Question

代码：

@{
....    
var GotAllDetails = ViewData["GetAllDetails"] as List<PRJ.DetailViewsModel>;
....
}

和

@foreach (var goteem in GotAllDetails){Print my pretty list}

我做了很多调试。我在var GetAllDetails之后放了一个断点，我可以确认ViewData["GetAllDetails"]已正确填充。我要进行类型转换的模型具有与ViewData.相同的值名称和类型但是，不知何故，GotAllDetails是null。当页面点击我的foreach时，它会通过我

异常详细信息：System.NullReferenceException：未将对象引用设置为对象的实例。

根据要求，我会分享更多代码，但首先告诉我是否有任何明显的错误。

EDIT：Per Request，这是指定ViewData的代码[“GetAllDetails”]

ViewData["GetAllDetails"] = getAllDetails.OrderBy(q => PadString(q.TaskDescription, ".")).ToList();

和LINQ查询

var getAllDetails =
    (from m in _db.MyStandards.AsEnumerable()
     join t in _db.Tasks on m.TaskId equals t.TaskId
     join p in _db.Products on t.ProductId equals p.ProductId
     join ce in _db.CompetencyElements on p.CompetencyElementId equals ce.CompetencyElementId
     join comp in _db.Competencies on ce.CompetencyId equals comp.CompetencyId
     join fu in _db.FunctionalUnitOfCompetences on comp.FunUnitOfCompetenceId equals fu.FunUnitOfCompetenceId
     join c in _db.Careers on fu.CareerId equals c.CareerId
     join rx in _db.RubricRefs on m.RubricStandardId equals rx.Id
     where (t.TaskId == m.TaskId && m.Email == getUserById && p.ProductId == proId)
     group new { t.TaskDescription, m.RubricStandardId, m.Comments }
     by new
     {
         c.CareerDescription,
         fu.FunUnitOfCompetenceDesc,
         comp.CompetencyDescription,
         ce.CompetencyElementdesc,
         p.ProductDescription,
         t.TaskDescription,
         m.RubricStandardId,
         m.Comments,
         m.StandardId,
         m.TaskId,
         m.ActiveInd,
         rx.RubricHexColor,
         rx.RubricSymbol
     } into g
     select new
     {
         ActiveInd = g.Key.ActiveInd,
         CareerDescription = g.Key.CareerDescription,
         Comments = g.Key.Comments,
         CompetencyDescription = g.Key.CompetencyDescription,
         CompetencyElementdesc = g.Key.CompetencyElementdesc,
         FunUnitOfCompetenceDesc = g.Key.FunUnitOfCompetenceDesc,
         ProductDescription = g.Key.ProductDescription,
         StandardId = g.Key.StandardId,
         RubricHexColor = g.Key.RubricHexColor,
         RubricStandardId = g.Key.RubricStandardId,
         RubricSymbol = g.Key.RubricSymbol,
         TaskDescription = g.Key.TaskDescription,
         TaskId = g.Key.TaskId,
     });

和模型

public class DetailViewsModel
{
    public string ActiveInd { get; set; }
    public string  CareerDescription {get;set;}
    public string Comments {get;set;}
    public string CompetencyDescription {get;set;}
    public string CompetencyElementdesc {get;set;}
    public string FunUnitOfCompetenceDesc {get;set;}
    public string ProductDescription {get;set;}
    public int StandardId {get;set;}
    public string RubricHexColor {get;set;}
    public int RubricStandardId {get;set;}
    public string RubricSymbol {get;set;}
    public string TaskDescription {get;set;}
    public int TaskId {get;set;}
}

Answer 1

此代码：

new  {
    ActiveInd = g.Key.ActiveInd,
    CareerDescription = g.Key.CareerDescription,
    Comments = g.Key.Comments,
    CompetencyDescription = g.Key.CompetencyDescription,
    CompetencyElementdesc = g.Key.CompetencyElementdesc,
    FunUnitOfCompetenceDesc = g.Key.FunUnitOfCompetenceDesc,
    ProductDescription = g.Key.ProductDescription,
    StandardId = g.Key.StandardId,
    RubricHexColor = g.Key.RubricHexColor,
    RubricStandardId = g.Key.RubricStandardId,
    RubricSymbol = g.Key.RubricSymbol,
    TaskDescription = g.Key.TaskDescription,
    TaskId = g.Key.TaskId,
 });

...不会产生DetailViewsModel。它产生的是一个具有相同属性名称和可能属性类型的类，但它不会产生SAME类。

如果您希望将DetailsViewModel获得的内容转换为getAllDetails，则必须实例化List<DetailViewsModel>。

幸运的是，您已经完成了困难的部分：设置所有属性值！你应该可以用这个代替：

  new DetailsViewModel() {
      ActiveInd = etc.

