将列表展平为带分隔符的字符串的 Linq 投影

Question

我正在尝试连接并以逗号分隔(或空格)列表并将其投影。我在下面有一些示例代码。

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

使用 string.Join(注意列表首先需要在内存中)，以连接 Friend 对象中的选定属性。如果您使用的是 .NET 3.5，则需要使用 ToArray() 以及 string.Join 的重载。3.5 中的 Join 需要一个数组。

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 ) )
              };