用一條LINQtoSQL語句返回不同的對象

Question

這是我的問題：我有一個由Employee和Student實現的IPerson。什麼我真的想要的是你在下面看到。一個LINQ語句來獲取每種類型的IPerson。這很好，直到我調用方法;）。這對我爲什麼會遇到錯誤是有道理的，但我真的很努力地找到一種體面的方式來從數據庫中提取所有IPerson對象，並避免將切換語句放在我的應用程序中。

public IQueryable<IPerson> getPersons() { 

     // gives Types in Union or Concat have different members assigned error 

     var people = from p in db.Persons select p; 

     var students = (from s in people 
         where s.TypeId == (int)PersonType.Student 
         select new Student 
         { 
          Id = s.Id, 
          Age = s.Age.GetValueOrDefault(0), 
          Name = s.Name, 
          Major = s.Student.Major ?? "None", 
          CreditHours = s.Student.CreditHours.GetValueOrDefault(0), 
          PersonType = (PersonType)s.TypeId 
         }).Cast<IPerson>(); 
     var employees = (from e in people 
         where e.TypeId == (int)PersonType.Employee 
         select new Employee 
         { 
          Id = e.Id, 
          Age = e.Age.GetValueOrDefault(0), 
          Name = e.Name, 
          PersonType = (PersonType)e.TypeId, 
          Salary = e.Employee.Salary.GetValueOrDefault(0) 
         }).Cast<IPerson>(); 

     return students.Concat<IPerson>(employees); 
     //return (students.ToList()).Concat<IPerson>(employees.Cast<IPerson>().ToList()).AsQueryable<IPerson>(); 
    } 

上面有一個註釋掉return語句 - 基本上做了.ToList（）和內放棄整個延遲執行的事情，創建2個SQL語句 - 並不理想。

任何幫助表示讚賞！

Answer 1

如何：

public IQueryable<IPerson> getPersons() { 

    // gives Types in Union or Concat have different members assigned error 

    var people = from p in db.Persons select p; 

    return (from s in people 
        where s.TypeId == (int)PersonType.Student 
        select new Student 
        { 
         Id = s.Id, 
         Age = s.Age.GetValueOrDefault(0), 
         Name = s.Name, 
         Major = s.Student.Major ?? "None", 
         CreditHours = s.Student.CreditHours.GetValueOrDefault(0), 
         PersonType = (PersonType)s.TypeId 
        }).Cast<IPerson>().Union((from e in people 
         where e.TypeId == (int)PersonType.Employee 
         select new Employee 
         { 
          Id = e.Id, 
          Age = e.Age.GetValueOrDefault(0), 
          Name = e.Name, 
          PersonType = (PersonType)e.TypeId, 
          Salary = e.Employee.Salary.GetValueOrDefault(0) 
         }).Cast<IPerson>()); 
} 

這不是好多了，但是你得到它的一個電話。或者，我會做的是這樣的：

public IPerson GetPerson(Person p) //I'm guessing that the objects in collection db.Persons is of type Person 
{ 
    IPerson ret; 
    switch(p.TypeId) 
    { 
     case (int)PersonType.Student: ret = .......break; 
     case (int)PersonType.Employee: ret = ......break; 
    } 
    return ret; 
} 

public IQueryable<IPerson> getPersons() { 
    return (from p in db.Persons select p).ToList().Select(p => GetPerson(p)).AsQueryable(); 
} 

但是，然後再次得到switch-statement。另外，如果你不喜歡在數據庫上執行ToList（）（如果我記得LinqToSQL不支持使用具有變量的構造函數的函數），你可以嘗試添加GetPerson方法（我可能會重命名它）到由LinqToSQL（部分類）生成的Person類，但我不確定這是否合法。

但是，如何在不使用開關的情況下使用來自getPersons的IQueryable，我不知道。

Answer 2

這裏是我落得這樣做：

庫

public IQueryable<Database.Person> getDbPersons() { 
    return from p in db.Persons select p; 
} 

// Called by Service layer when viewing all People 
public IQueryable<Person> getPersons() { 
    return from p in getDbPersons() select new Person { //yada yada }; 
} 

服務層重新

public IList<Person> getPersons() { 
    return from p in repository.getPersons() return p; 
} 

public IPerson getPerson(int id) { 
    return repository.getDbPersons().withPersonId(id); 
} 

// Person Filter Class 
public static class PersonFilters 
{ 
    public static IPerson WithPersonId(this IQueryable<SqlServer.Person> qry, int Id) 
    { 
     return (from p in qry 
       where p.Id == Id 
       select p).Select(p => ThisPerson(p)).SingleOrDefault(); 
    } 

    private static IPerson ThisPerson(OneToOne.Data.SqlServer.Person x) 
    { 
     IPerson ret; 
     switch (x.TypeId) 
     { 
      case (int)PersonType.Employee: 
       var e = new Employee(); 
       e.Id = x.Id; 
       e.Name = x.Name; 
       e.Age = x.Age.GetValueOrDefault(0); 
       e.Salary = x.Employee.Salary.GetValueOrDefault(0); 
       e.PersonType = PersonType.Employee; 
       ret = e; 
       break; 
      case (int)PersonType.Student: 
       var s = new Student(); 
       s.Id = x.Id; 
       s.Name = x.Name; 
       s.Age = x.Age.GetValueOrDefault(0); 
       s.Major = x.Student.Major; 
       s.PersonType = PersonType.Employee; 
       ret = s; 
       break; 
      default: 
       throw new Exception("Bad Person Type"); 
     } 
     return ret; 
    } 
} 

感謝，Alxandr指着我在正確的方向！