Linq投影将列表平整为已定义字符串

问题描述

| 我正在尝试合并并用逗号分隔（或空格）一个列表并对其进行投影。我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           
                };

    }
}

    

解决方法

        string.Join是更好的方法，但是滥用LINQ太有趣了。

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)
              };

    ,        使用string.Join（请注意，列表必须首先在内存中），以合并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（）以及字符串的重载。3.5中的联接需要一个数组。

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,FriendsList = string.Join( \",\",q.Friends.Select( f => f.Name ) )
              };