的DropDownList在包裝類兩類

Question

我的問題，當我換三類 第一類是開始

[Bind(Exclude="ID")] 
public class Material 
{ 
    public string MaterialName { get; set; } 
} 

第二類是

[Bind(Exclude="ID")] 
public class ExameState 
{ 
    public string ExamState { get; set; } 
} 

第三類是

[Bind(Exclude="ID")] 
public class Exams 
{ 
    public string ExamsName { get; set; } 
    public DateTime CreationDate { get; set; } 
    public DateTime StartingDate { get; set; } 
    public int Period { get; set; } 
    public int ExamStateID { get; set; } 
    public int MaterialID { get; set; } 
    public int GroupID { get; set; } 
    public int QuestionState { get; set; } 
    public int TeacherID { get; set; } 
    public int ExamMarkState { get; set; } 
    public string Password { get; set; } 
} 

包裝類是

public class Examswrapper 
{ 
    public Material material { get; set; } 
    public ExameState examstate { get; set; } 
    public Exam exam { get; set; } 
} 

我需要datavalue = MaterialName 並鑑於關鍵= ID建立在Examswrapper類顯示下拉列表物料 我想這

如何使它 ，感謝您的諮詢

新錯誤： System.NullReferenceException：未將對象引用設置爲對象的實例。

Answer 1

沒有您的視圖模型一個問題，因爲Material屬性應該是一個集合，如果你想有一個下拉列表中。此外您的材料類缺少ID屬性。所以：

public class Material 
{ 
    public string ID { get; set; } 
    public string MaterialName { get; set; } 
} 

public class ExamsViewModel 
{ 
    public string SelectedMaterialId { get; set; } 
    public IEnumerable<Material> Materials { get; set; } 

    ... // some other properties 
} 

，然後在你的強類型的視圖：

<%= Html.DropDownListFor(
    // This is the property that will hold the selected value 
    x => x.SelectedMaterialId, 
    // Build the select list based on the Model.Materials collection 
    new SelectList(
     Model.Materials.Select(material => new SelectListItem 
     { 
      Value = material.ID, 
      Text = material.MaterialName 
     }), 
     "Value", "Text" 
    ) 
) %> 

Answer 2

@Html.DropDownListFor(model => model.material, 
     Model.material.Select(
     x => new SelectListItem 
     { 
      Text = x.MaterialName, 
      Value = x.Id 
     } 
    ))