Answer 2

问题来自这种语法

var GotAllDetails = ViewData["GetAllDetails"] as List<PRJ.DetailViewsModel>;

根据MSDN

as运算符就像一个强制转换操作。但是，如果无法进行转换，则返回null而不是引发异常。

由于您说ViewData["GetAllDetails"]不为空，因此GotAllDetails为空，因为ViewData["GetAllDetails"]的类型不是List<PRJ.DetailViewsModel>。您需要确保ViewData["GetAllDetails"]的类型为List<PRJ.DetailViewsModel>。

根据您的问题中生成getAllDetails的linq查询，如下所示

var getAllDetails = ....
                    ....
                 select new
                 {
                     ActiveInd = g.Key.ActiveInd,
                     CareerDescription = g.Key.CareerDescription,
                     Comments = g.Key.Comments,
                     CompetencyDescription = g.Key.CompetencyDescription,
                     CompetencyElementdesc = g.Key.CompetencyElementdesc,
                     FunUnitOfCompetenceDesc = g.Key.FunUnitOfCompetenceDesc,
                     ProductDescription = g.Key.ProductDescription,
                     StandardId = g.Key.StandardId,
                     RubricHexColor = g.Key.RubricHexColor,
                     RubricStandardId = g.Key.RubricStandardId,
                     RubricSymbol = g.Key.RubricSymbol,
                     TaskDescription = g.Key.TaskDescription,
                     TaskId = g.Key.TaskId,
                 });

以及如何设置ViewData["GetAllDetails"]

ViewData["GetAllDetails"] = getAllDetails.OrderBy(q => PadString(q.TaskDescription, ".")).ToList();

很明显ViewData["GetAllDetails"]的类型是List<anonymous>而不是List<PRJ.DetailViewsModel>。

您需要将linq查询更改为以下

var getAllDetails = ....
                    ....
                 select new DetailViewsModel
                 {
                     ActiveInd = g.Key.ActiveInd,
                     CareerDescription = g.Key.CareerDescription,
                     Comments = g.Key.Comments,
                     CompetencyDescription = g.Key.CompetencyDescription,
                     CompetencyElementdesc = g.Key.CompetencyElementdesc,
                     FunUnitOfCompetenceDesc = g.Key.FunUnitOfCompetenceDesc,
                     ProductDescription = g.Key.ProductDescription,
                     StandardId = g.Key.StandardId,
                     RubricHexColor = g.Key.RubricHexColor,
                     RubricStandardId = g.Key.RubricStandardId,
                     RubricSymbol = g.Key.RubricSymbol,
                     TaskDescription = g.Key.TaskDescription,
                     TaskId = g.Key.TaskId,
                 });

然后改变这个

var GotAllDetails = ViewData["GetAllDetails"] as List<PRJ.DetailViewsModel>;

到这个

var GotAllDetails = (List<PRJ.DetailViewsModel>)ViewData["GetAllDetails"];

所以你会知道转换是否失败。

Answer 3

ViewData链接到Session。如果您想删除错误，请尝试此操作。

app.get('/path', function(req, res) {
  if (typeof req.param('param1') !== 'undefined' && typeof req.param('param2') !== 'undefined'){

    var param1 = req.param('param1');
    var param2 = req.param('param2');

    if (param1 === param2) {
      // explicitly return
      return User.findOne({ 'myfield' : param1 }, function (err, user) {
        // Do some async stuff, doesn't matter
      })
    }
  }

  return res.redirect('/login');
});

无法将ViewData分配/类型转换为变量，未设置对象引用

3 个答案: