在 linq 中合并为等于

Question

我正在尝试在 linq 中加入两个对象，如下所示：

var clients = from cts in ctList
              join pc in personList on ct.name equals pc.name
              select new 
              {
                ip = pc.Ip,
                clts = cts
              };

实际结果：当 personList 为 null 时，我得到的客户没有结果。

预期：当 personList 为 null 时，我们仍然应该接收客户端结果，但是当 personList 中没有结果时，将 ip 分配为“-”。

我想实现如下，但它不起作用：

join pc in personList on ct.name equals pc?.name ?? ct.name

我非常感谢您的帮助。 谢谢！

Answer 1

通过使用左连接，查询将是：

var clients = from cts in ctList
              join pc in personList on cts.name equals pc.name into lefedResults
              from lefedResult in lefedResults.DefaultIfEmpty()
              select new 
              {
                ip = lefedResult?.Ip ?? "-",
                clts = cts
              };

我希望你觉得这有帮助。

Answer 2

解决方案1：

var clients = from cts in ctList
    from pc in personList.Where(x => x.name == cts.name).DefaultIfEmpty()
    select new
    {
        ip = pc?.Ip ?? "-",
        clts = cts
    };

解决方案2：

var clients = from cts in ctList
              select new
              {
                  ip = personList.Where(x => x.name == cts.name).Select(x => x.Ip).FirstOrDefault() ?? "-",
                  clts = cts
              };

解决方案3：

var clients = ctList.Select(cts => new
{
    clts = cts,
    ip = personList.Where(x => x.name == cts.name).Select(x => x.Ip).DefaultIfEmpty("-").First()
});

Answer 3

假设输入列表可能未经过清理，请在下面找到一个稍作更改以提高稳健性的示例：

更多详细信息，请访问https://docs.microsoft.com/en-us/dotnet/csharp/linq/perform-left-outer-joins 。

--- Output（控制台应用程序）---

John -> 10.0.0.5
Sally -> 10.0.0.7
Sally -> 10.0.0.9
Sally -> -
<Unknown User> -> 10.1.1.100
<Unknown User> -> -
<Unknown User> -> -

--- 课程 ---

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

class Client
{
    public string OwnerName { get; set; }

    public string Ip { get; set; }
}

--- LINQ 查询 ---

        List<Person> people = new List<Person>{
            new Person { Name = "John" },
            new Person { Name = "Mary" },
            new Person { Name = "Sally" },
            new Person { Name = null },
            null
        };

        List<Client> machines = new List<Client>{
            new Client { OwnerName = "John", Ip = "10.0.0.5" },
            new Client { OwnerName = "Sally", Ip = "10.0.0.7" },
            new Client { OwnerName = "Sally", Ip = "10.0.0.9" },
            new Client { OwnerName = "Sally", Ip = null }, // Sally -> -
            new Client { OwnerName = null, Ip = "10.1.1.100" }, // <Unknown User> -> 10.1.1.100
            new Client { OwnerName = null, Ip = null }, // <Unknown User> -> -
            null // <Unknown User> -> -
        };

        if (people != null && machines != null)
        {
            var query = from machine in machines
                        join person in people on machine?.OwnerName equals person?.Name into gj
                        select new { Name = machine?.OwnerName?? "<Unknown User>", Ip = machine?.Ip ?? "-" };

            foreach (var result in query)
            {
                Console.WriteLine($"{result.Name} -> {result.Ip}");
            }
        }