Linq投影，將一個列表變爲一個刪除的字符串

Question

我想要連接和逗號消除（或空間）的列表和項目它。下面有一些示例代碼。

public class Friend 
{ 
    public string Name { get; set; } 
} 

public class Person 
{ 
    public int PersonID { get; set; } 
    public string FirstName { get; set; } 
    public string Surname { get; set; } 
    List<Friend> Friends { get; set; } 
} 

public class ProjectedPerson 
{ 
    public int PersonID { get; set; } 
    public string FirstName { get; set; } 
    public string Surname { get; set; } 
    public string FriendsList { get; set; } 
} 

public class Test 
{ 
    public void MyTest() 
    { 
     var query = from p in MyDataStore.Person 
       select p; 

     var results = from q in query 
      select new ProjectedPerson 
       { 
        PersonID = q.PersonID, 
        FirstName = q.FirstName, 
        Surname = q.Surname, 
        FriendsList = q.FriendsList.Concat() //??? How can I concat this and return a string   
       }; 

    } 
} 

Answer 1

string.Join是更好的方法，但濫用LINQ是如此有趣。

var query = (from p in MyDataStore.Person 
      select p).ToList(); // <-- bring into memory with ToList() 

var results = from q in query 
       select new ProjectedPerson 
       { 
        PersonID = q.PersonID, 
        FirstName = q.FirstName, 
        Surname = q.Surname, 
        FriendsList = q.Friends.Aggregate<Friend, string>(null, (accum, f) => accum + (accum == null ? accum : ", ") + f.Name) 
       }; 

Answer 2

使用string.Join（注意列表需要首先在內存中），從Friend對象中提取選定的屬性。如果您使用的是.NET 3.5，則需要使用ToArray（）以及string.Join中的重載需要一個數組。

var query = (from p in MyDataStore.Person 
      select p).ToList(); // <-- bring into memory with ToList() 

var results = from q in query 
       select new ProjectedPerson 
       { 
        PersonID = q.PersonID, 
        FirstName = q.FirstName, 
        Surname = q.Surname, 
        FriendsList = string.Join(", ", q.Friends.Select(f => f.Name)) 
       };