Linq 左连接和在哪里

Question

i'm learning the book:

LINQ to Objects 使用 C# 4.0

好的，我的问题：

我有两个类：Contact 和 CallLog。它看起来像：

public class Contact
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Email { get; set; }
        public string Phone { get; set; }
        public DateTime DateOfBirth { get; set; }
        public string State { get; set; }
};

public class CallLog
    {
        public string Number { get; set; }
        public int Duration { get; set; }
        public bool Incoming { get; set; }
        public DateTime When { get; set; }
    }

关系：Contact.Phone 等于 CallLog.Number

两个类有方法：SampleData()。此方法返回一个简单的列表，其中包含联系人和通话记录。

List<Contact> contacts = Contact.SampleData();
List<CallLog> callLogs = CallLog.SampleData();

我的查询是：每个联系人打了多少电话。

var query2 = (from contact in contacts
                         join callLog in callLogs on contact.Phone equals callLog.Number into joined
                         from callLog in joined.Where(p=>p.Incoming == false).DefaultIfEmpty()

                         select new
                         {
                             who = contact.FirstName + " " + contact.LastName + " " + contact.Phone,
                             how_many = callLog != null ? callLogs.Where(s =>s.Number == contact.Phone).Count() : 0
                         }).Select(p=>p).Distinct();
            foreach (var q in query2)
            {
                Console.WriteLine(q.who + " " + q.how_many);
            }

结果：

Tom XXXX 555-555-555 拨打 2 次​​p>

苏菲 YYYY 333-333-333 打了 3 次电话

Mark ZZZZ 111-111-111 调用 0 次

现在我只想选择传入 == true 的 callLog：

var query2 = (from contact in contacts
                         join callLog in callLogs.Where(p=>p.Incomming == true) on contact.Phone equals callLog.Number into joined
                         from callLog in joined.Where(p=>p.Incoming == false).DefaultIfEmpty()

                         select new
                         {
                             who = contact.FirstName + " " + contact.LastName + " " + contact.Phone,
                             how_many = callLog != null ? callLogs.Where(s =>s.Number == contact.Phone).Count() : 0
                         }).Select(p=>p).Distinct();
            foreach (var q in query2)
            {
                Console.WriteLine(q.who + " " + q.how_many);
            }

（查看新查询2中的第二行） 但结果是一样的：

Tom XXXX 555-555-555 拨打 2 次​​p>

苏菲 YYYY 333-333-333 打了 3 次电话

Mark ZZZZ 111-111-111 调用 0 次

我该如何解决？

Answer 1

试试这个：

  var query2 = (from contact in contacts
                                 join callLog in callLogs on contact.Phone equals callLog.Number into joined
                                 from callLog in joined.Where(p=>p.Incoming == true).DefaultIfEmpty()

                                 select new
                                 {
                                     who = contact.FirstName + " " + contact.LastName + " " + contact.Phone,
                                     how_many = callLog != null ? callLogs.Where(s =>s.Number == contact.Phone).Count() : 0
                                 }).Select(p=>p).Distinct();
foreach (var q in query2)
{
  Console.WriteLine(q.who + " " + q.how_many);
}

Answer 2

查看问题而不是代码，我认为以下内容更好地表达了您正在尝试做的事情：

var contacts = new List<Contact>
{
    new Contact { FirstName = "Tom", LastName = "Jones", Phone = "555-555-555" },
    new Contact { FirstName = "Sophie", LastName = "Monk", Phone = "333-333-333" },
    new Contact { FirstName = "Mark", LastName = "Twain", Phone = "111-111-111" }
};

var callLogs = new List<CallLog>
{
    new CallLog { Number = "555-555-555", Incoming = true },
    new CallLog { Number = "555-555-555", Incoming = true },
    new CallLog { Number = "333-333-333", Incoming = true },
    new CallLog { Number = "333-333-333", Incoming = true },
    new CallLog { Number = "333-333-333", Incoming = true }
};

var data = contacts.Select(c => new 
    {
        who = string.Format("{0} {1} {2}", c.FirstName, c.LastName, c.Phone),
        how_many = callLogs.Count(l => l.Number == c.Phone && l.Incoming)
    }).ToList();

data.Dump();

}

public class Contact
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public string Phone { get; set; }
    public DateTime DateOfBirth { get; set; }
    public string State { get; set; }
}

public class CallLog
{
    public string Number { get; set; }
    public int Duration { get; set; }
    public bool Incoming { get; set; }
    public DateTime When { get; set; }

用 Linqpad 编写的示例，将其复制/粘贴到编辑器中，您应该会看到以下输出：

• 谁有多少
• 汤姆·琼斯 555-555-555 2
• 苏菲·蒙克 333-333-333 3
• 马克吐温 111-111-111 0