MVC：通過SelectListItem，我得到“ System.NullReferenceException：對象引用未設置為對象的實例。”錯誤

Question

通過SelectListItem，我得到“ System.NullReferenceException：對象引用未設置為對象的實例。” 錯誤。 我怎么了

這是我的xml：

<?xml version="1.0" encoding="utf-8"?>
<Artiesten>
  <Item ID_Artiest="509998" Artiest="-" Product="-" Land="" Plaats="" Website="" Genre="Onbekend" />
  <Item ID_Artiest="970119" Artiest="(hed)p.e." Product="-" Land="" Plaats="" Website="" Genre="Onbekend" />
  <Item ID_Artiest="970080" Artiest="(urusei) yatsura" Product="-" Land="" Plaats="" Website="" Genre="Noise" />
  <Item ID_Artiest="1010302" Artiest="...and you will know us by the trail of dead" Product="-" Land="Verenigde Staten" Plaats="Austin (TX)" Website="" Genre="Noise" />
...
</Artiesten>

這是我的模特

public class ArtiestInfoModel
    {
        [DisplayName("Selecteer artiest: ")]
        public int ID_Artiest { get; set; }
        public string SelectedArtiest { get; set; }
        public IEnumerable<SelectListItem> ArtiestenLijst { get; set; }
    }

控制器：

 public ActionResult Index(int? AID_Artiest=810000)
        {
            var document = XDocument.Load(Server.MapPath("~/App_Data/Artiesten.xml"));
            var query = new ArtiestInfoModel
            {
                ArtiestenLijst =
                    (from artiest in document.Descendants("Artiesten").Elements("Item")
                    select new SelectListItem
                    {
                        Value = artiest.Attribute("ID_artiest").Value,
                        Text = artiest.Attribute("Artiest").Value,
                        Selected = false //  artiest.Attribute("ID_artiest").Value == AID_Artiest.ToString()
                    }).ToList()
            };

            ViewBag.ID_Artiest = AID_Artiest.Value;
            ViewBag.ArtiestenLijst = query.ArtiestenLijst;

            return View();
        }

下一個DropDownList調用Controller：

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">    
    <% using (Html.BeginForm("Index", "Hitdossier", FormMethod.Post, new { id = "frmArtiest" })) %>
    <% { %>
        <table><tr><th width="1000" align="left">Selecteer artiest:</th></tr>
            <tr>
                <td><%= Html.DropDownList("ID_Artiest",
                    new SelectList(ViewBag.ArtiestenLijst, "Value", "Text"),
                    "-- Selecteer artiest --",
                    new
                    {
                        @onchange = "document.getElementById('frmArtiest').submit();"
                    })%>
                </td>
            </tr>
        </table>
        <br />
        <br />
        <div id="divPartialView">
            <%= Html.Action("Detail_Hitdossier", new { AID_Artiest = ViewBag.ID_Artiest })%> <br />
        </div>
    <%} %> 
</asp:Content>

謝謝

Answer 1

在您的控制器中，您將節點ID_artiest讀為小寫，而xml顯示駝峰式。

更改

Value = artiest.Attribute("ID_artiest").Value,

至

Value = artiest.Attribute("ID_Artiest").Value,

如果artiest.Attribute("ID_artiest")結果為NULL，則讀取屬性Value將拋出null-reference-exception。

Answer 2

請初始化為這樣的新對象，例如ArtiestenLijst = new ArtiestenLijst(); 然后使用后