c# - 如何将 System.Data.Entity.Infrastructure.DbQuery 类型转换为 System.Collections.Generic.List 类型？

Question

我有两个简单的列表:

public partial class Visitor
{
    public Visitor()
    {
        this.Visits = new HashSet<Visit>();
    }

    public int Id { get; set; }
    public int PermitId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string MiddleName { get; set; }
    public bool IsValid { get; set; }
    public System.DateTime RegistrationDate { get; set; }
    public byte[] Picture { get; set; }
    public virtual ICollection<Visit> Visits { get; set; }
}

public partial class Visit
{
    public int Id { get; set; }
    public int VisitType { get; set; }
    public System.DateTime VisitDate { get; set; }
    public Nullable<int> Visitor_Id { get; set; }

    public virtual Visitor Visitor { get; set; }
}

在 WCF 方法中，我编写了查询并尝试返回结果:

public List<Visitor> AllVisitors()
        {
            using (var te = new TurnstileDbEntities())
            {
                te.Configuration.ProxyCreationEnabled = false;


                return (List<Visitor>) te.Visitors.SelectMany(visitors => te.Visits
                    .Where(o => o.Visitor_Id == visitors.Id)
                    .DefaultIfEmpty(), (visitors, visit) => new {visitors, visit});
            }
        }

好吧，正如预期的那样，我收到了一个异常:

Additional information: Failed to cast the type of object "System.Data.Entity.Infrastructure.DbQuery1[<>f__AnonymousType02[TurnstileWcfService.Visitor,TurnstileWcfService.Visit]]" to type "System.Collections.Generic.List`1[TurnstileWcfService.Visitor]".

好的。我重写这个。声明新类。

public class JoinResult
{
    public Visitor Visitors { get; set; }
    public Visit Visits { get; set; }
}

并重写方法。

   public IQueryable AllVisitors()
    {
        using (var te = new TurnstileDbEntities())
        {
            te.Configuration.ProxyCreationEnabled = false;

            return te.Visitors.Join(te.Visits, 
                r => r.Id, a => a.Visitor_Id, (r, a) => new JoinResult
            {
                Visits = a,
                Visitors = r
            });
        }
    }

这是可行的，但我想使用 Visitor 类。访客类包含 ICollectionVisits，我想填充它。如何在不声明新类的情况下在 Linq 语句中执行此操作？

我也可以在下面的代码中得到我想要的。但我不喜欢这样。

var visitors = te.Visitors.ToList();
foreach (var item in visitors)
{
      item.Visits = te.Visits.Where(v => v.Visitor_Id == item.Id).ToList();
}

Answer 1

Gert Arnold 是对的，试试看:

List<Visitor> visitors = te.Visitors.Include(v => v.Visits).ToList();

编辑我也让它按如下方式工作:

List<Visitor> visitors = te.Visitors.Include("Visits").ToList();