Linq projection that flattens a list into a deliminated string - linq

Question

I am trying to concat and comma deliminated (or space) a list and project it. I have some sample code below.
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

Use string.Join (note that the list will need to be in memory first), to concatentate the selected properties from your Friend objects. If you are using .NET 3.5, you'll need to use ToArray() as well as the overloads on string.Join in 3.5 require an array.
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 ) )
};

Answer 2

string.Join is the better way but abusing LINQ is just so fun.
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)
};