只有实体的无参数构造函数和初始化器？

Question

我收到此错误，

LINQ 仅支持无参数构造函数和初始化程序 到实体。

经过研究，这似乎是一个有很多答案的常见问题。除了我不能对很多答案进行逆向工程，以便我自己解决这个问题。

我意识到答案在错误中，但我看不到需要使用无参数构造函数吗？或者在这个例子中，我什至不知道参数在哪里？？？

这个 LINQ 查询给我带来了麻烦：

public static ObservableCollection<Employee> ReturnEmployeesInSection(string section)
{
    using (StaffShiftManagerEntities dataBaseEntities = new  StaffShiftManagerEntities())
    {
          return new ObservableCollection<Employee>(dataBaseEntities.Staff_Data_TBL
             .Where(p => p.Section_Data == section && p.Staff_Bool == true)
             .Select(staff => new Employee(staff.Staff_No ?? -1,
                staff.Staff_Name_First, staff.Staff_Name_Second))
             .ToList()
             .GroupBy(staff => staff.Key)
             .Select(staff => staff.First())
             .OrderBy(staff => staff.Key)
             .ToList());
    }
}    

还有Employee 类：

public class Employee
{
    public int Key { get; set; }
    public string FirstName { get; set; }
    public string SecondName { get; set; }
    public string FullName => FirstName + " " + SecondName;

    public Employee(int key, string first = null, string second = null)
    {
        Key = key;
        FirstName = first;
        SecondName = second;
    }
}

这是实体框架自动生成的类：

public partial class Staff_Data_TBL
{
    public long ID { get; set; }
    public byte[] Image_Col { get; set; }
    public Nullable<int> Staff_No { get; set; }
    public string Staff_Name_First { get; set; }
    public string Staff_Name_Second { get; set; }
    public string Section_Data { get; set; }
    public string Lic_Data { get; set; }
    public Nullable<System.DateTime> Start_Date { get; set; }
    public Nullable<System.DateTime> End_Date { get; set; }
    public Nullable<long> Night_1 { get; set; }
    public Nullable<long> Night_2 { get; set; }
    public Nullable<long> Lunch_Data { get; set; }
    public Nullable<long> Unples_Data { get; set; }
    public string Staff_Part { get; set; }
    public bool Staff_Kaizen { get; set; }
    public int StaffKaizenPercentage { get; set; }
    public Nullable<bool> Staff_Bool { get; set; }
    public string Group_Section_Data { get; set; }
    public string Notes { get; set; }
}

请原谅命名约定，我正在将所有内容更改为更标准的命名。

Answer 1

为Employee 类创建公共无参数构造函数。

public Employee() {}

使用对象初始化器而不是带参数的构造器：

     dataBaseEntities.Staff_Data_TBL
         .Where(s => s.Section_Data == section && s.Staff_Bool == true)
         .GroupBy(s => s.Staff_No) // Important, see notes below
         .Select(g => g.FirstOrDefault())
         .OrderBy(s => s.Staff_No)
         .Select(s => new Employee {
            Key = s.Staff_No ?? -1,
            FirstName = s.Staff_Name_First,
            LastName = s.Staff_Name_Second
          })
         .ToList()

注意：如您所见，我在结果投影之前移动了分组和排序。因此，您将在服务器端完成所有工作，而不是将表数据加载到内存中。

Answer 2

改变

public class Employee
{
    public int Key { get; set; }
    public string FirstName { get; set; }
    public string SecondName { get; set; }
    public string FullName => FirstName + " " + SecondName;

    public Employee(int key, string first = null, string second = null)
    {
        Key = key;
        FirstName = first;
        SecondName = second;
    }
}

到

public class Employee
{
    public Employee(){} //Constructor with no parameters that needs to be added

    public int Key { get; set; }
    public string FirstName { get; set; }
    public string SecondName { get; set; }
    public string FullName => FirstName + " " + SecondName;

    public Employee(int key, string first = null, string second = null)
    {
        Key = key;
        FirstName = first;
        SecondName = second;
    }
}