MVC3在where子句

Question

我有我的控制器

var workshop = registerDB.Workshops.Single(w => w.WorkshopID == id); 
ViewBag.Enrollments = registerDB.Carts.Where(x => x.Username.Equals(User.Identity.Name)); 

以下多個條件，這在我看來

@{ 
    //var carts = Model.Carts.Where(x => x.Username.Equals(User.Identity.Name)); 
    var carts = ViewBag.Enrollments; 
    var timeSlot = Model.TimeSlot; 
    } 

    @{ 
foreach (var item in carts) 

{ 
    if (item != null) 
    { 
    if (timeSlot == item.Workshop.TimeSlot) 
    { 
     <h3>@timeSlot</h3> 
    } 
    } 
    else 
    { 
     <h3>Does not Exist</h3> 
    } 

} 
} 

每次ViewBag.Enrollments = registerDB.Carts.Where(x => x.Username.Equals(User.Identity.Name));回報沒有結果，我得到一個錯誤說System.InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first.這行突出顯示

if (timeSlot == item.Workshop.TimeSlot) 

Answer 1

嘗試調用.ToList()在控制器中預先抓取結果：

ViewBag.Enrollments = registerDB 
    .Carts 
    .Where(x => x.Username.Equals(User.Identity.Name)) 
    .ToList(); 

Answer 2

您檢查item != null但不是item.Workshop != null嘗試使用它之前。這似乎也許是錯誤，但爲什麼它提高InvalidOperationException，而不是NullReferenceException我不知道。

嘗試：

if (item != null && item.Workshop != null) 
{ 
    if (timeSlot == item.Workshop.TimeSlot) 
    { 
     <h3>@timeSlot</h3> 
    } 
} 

Answer 3

你能不能把單一的呼叫模型：

workshops workshop = registerDB.Workshops.Single(w => w.WorkshopID == id); 

然後在你的控制器設置ViewBag：

try 
{ 
ViewBag.Enrollments = workshop.Carts.Where(x => x.Username.Equals(User.Identity.Name)); 
} 
catch 
{ 
ViewBag.Enrollments = "There are no results"; 
}