Lambda表達式，是用來寫匿名方法的。 在委托用得比較多，因為委托是傳遞方法的。 定義幾個委托： public delegate void DoNoThing();//無參無返回值 public delegate void DoNoThingWithPara(sting name,int age) ...
Lambda表達式，是用來寫匿名方法的。 在委托用得比較多，因為委托是傳遞方法的。 定義幾個委托： public delegate void DoNoThing();//無參無返回值 public delegate void DoNoThingWithPara(sting name,int age);//有參無返回值 public delegate sting DoNoThingWithReturn();//無參有返回值 public delegate int DoNoThingWithParaAndReturn(stiing name,int age);//有參有返回值 實例化委托 DoNothing dnt = ()=>{}; //無參無返回值方法 DoNoThingWithPara dtwp = (x,y)=>{};//有參無返回值 DoNoThingWithReturn dtwr = ()=>"Hello"; //無參有返回值 DoNoThingWithParaAndReturn dntwpr = (x,y)=> 123; //有參有返回值 這就是Lambda表達式的寫法，本質就是方法。
Linq To Object 準備一個類：學生類student
public class Student { public int Id { get; set; } public string Name { get; set; } public int Age { get; set; } public int ClassId { get; set; } }
準備學生數據：
List<Student> stus= new List<Student> { new Student(){Id=1,Name="張三1",Age=27,ClassId=1 }, new Student(){Id=2,Name="張三2",Age=27,ClassId=1 }, new Student(){Id=3,Name="張三3",Age=27,ClassId=1 }, new Student(){Id=4,Name="張三4",Age=27,ClassId=1 }, new Student(){Id=5,Name="張三5",Age=27,ClassId=1 }, new Student(){Id=6,Name="張三6",Age=27,ClassId=2}, new Student(){Id=7,Name="張三7",Age=19,ClassId=2}, new Student(){Id=8,Name="張三8",Age=19,ClassId=2}, new Student(){Id=9,Name="張三9",Age=19,ClassId=2}, new Student(){Id=10,Name="李四",Age=32,ClassId=2 }, new Student(){Id=11,Name="李四1",Age=32,ClassId=2 }, new Student(){Id=12,Name="李四2",Age=32,ClassId=3 }, new Student(){Id=13,Name="李四3",Age=32,ClassId=3 }, new Student(){Id=14,Name="李四4",Age=32,ClassId=3 }, new Student(){Id=15,Name="李四5",Age=32,ClassId=3 }, new Student(){Id=16,Name="李四6",Age=32,ClassId=3 }, new Student(){Id=17,Name="李四7",Age=37,ClassId=3 }, new Student(){Id=18,Name="李四8",Age=37,ClassId=4 }, new Student(){Id=19,Name="王五",Age=37,ClassId=4 }, new Student(){Id=20,Name="王五1",Age=37,ClassId=4 }, new Student(){Id=21,Name="王五2",Age=37,ClassId=4 }, new Student(){Id=22,Name="王五3",Age=37,ClassId=4 }, new Student(){Id=23,Name="王五4",Age=37,ClassId=4 }, new Student(){Id=24,Name="王五5",Age=37,ClassId=4 }, new Student(){Id=25,Name="王五6",Age=37,ClassId=4 }, new Student(){Id=26,Name="王五7",Age=37,ClassId=4 } };
查詢班級Id是1的 var list = from s in stus where s.ClassId == 1 select new { Name = s.Name, ClassId = s.ClassId }; foreach (var item in list) { Console.WriteLine(item.Name + "---" + item.ClassId); } //或者用框架的方法,查詢年齡大於30的學生 Console.WriteLine("**********************"); var list1 = stus.Where(s => s.Age > 30).Select(s => new { Id = s.Id, Name = s.Name, Age = s.Age }); foreach (var item in list1) { Console.WriteLine(item.Id + "---" + item.Name + "--" + item.Age); }
下麵寫一個分頁的Linq
var list = stus.Where(s => s.Age > 30)//條件篩選 .Select(s => new //投影 { Name = s.Name, Age = s.Age, ClassId = s.ClassId }).OrderBy(s => s.Age)//排序 .Skip(2)//跳過幾條 .Take(3);//獲取幾條 ，用於分頁 foreach (var item in list) { Console.WriteLine(item.Name + "--" + item.Age); }
內連接，準備另外一個班級類
public class ClassInfo { public int Id { get; set; } public string ClassName { get; set; } } List<ClassInfo> classes = new List<ClassInfo>() { new ClassInfo(){Id=1,ClassName="初級班" }, new ClassInfo(){Id=2,ClassName="中級班" }, new ClassInfo(){Id=3,ClassName="高級班" }, // new ClassInfo(){Id=4,ClassName="超級班" }, };
var list = from s in stus join cla in classes on s.ClassId equals cla.Id select new { name = s.Name, className = cla.ClassName, age = s.Age }; foreach (var item in list) { Console.WriteLine(item.name+"========"+item.className+"==="+item.age); }
左外連接
var list1 = from s in stus join c in classes on s.ClassId equals c.Id into slist from sc in slist.DefaultIfEmpty() select new { name = s.Name, className = sc== null ? "沒有班級":sc.ClassName, age = s.Age }; foreach (var item in list1) { Console.WriteLine(item.name + "========" + item.className + "===" + item.age); }
