Asp.net mvc 两个类的包装类中的DropDownList

我的问题是从我结束三节课开始的 第一节课是

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

[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; }
}

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

第二节课是

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

第三类是

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

[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; }
}

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

包装类是

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

[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; }
}

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

我需要为datavalue=MaterialName的物料显示dropdownlist 在Examswrapper类上构建的视图中 我正在尝试这个

怎么做 谢谢你的建议

新错误：

---

System.NullReferenceException:对象引用未设置为对象的实例。

视图模型存在问题，因为如果需要下拉列表，则

材质属性应为集合。此外，材质类缺少ID属性。因此：

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

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
}

公共类材料
{
公共字符串ID{get；set；}
公共字符串MaterialName{get；set；}
}
公共类ExamView模型
{
公共字符串SelectedMaterialId{get；set；}
公共IEnumerable材质{get；set；}
…//其他一些属性
}

然后在强类型视图中：

<%= 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"
    )
) %>

x.SelectedMaterialId，
//基于Model.Materials集合构建选择列表
新选择列表(
Model.Materials.Select（material=>newselectListItem
{
值=material.ID，
Text=material.MaterialName
}),
“值”、“文本”
)
) %>
类材料中没有Id，丢失？有一个错误：方法“Select”没有重载3arguments@Abdalmohaymen是的，的确，我犯了一个错误。我已经更新了我的帖子。@Abdalmohaymen，您是否在控制器操作中初始化了
材料
集合？