Linq查询

//Linq查询
List<A1> a1 = new List<A1>();
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<DogToy> a2 = new List<DogToy>();
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...by
var 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 by
var 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<T> GetPagedListByIndex<T>(List<T> 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;