使用list< T>时如何防止数据丢失数据？其来源是iqueryable< T>类型。 [英] How do I prevent data loss data while using list<T> whose source is of type iqueryable<T>

问题描述

我正在使用

 IQueryable 

和列表。这是我的代码：

< pre lang =   C＃>公共ActionResult列表（chap =  false ）
 {
 

 IQueryable< UserInfo> userQuery = context.UserInfoes.Include（  aspnet_Users）
 .Include（  aspnet_Users.aspnet_Membership）
 .Include（  Pshycologists）。OrderBy（m = >  m.aspnet_Users.UserName）; 

列表< UserInfo> userInfoList =  new  List< UserInfo>（）; 
 UserInfo userInfoRow =  new  UserInfo（）; 

< pre lang =C＃> var userQueryForChapter = userQuery.OrderBy（a = > a.aspnet_Users。用户名）;
foreach （ var 行 userQueryForChapter）
{
var stagesUserCompleted =（ from au db.aspnet_Users
join spu in db .stagePageUsers on new {UserId = au.UserId}等于 new {UserId =（Guid）spu.UserId }
join sp db.StagePages new {StagePageId =（ Int32 ）spu.StagePageId}等于 new {StagePageId = sp.StagepageId}
join s db.Stages on new {StageID =（ Int32 ）sp.StageID}等于 new {StageID = s.StageId}
其中
au.UserId == row.UserId
选择 s
）.ToList（）;
if （stages.Count == stagesUserCompleted.Count）
{
userInfoRow = row;
}

userInfoList.Add（userInfoRow）;

}

var users = userInfoList.ToPagedList（page，100）;



ViewBag.searchKey = user;

返回查看（用户）;

}







使用此代码时，我收到错误。错误是

 对象引用而不是 set 到<的实例< span class =code-keyword> object 。

。错误在这一行

 @ item.aspnet_Users。用户名

我想这个错误的原因是因为我正在转换自

 IQueryable< UserInfo> 

到

列表<   UserInfo  >  

。这导致aspnet_Users.UserName的值丢失。如何转换为列表仍然没有丢失aspnet_Users.UserName的值我调试并发现aspnet_Users.UserName在行var user = userInfoList.ToPagedList（page，100）上为null;（userInfoList.aspnet_Users.UserName的值为null，其中userQuery中有值。 aspnet_Users.UserName）。





有什么帮助吗？ ？



我尝试了什么：



我添加了代码var用户= userInfoList.ToPagedList（page，100）.AsQueryable（）;使它成为可查询但我得到错误传入字典的模型项是'System.Linq.EnumerableQuery`1 [Kats.DAL.UserInfo]'类型，但这个字典需要一个类型为'PagedList.IPagedList的模型项`1 [Kats.DAL.UserInfo]'。

解决方案

Quote：

  if （stages.Count == stagesUserCompleted.Count）
 {
 userInfoRow = row; 
} 
 
 userInfoList.Add（userInfoRow）; 

我怀疑这就是问题所在。



对于查询的第一行，如果 stages.Count！= stagesUserCompleted.Count ，你就是将空白 UserInfo 记录添加到列表中。没有任何相关实体将被填充，因此当您尝试访问它们时，您将获得 NullReferenceException 。



对于后续行，如果 stages.Count！= stagesUserCompleted.Count ，则表示您要添加上一行的另一个副本。这对我来说是不对的。



我怀疑你打算做的是：

  if （stages.Count == stagesUserCompleted.Count）
 {
 userInfoList.Add（row）; 
} 

I am using

IQueryable

and list. This is my code :

<pre lang="C#">public ActionResult List(chap = false)
 {

IQueryable<UserInfo> userQuery = context.UserInfoes.Include("aspnet_Users")
                                   .Include("aspnet_Users.aspnet_Membership")
                                   .Include("Pshycologists").OrderBy(m => m.aspnet_Users.UserName);

List<UserInfo> userInfoList = new List<UserInfo>();
    UserInfo userInfoRow = new UserInfo();

var userQueryForChapter = userQuery.OrderBy(a => a.aspnet_Users.UserName);
                  foreach (var row in userQueryForChapter)
                  {
                      var stagesUserCompleted = (from au in db.aspnet_Users
                                                 join spu in db.StagePageUsers on new { UserId = au.UserId } equals new { UserId = (Guid)spu.UserId }
                                                 join sp in db.StagePages on new { StagePageId = (Int32)spu.StagePageId } equals new { StagePageId = sp.StagepageId }
                                                 join s in db.Stages on new { StageID = (Int32)sp.StageID } equals new { StageID = s.StageId }
                                                 where
                                                   au.UserId == row.UserId
                                                 select s
                          ).ToList();
                      if (stages.Count == stagesUserCompleted.Count)
                      {
                          userInfoRow = row;
                      }

                   userInfoList.Add(userInfoRow);

                  }

var users = userInfoList.ToPagedList(page, 100);

ViewBag.searchKey = user;
return View(users);
}



In using this code, I am getting error in view. The error is "

Object reference not set to an instance of an object.

. The error is on this line

@item.aspnet_Users.UserName

I guess the cause of the bug is because I am converting from

IQueryable<UserInfo>

to

List<UserInfo>

. This resulted to loss in value of aspnet_Users.UserName. How can I convert to list and still not loss value of aspnet_Users.UserName ?? I debugged and found out that aspnet_Users.UserName is null on line var users = userInfoList.ToPagedList(page, 100); (value of userInfoList.aspnet_Users.UserName is null where as value is there in userQuery.aspnet_Users.UserName ).


Any help??

What I have tried:

I added code var users = userInfoList.ToPagedList(page, 100).AsQueryable(); to make it as queryable but I got error "The model item passed into the dictionary is of type 'System.Linq.EnumerableQuery`1[Kats.DAL.UserInfo]', but this dictionary requires a model item of type 'PagedList.IPagedList`1[Kats.DAL.UserInfo]'."

解决方案

Quote:

if (stages.Count == stagesUserCompleted.Count)
{
    userInfoRow = row;
}

userInfoList.Add(userInfoRow);

I suspect that's where the problem lies.

For the first row in the query, if stages.Count != stagesUserCompleted.Count, you're adding a blank UserInfo record to the list. None of the related entities will be populated, so when you try to access them, you'll get a NullReferenceException.

For subsequent rows, if stages.Count != stagesUserCompleted.Count, you're adding another copy of the previous row. That looks wrong to me.

I suspect what you meant to do was:

if (stages.Count == stagesUserCompleted.Count)
{
    userInfoList.Add(row);
}

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