asp.net mvc c# 中的数据类型转换问题

Question

我必须将 linq 的结果保存在缓存中，然后检索它。但是我收到以下代码的错误：

public PartialViewResult showall()
{
    //initially getting data from DB
    var task1 = from r in _db.Task1
            where r.Status == true
            orderby r.Post_Date descending
            select new
            {
             id = SqlFunctions.StringConvert((double)r.Task1_ID),
             dt = r.Post_Date,
             typ = "Task1"
            };
    HttpContext.Cache.Insert(key: "Task1Data",
                         value: task1, 
                         dependencies: null, 
                         absoluteExpiration: DateTime.Now.AddHours(5), 
                         slidingExpiration: System.Web.Caching.Cache.NoSlidingExpiration
                         );
    var task1Cache = (System.Web.HttpContext.Current.Cache["Task1Data"] as IQueryable);
    var task1list = from a in task1Cache.Cast<Task1data>()
                select new
                {
                 id = SqlFunctions.StringConvert((double)a.Task1_ID),
                 dt = a.Post_Date,
                 typ = a.typ 
                }; 
    var task2 = from r in _db.Task2
            orderby r.Post_Date descending
            select new
            {
             id = SqlFunctions.StringConvert((double)r.Task2_ID),
             dt = r.Post_Date,
             typ = "Task2"
            };
    HttpContext.Cache.Insert(key: "Task2Data",
                         value: task2, 
                         dependencies: null, 
                         absoluteExpiration: DateTime.Now.AddHours(5), 
                         slidingExpiration: System.Web.Caching.Cache.NoSlidingExpiration
                         );
    var task2Cache = (System.Web.HttpContext.Current.Cache["Task2Data"] as IQueryable);
    var task2list = from a in task2Cache.Cast<Task2data>()
                select new
                {
                 id = SqlFunctions.StringConvert((double)a.Task2_ID),
                 dt = a.Post_Date,
                 typ = a.typ 
                }; 
    var listc = task1list.Union(task2list);
    var q3 = (from r in listc.Cast<Totaldata>() 
                 select new
                 {
                     mix = r.typ + "~" + r.id
                 }).ToList();
    return PartialView("~/Views/Ex/_ShowAll.cshtml", q3);
}

我在最后一行得到以下异常：

An exception of type 'System.NotSupportedException' occurred in System.Data.Entity.dll but was not handled in user code

Additional information: Unable to cast the type 'Anonymous type' to type 'Models.Task1data'. LINQ to Entities only supports casting EDM primitive or enumeration types.

public class Task1data
{
    public double Task1_ID { get; set; }
    public DateTime Post_Date { get; set; }
    public string typ { get; set; }
}

public class Task2data
{
    public double Task2_ID { get; set; }
    public DateTime Post_Date { get; set; }
    public string typ { get; set; }
}

public class Totaldata
{
    public double id { get; set; }
    public DateTime Post_Date { get; set; }
    public string typ { get; set; }
}

Answer 1

我已经摆脱了 task1data 和 task2data 转换，因为它们不是必需的（而且不正确）并且只使用了总数据 class。另外，我创建了一个新类型来存储名为 something 的结果，因为你不能传递匿名类型。

public something() { public string mix {get;set;}}
public PartialViewResult showall()
{
    //initially getting data from DB
    var task1 = from r in _db.Task1
            where r.Status == true
            orderby r.Post_Date descending
            select new Totaldata
            {
             id = SqlFunctions.StringConvert((double)r.Task1_ID),
             dt = r.Post_Date,
             typ = "Task1"
            };
    HttpContext.Cache.Insert(key: "Task1Data",
                         value: task1, 
                         dependencies: null, 
                         absoluteExpiration: DateTime.Now.AddHours(5), 
                         slidingExpiration: System.Web.Caching.Cache.NoSlidingExpiration
                         );
    var task1Cache = (System.Web.HttpContext.Current.Cache["Task1Data"] as IQueryable<Totaldata>);
    var task1list = from a in task1Cache
                select new Totaldata
                {
                 id = SqlFunctions.StringConvert((double)a.Task1_ID),
                 dt = a.Post_Date,
                 typ = a.typ 
                }; 
    var task2 = from r in _db.Task2
            orderby r.Post_Date descending
            select new Totaldata
            {
             id = SqlFunctions.StringConvert((double)r.Task2_ID),
             dt = r.Post_Date,
             typ = "Task2"
            };
    HttpContext.Cache.Insert(key: "Task2Data",
                         value: task2, 
                         dependencies: null, 
                         absoluteExpiration: DateTime.Now.AddHours(5), 
                         slidingExpiration: System.Web.Caching.Cache.NoSlidingExpiration
                         );
    var task2Cache = (System.Web.HttpContext.Current.Cache["Task2Data"] as IQueryable<Totaldata>);
    var task2list = from a in task2Cache
                select new Totaldata
                {
                 id = SqlFunctions.StringConvert((double)a.Task2_ID),
                 dt = a.Post_Date,
                 typ = a.typ 
                }; 
    var listc = task1list.Union(task2list);
    var q3 = (from r in listc 
                 select new something
                 {
                     mix = r.typ + "~" + r.id
                 }).ToList();
    return PartialView("~/Views/Ex/_ShowAll.cshtml", q3);
}

Answer 2

问题归根结底是你在这里对类型玩得又快又松。首先，当您从数据库中提取任务时，您是将它们选择到一个 匿名对象 中，即 select new { ... }。现在的结果是 DbQuery<[AnonymousType]>。这是您在缓存中设置的内容。然后，当您将其拉出时，您正试图将其转换为 IQueryable。那应该没问题，但现在您甚至不再具有匿名类型的动态特性。

接下来，您将整个内容再次投射到 Task1Data。这行不通有两个原因。首先，您在这里有一个项目集合，您永远无法像 Task1Data 那样将其转换为非集合 class。其次，即使您将其更正确地转换为 IEnumerable<Task1Data> 之类的内容，集合中项目的成员也不匹配 Task1Data 中的属性。它们甚至都不是一回事，所以你永远不能转换回 Task1Data.

asp.net mvc c# 中的数据类型转换问题

Data Type casting issue in asp.net mvc c#

c#

linq

asp.net

caching

asp.net-mvc-4