LINQ to OBJECT语句之Join操作符的使用

朱雀 2021-09-16 04:50 275阅读 0赞

实例：将员工信息与部门信息关联。

要求：1、join实现员工信息与部门信息关联。

2、orderby实现以员工入职时间倒序排序。

3、let投影出员工详细地址。

**主代码：**

/// <summary>
    /// 获取员工并关联部门名称
    /// </summary>
    /// <returns></returns>
    public static List<Emplayee> JoinEmplayeeList()
    {
        List<Emplayee> resultList = new List<Emplayee>();
        List<Emplayee> emplayeeList = GetEmplayeeList();        //获取员工信息列表
        List<Department> departmentList = GetDepartmentList();  //获取部门信息列表
        //将员工信息与部门信息关联
        resultList = (from emplayee in emplayeeList
                      join department in departmentList on emplayee.DepartmentId equals department.DepartmentId into t1
                      from depInfo in t1.DefaultIfEmpty()
                      orderby emplayee.AddDate descending
                      let z = emplayee.Country + emplayee.City
                      select new Emplayee()
                      {
                          Name = emplayee.Name,
                          AddDate = emplayee.AddDate,
                          City = emplayee.City,
                          Country = z,
                          DepartmentId = emplayee.DepartmentId,
                          DepartmentName = depInfo == null ? "" : depInfo.DepartmentName  //若关联到部门信息，则返回空字符串
                      }).ToList();
        
        return resultList;
    }

**其他代码：**

/// <summary>
    /// 获取员工信息列表
    /// </summary>
    /// <returns></returns>
    public static List<Emplayee> GetEmplayeeList()
    {
        List<Emplayee> emplayeeList = new List<Emplayee>();
        Emplayee emplayee1 = new Emplayee() { Name = "张三", AddDate = new DateTime(2011, 2, 17), Country= "中国", City="深圳", DepartmentId = 1 };
        Emplayee emplayee2 = new Emplayee() { Name = "李四", AddDate = new DateTime(2012, 10, 23), Country = "美国", City = "洛杉矶", DepartmentId = 2 };
        Emplayee emplayee3 = new Emplayee() { Name = "王五", AddDate = new DateTime(2012, 8, 6), Country = "法国", City = "巴黎", DepartmentId = 99 };
        emplayeeList.Add(emplayee1);
        emplayeeList.Add(emplayee2);
        emplayeeList.Add(emplayee3);
        return emplayeeList;
    }
    
    /// <summary>
    /// 获取部门信息列表
    /// </summary>
    /// <returns></returns>
    public static List<Department> GetDepartmentList()
    {
        List<Department> departmentList = new List<Department>();
        Department department1 = new Department() { DepartmentId = 1, DepartmentName = "研发部" };
        Department department2 = new Department() { DepartmentId = 2, DepartmentName = "人事部" };
        Department department3 = new Department() { DepartmentId = 3, DepartmentName = "财务部" };
        departmentList.Add(department1);
        departmentList.Add(department2);
        departmentList.Add(department3);
        return departmentList;
    }

**实体类：**

/// <summary>
    /// 员工信息类
    /// </summary>
    public class Emplayee 
    {
        /// <summary>
        /// 姓名
        /// </summary>
        public string Name { get; set; }
    
        /// <summary>
        /// 入职时间
        /// </summary>
        public DateTime AddDate { get; set; }
    
        /// <summary>
        /// 国家
        /// </summary>
        public string Country { get; set; }
    
        /// <summary>
        /// 城市
        /// </summary>
        public string City { get; set; }
    
        /// <summary>
        /// 部门ID
        /// </summary>
        public int DepartmentId { get; set; }
    
        /// <summary>
        /// 部门名称
        /// </summary>
        public string DepartmentName { get; set; }
    }
    
    /// <summary>
    /// 部门信息类
    /// </summary>
    public class Department
    {
        /// <summary>
        /// 部门ID
        /// </summary>
        public int DepartmentId { get; set; }
    
        /// <summary>
        /// 部门名称
        /// </summary>
        public string DepartmentName { get; set; }
    }