplayFor(modelItem => item.Last_Posts) %>
        </td>
         <td>
            <%: Html.DisplayFor(modelItem => item.Next_Posts) %>
        </td>
        <td>

            <%: Html.ActionLink("Modifier", "Update", new { id=item.ID_Gamme }) %> 

        </td>
    </tr>


<% } %>

</table>
 </fieldset>     
</asp:Content>

and this a part of the code of the controller ProfileGa :

public ActionResult Update(string id)
        {
            FlowViewModel flv = db.FlowViewModels.Find(id);
            return View(flv);
        }

        [HttpPost]
        public ActionResult Update(FlowViewModel model)
        {
            Console.WriteLine("" + model.Nbr_Passage);

                Gamme G = new Gamme();
                ListG.Remove(G);
                db.Gammes.Remove(G); 
                G.ID_Gamme = model.SelectedProfile_Ga;
                G.ID_Poste = model.SelectedPoste;
                G.Last_Posts = model.PostePrecedentSelected;
                G.Next_Posts = model.PosteSuivantSelected;
                G.Nbr_Passage = int.Parse(model.Nbr_Passage);
                G.Position = int.Parse(model.Position);
                ListG.Add(G);
                db.Gammes.Add(G);
                db.SaveChanges();

            return RedirectToAction("Index");

        } 

and this is the code of the model FlowViewModel :

public class FlowViewModel
    {

        [Key]
        public string IDv { get; set; }
        [NotMapped]
        public SelectList PostesItems { get; set; }
        public List<Profile_Ga> Profile_GaItems { get; set; }
     public List<Gamme> GaItems { get; set; }



        public string SelectedProfile_Ga { get; set; }


      public string SelectedPoste { get; set; }

      public string PostePrecedentSelected { get; set; } 
      public string PosteSuivantSelected { get; set; }


        public string Position { get; set; }
        public string  Nbr_Passage { get; set; }

        public List<Gamme> ListG = new List<Gamme>();

    }

and finaly the code of the View 'UPDATE' :

asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    Update
</asp:Content>

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

<h2>Modifier une gamme</h2>

<script src="<%: Url.Content("~/Scripts/jquery.validate.min.js") %>" type="text/javascript"></script>
<script src="<%: Url.Content("~/Scripts/jquery.validate.unobtrusive.min.js") %>" type="text/javascript"></script>

<% using (Html.BeginForm()) { %>
    <%: Html.ValidationSummary(true) %>
    <fieldset>
        <legend>Gamme</legend>
        <div><%:Html.Label("Gamme :")%><%: Html.DropDownListFor(model => model.SelectedProfile_Ga, new SelectList(Model.Profile_GaItems, "ID_Gamme", "ID_Gamme"))%></div>

         <div><%:Html.Label("Poste :")%><%: Html.DropDownListFor(model => model.SelectedPoste, Model.PostesItems)%></div>


         <div><%:Html.Label("Nombre de Passage :")%><%: Html.EditorFor(model=>model.Nbr_Passage)%></div><div class="editor-field"> <%: Html.ValidationMessageFor(model => model.Nbr_Passage)%></div>
        <div><%:Html.Label("Position :")%><%: Html.EditorFor(model=>model.Position)%></div>
        <div><%:Html.Label("Poste Précédent :")%><%: Html.DropDownListFor(model => model.PostePrecedentSelected, Model.PostesItems)%></div>
        <div><%:Html.Label("Poste Suivant :")%><%: Html.DropDownListFor(model => model.PosteSuivantSelected, Model.PostesItems)%></div>
        <p>
            <input type="submit" value="Enregistrer" />
        </p>
    </fieldset>
<% } %>

<div>
    <%: Html.ActionLink("Reour à la liste", "Index") %>
</div>  

回答1:

On the following line of code in your controller you seem to be fetching the model from somewhere:

FlowViewModel flv = db.FlowViewModels.Find(id);

Make sure (by placing a breakpoint here) that all properties that you are using in your Update view are properly initialized here. Possible causes for this exception are that either the flv is null or some of its properties that are being used.

Usually when you get a NullReferenceException at runtime, the YSOD is pointing to the exact location of your view in which you are getting this exception. This will give you a hint at which property you need to populate.