LINQ笔记-复杂案例

实体：

    public class Person
    {
        public string Name { get; set; }
        public int CompanyID { get; set; }
    }
    public class Company
    {
        public int ID { get; set; }
    }

List<Company> companyList = new List<Company> {
                new Company{ ID=1},
                new Company{ ID=2},
                new Company{ ID=3}
            };
            List<Person> personList = new List<Person> {
                new Person{ CompanyID=1,Name="zhangsan"},
                new Person{ CompanyID=1,Name="lisi"},
                new Person{ CompanyID=2,Name="wangwu"}
            };

内连接：

var list1 = from c in companyList
                       join p in personList on c.ID equals p.CompanyID
                       select new { CompanyID = c.ID, PersonName = p.Name };

左连接：personList的数据放入临时变量personTempList中，并调用DefaultIfEmpty(),意思是为空就使用默认值

var list1 = from c in companyList
                       join p in personList on c.ID equals p.CompanyID into personTempList
                       from pTemp in personTempList.DefaultIfEmpty()
                       select new { CompanyID = c.ID, PersonName = pTemp?.Name };

也可以指定默认值：

var list1 = from c in companyList
                       join p in personList on c.ID equals p.CompanyID into personTempList
                       from pTemp in personTempList.DefaultIfEmpty(new Person { Name="DefaultName"})
                       select new { CompanyID = c.ID, PersonName = pTemp.Name };

未完待续...