linq to object使用
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//linq to object public class LinqShow { #region Data Init private List<Student> GetStudentList() { #region 初始化数据 List<Student> studentList = new List<Student>() { new Student() { Id=1, Name="张三", ClassId=6, Age=35 }, new Student() { Id=1, Name="李四", ClassId=2, Age=23 }, new Student() { Id=1, Name="王炸", ClassId=2, Age=27 }, new Student() { Id=1, Name="疯子科学家", ClassId=2, Age=26 }, new Student() { Id=1, Name="灭", ClassId=2, Age=25 }, new Student() { Id=1, Name="王五", ClassId=2, Age=24 }, new Student() { Id=1, Name="故乡的风", ClassId=2, Age=21 }, new Student() { Id=1, Name="晴天", ClassId=2, Age=22 }, new Student() { Id=1, Name="旭光", ClassId=2, Age=34 }, new Student() { Id=1, Name="oldkwok", ClassId=2, Age=30 }, new Student() { Id=1, Name="乐儿", ClassId=2, Age=30 }, new Student() { Id=1, Name="暴风轻语", ClassId=2, Age=30 }, new Student() { Id=1, Name="一个人的孤单", ClassId=2, Age=28 }, new Student() { Id=1, Name="小张", ClassId=2, Age=30 }, new Student() { Id=3, Name="阿亮", ClassId=3, Age=30 }, new Student() { Id=4, Name="37度", ClassId=4, Age=30 } , new Student() { Id=4, Name="关耳", ClassId=4, Age=30 } , new Student() { Id=4, Name="耳机侠", ClassId=4, Age=30 }, new Student() { Id=4, Name="Wheat", ClassId=4, Age=30 }, new Student() { Id=4, Name="Heaven", ClassId=4, Age=22 }, new Student() { Id=4, Name="等待你的微笑", ClassId=4, Age=23 }, new Student() { Id=4, Name="畅", ClassId=4, Age=25 }, new Student() { Id=4, Name="混无痕", ClassId=4, Age=26 }, new Student() { Id=4, Name="37度", ClassId=4, Age=28 }, new Student() { Id=4, Name="新的世界", ClassId=4, Age=30 }, new Student() { Id=4, Name="Rui", ClassId=4, Age=30 }, new Student() { Id=4, Name="帆", ClassId=4, Age=30 }, new Student() { Id=4, Name="肩膀", ClassId=4, Age=30 }, new Student() { Id=4, Name="根号三", ClassId=4, Age=30 } }; #endregion return studentList; } #endregion public void Show() { List<Student> studentList = this.GetStudentList(); #region 投影成一个新的对象 { var list = studentList.Where<Student>(s => s.Age < 30).Select(s => new { IdName = s.Id + s.Name, ClassName = s.ClassId == 1 ? "一班" : "二班" }); } { var list = from s in studentList where s.Age < 30 select new { IdName = s.Id + s.Name, ClassName = s.ClassId == 1 ? "一班" : "二班" }; } #endregion #region In查询 { var list = studentList.Where<Student>(s => new int[] { 1, 2, 3 }.Contains(s.ClassId)).Select(s => new { IdName = s.Id + s.Name, ClassName = s.ClassId == 1 ? "一班" : "二班" }); } #endregion #region 排序分页 { var list = studentList.Where<Student>(s => s.Age < 30)//条件过滤 .Select(s => new//投影 { s.Id, s.ClassId, IdName = s.Id + s.Name, ClassName = s.ClassId == 1 ? "一班" : "二班" }) .OrderBy(s => s.Id)//排序 //.ThenBy//2个都生效 .OrderByDescending(s => s.ClassId)//倒排 最后一个生效 .Skip(2)//跳过几条 .Take(3);//获取几条 } #endregion #region group by { var list = from s in studentList where s.Age < 30 group s by s.ClassId into sg select sg; foreach (var data in list) { Console.WriteLine(data.Key); foreach (var item in data) { Console.WriteLine($"{item.Id} {item.Name} {item.Age}"); } } } { var list = from s in studentList where s.Age < 30 group s by s.ClassId into sg select new { key = sg.Key, maxAge = sg.Max(t => t.Age) }; foreach (var item in list) { Console.WriteLine($"key={item.key} maxAge={item.maxAge}"); } //group by new {s.ClassId,s.Age} //group by new {A=s.ClassId>1} } { var list = studentList.GroupBy(s => s.ClassId).Select(sg => new { key = sg.Key, maxAge = sg.Max(t => t.Age) }); //new { s.ClassId, s.Age } } #endregion List<Class> classList = new List<Class>() { new Class() { Id=1, ClassName="一班" }, new Class() { Id=2, ClassName="二班" }, new Class() { Id=3, ClassName="三班" }, }; #region 左连接 { var list = from s in studentList join c in classList on s.ClassId equals c.Id//不能用==只能equals select new { Name = s.Name, CalssName = c.ClassName }; foreach (var item in list) { Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}"); } } { var list = studentList.Join(classList, s => s.ClassId, c => c.Id, (s, c) => new { Name = s.Name, CalssName = c.ClassName }); foreach (var item in list) { Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}"); } } { var list = from s in studentList join c in classList on s.ClassId equals c.Id into scList from sc in scList.DefaultIfEmpty()// select new { Name = s.Name, CalssName = sc == null ? "无班级" : sc.ClassName//c变sc，为空则用 }; foreach (var item in list) { Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}"); } Console.WriteLine(list.Count()); } { var list = studentList.Join(classList, s => s.ClassId, c => c.Id, (s, c) => new { Name = s.Name, CalssName = c.ClassName }).DefaultIfEmpty();//为空就没有了 foreach (var item in list) { Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}"); } } #endregion } } /// <summary> /// 学生实体 /// </summary> public class Student { public int Id { get; set; } public int ClassId { get; set; } public string Name { get; set; } public int Age { get; set; } } /// <summary> /// 班级实体 /// </summary> public class Class { public int Id { get; set; } public string ClassName { get; set; } }
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