使用LinqWhy执行左外部联接时获得错误结果

本文关键字：错误 结果 LinqWhy 执行 外部 使用 | 更新日期: 2023-09-27 18:24:57

我有以下

public class Person
    { 
        // Properties
        public string Name { get; set; }
        public string Area { get; set; }
        public string Gender { get; set; }
        public int Age { get; set; }
        public List<Person> GetPersonData()
        {
            List<Person> personLst = new List<Person>  {
           new Person { Name="Shashidhar Niketani", Age=20, Gender="Male" , Area = "Assam"},
           new Person { Name="Ahmed Ali Khan", Age=25 ,Gender="Male", Area = "Assam" },
           new Person { Name="S. Mirja", Age=20, Gender="Female", Area = "Assam"},
           new Person { Name="Neru Kumar", Age=18, Gender="Female", Area = "Colombo"},
           new Person { Name="Chidam P", Age=19, Gender="Male", Area = "Colombo"},
           new Person { Name="H Kontala", Age=19, Gender="Male", Area = "Bombay"},
           new Person { Name="Priya Pankhraj", Age=23, Gender="Female", Area = "North Punjab"},
             new Person { Name="Ambla", Age=20, Gender="Female", Area = "Madras"},
           new Person { Name="H Kontala", Age=25, Gender="Male", Area = "Bombay"},
           new Person { Name="Sirisha Chalukuri", Age=30, Gender="Female", Area = "Bombay"}};
            return personLst;
        }
        public override string ToString()
        {
            return string.Format("Patient Name: =  {0} , Age:= {1}, Gender:= {2}, Area: = {3}", Name, Age, Gender, Area);
        }
    }
    public class Hobbies
    {
        public string Owner { get; set; }
        public string HobbyName { get; set; }
        public List<Hobbies> GetHobbies()
        {
            List<Hobbies> hobbyList = new List<Hobbies>  {
           new Hobbies { Owner="Sirisha Chalukuri", HobbyName = "Singing"},
           new Hobbies { Owner="Priya Pankhraj", HobbyName = "Cooking" },
           new Hobbies { Owner="S. Mirja", HobbyName="Playing"},
           new Hobbies { Owner="Neru Kumar", HobbyName="Programing"}};
           return hobbyList;
        }
    }

我正在尝试执行左加入

//提取那些记录

Person p = new Person();
var personSource = p.GetPersonData();
Hobbies h = new Hobbies();
var hobbySource = h.GetHobbies();

//左侧外部连接

var res8 = (from person in personSource                       
                        join hobby in hobbySource
                        on p.Name equals hobby.Owner into temp
                        from hobby in temp.DefaultIfEmpty()
                        select new
                            {
                                PersonName = p.Name,
                                PersonAge = p.Age,
                                Gender = p.Gender,
                                LivesIn = p.Area,
                                Hobby = (hobby == null) ? "N/A" : hobby.HobbyName
                            });

但是我不能得到正确的结果。。。所有记录都是空/不匹配的记录。。至少有4条匹配记录，其余将是不匹配记录。。。

我想找的是，给定这两个数据源，如果我们对PersonName和HobbyOwner执行左联接，我们会期望得到的结果。

需要的帮助

使用LinqWhy执行左外部联接时获得错误结果

当你提到业余爱好时。HobbyName在您选择的投影中，当一个人没有匹配的爱好时，hobby为null（对于空的爱好集合，DefaultIfEmpty返回null）。您需要检查hobby是否为null，如果为null，则为选择投影中的hobby属性指定一个默认值。例如：

select new
{
  PersonName = p.Name,
  PersonAge = p.Age,
  Gender = p.Gender,
  LivesIn = p.Area,
  Hobby = (hobby == null) ? "N/A" : hobby.HobbyName
});

此外，不知道这是否是拼写错误，但在您的查询中，当您应该使用person:时，您使用了p

var res8 = (from person in personSource                       
        join hobby in hobbySource
        on person.Name equals hobby.Owner into temp
        from hobby in temp.DefaultIfEmpty()
        select new
        {
         PersonName = person.Name,
         PersonAge = person.Age,
         Gender = person.Gender,
         LivesIn = person.Area,
         Hobby = (hobby == null) ? "N/A" : hobby.HobbyName
         });

根据定义，左外部联接将从第一个集合中获取所有记录，而不管右集合中是否存在匹配的记录。

我想你要找的是：

var res8 = (from person in personSource                       
                        join hobby in hobbySource
                        on p.Name equals hobby.Owner into temp
                        from hobby in temp.DefaultIfEmpty()
                        select new
                            {
                                PersonName = p.Name,
                                PersonAge = p.Age,
                                Gender = p.Gender,
                                LivesIn = p.Area,
                                Hobby = hobby == null ? null : hobby.HobbyName
                            });

我对您的代码所做的唯一一件事就是在您的最后一个块（以及我在下面复制、修改和粘贴的最后一块）中更改p到person的每一次出现：

var res8 = (from person in personSource
                    join hobby in hobbySource
                    on person.Name equals hobby.Owner into temp
                    from hobby in temp.DefaultIfEmpty()
                    select new {
                        PersonName = person.Name,
                        PersonAge = person.Age,
                        Gender = person.Gender,
                        LivesIn = person.Area,
                        Hobby = (hobby == null) ? "N/A" : hobby.HobbyName
                    });

此外，我不知道这段代码与实际代码有多相似，但您的GetPersonData（）和GetHobbies（）函数作为静态方法更有意义。这样，你就不需要创建一个无用的个人或爱好实例来获得你的列表。看起来上面的p Person实例让你失望了。

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