Linq查询-白红宇

Linq查询

阅读量：5086 次

发布时间：2019-06-13

本文共 3502 字，大约阅读时间需要 11 分钟。

//Linq查询List
     
       a1 = new List
      
       ();a1.Add(new A1 { Id = 10, Name = "aaa", Age = 101, Gender = true });a1.Add(new A1 { Id = 15, Name = "bbb", Age = 21, Gender = true });a1.Add(new A1 { Id = 12, Name = "ccc", Age = 21, Gender = false });a1.Add(new A1 { Id = 13, Name = "ddd", Age = 200, Gender = false });List
       
         a2 = new List
        
         ();a2.Add(new DogToy { DogId = 11, DogName = "小白" });a2.Add(new DogToy { DogId = 10, DogName = "小欢" });a2.Add(new DogToy { DogId = 12, DogName = "小美" });a2.Add(new DogToy { DogId = 15, DogName = "小王" });//查询所有语句var s1 = (from c in a1 select c).ToList();//查询Gender为false的成员var s2 = from c in a1 where c.Gender == false select c;//orderby排序var s3 = from c in a1 orderby c.Id select c;//Join连接两个集合相匹配元素,注意：连接条件使用equals关键字！！！var s4 = from c in a1 join t in a2 on c.Id equals t.DogId select new { A1Name = c.Name, DogName = t.DogName };//Join连接两个集合所有元素var s5 = from c in a1 join t in a2 on c.Id equals t.DogId select new { A1Name = c, DogName = t };//分组group...byvar s6 = from c in a1 group c by c.Gender;foreach (var group in s6){    Console.WriteLine(group.Key.ToString());    foreach (var item in group)    {        Console.WriteLine(item.ToString());    }}public class A1{    public int Id { get; set; }    public string Name { get; set; }    public int Age { get; set; }    public bool Gender { get; set; }    //重写ToString方法    public override string ToString()    {        return Id + "," + Name + "," + Age + "," + Gender;    }}public class DogToy{    public int DogId { get; set; }    public string DogName { get; set; }    //重写ToString方法    public override string ToString()    {        return DogId + "," + DogName;    }}

//Linq查询例子：var count = (from u in db.Users where u.Name == userName && u.Pwd == userPwd select u).ToList();//得到一个集合数量，获取用户输入的用户名与密码是否与数据库匹配var query = (from u in db.Users orderby u.Pwd ascending select u).ToList();//升序排序var query = (from u in db.Users orderby u.Pwd descending select u).ToList();//降序排序var query = (from u in db.Users group u by u.Gender).ToList();//将Gender分组foreach (var group in query){    Console.WriteLine("小组："+group.Key);//各小组    foreach (var item in group)    {        Console.WriteLine(item.ToString());//小组成员    }}MethodInfo[] methods = typeof(string).GetMethods();//第一种方法，获取所有string方法并去除重复的方法var result = (from m in methods             where m.IsStatic != true             select m.Name).Distinct().ToList();//第二种方法，用group byvar result1 = (from m in methods             where m.IsStatic != true             group m by m.Name into g             select new { MethodName = g.Key, Overload = g.Count() }).ToList();//分页var list1 = db.Users.ToList();var first = (from c in list1 select c).Skip(0).Take(3).ToList();//封装分页方法static List
     
       GetPagedListByIndex
      
       (List
       
         groupList,int pageIndex){    int pageSize = 3;    var first = (from c in groupList select c).Skip((pageIndex - 1) * pageSize).Take(pageSize).ToList();    //return groupList.Skip((pageIndex - 1) * pageSize).Take(pageSize).ToList();    return first;}//Join内连接，没有分类的产品查询不到var innerjoin = from p in ctx.Products 　　join c in ctx.Categories 　　on p.CategoryID equals c.CategoryID 　　select p.ProductName;//外连接，没有分类的产品也能查询到var leftjoin = from p in ctx.Products 　　join c in ctx.Categories 　　on p.CategoryID equals c.CategoryID 　　into pro 　　from x in pro.DefaultIfEmpty() 　　select p.ProductName;

转载于:https://www.cnblogs.com/genesis/p/4908995.html

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