Linq 对List的一些操作

代码：

    public class Person
    {
        public int ID { get; set; }

        public string Name { get; set; }

        public int Age { get; set; }
    }
    public class Dog
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public int PersonID { get; set; }
    }
    public class DogClothes
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public int DogID { get; set; }
    }
    public class ActionPerson
    {
        public List<Person> GetPersons()
        {
            List<Person> list = new List<Person>();
            for (var i = ; i < ; i++)
            {
                Person p = new Person {ID=i+, Name = "hongda" + (i + ), Age =  + i };
                list.Add(p);
            }
            return list;
        }
        public List<Dog> GetDogs()
        {
            List<Dog> dogs = new List<Dog>();
            for (var i = ; i < ; i++)
            {
                Dog dog = new Dog { ID = i, Name = "dogs" + i, PersonID = (i %)+ };
                dogs.Add(dog);
            }
            return dogs;
        }
        public List<DogClothes> GetDogClotheses()
        {
            List<DogClothes> DogClothes = new List<DogClothes>();
            for (var i = ; i < ; i++)
            {
                DogClothes clothes = new DogClothes() { ID = i, Name = "DogClothes" + i, DogID = (i % ) +  };
                DogClothes.Add(clothes);
            }
            return DogClothes;
        }
        public void Show<T>(List<T> list)
        {
            foreach (var l in list)
            {
                ShowObj(l);
            }
        }
        public void ShowObj<T>(T data)
        {
            Type t = data.GetType();
            PropertyInfo[] infos = t.GetProperties();
            StringBuilder strBuilder = new StringBuilder();
            foreach (var p in infos)
            {
                object value = p.GetValue(data, null);
                string name = p.Name.ToString();
                strBuilder.Append(name + ":" + value + ",");
            }
            Console.WriteLine(strBuilder.ToString());
        }
    }

对list操作

上面的代码可以不看，反正就是一些临时用的数据

ActionPerson ap = new ActionPerson();
            List<Person> list = ap.GetPersons();
            ap.Show(list);
            Console.WriteLine("========================================");
            List<Dog> dogs = ap.GetDogs();
            ap.Show(dogs);
            Console.WriteLine("========================================");

上面的是list,下面的是dogs

ap(dogClothes);

 1.取有狗的人

 var list2 = from a in list from b in dogs where a.ID == b.PersonID select a;
            ap.Show(list2.ToList());
            Console.WriteLine("========================================");
            ap.Show(list2.Distinct().ToList());
            Console.WriteLine("========================================");

发现，自连接就是左表一条条数据与右表一条条数据交叉，取得符合条件的（a.ID==b.PersonID)，再取得需要的字段（a），不会自动合并。

看这个就清楚了

var list4 = from a in list from b in dogs where a.ID == b.PersonID select new { a.ID, a.Name, a.Age, DogsID = b.ID, DogsName = b.Name };
            ap.Show(list4.ToList());

这样上面的就不要区分（distinct）了，之后再取需要的字段

2.join

  var list2 = from a in list join b in dogs on a.ID equals b.PersonID into c select new { a.ID, a.Name, a.Age,Count=c.Count() };
            ap.Show(list2.ToList());

在上面这种into一个对象时，会发现b不能够调用了，a还是可以使用的，当然新形成的对象c也可以，c有一些特殊的属性，例如count

            var list2 = from a in list join b in dogs on a.ID equals b.PersonID select new { a.ID, a.Name, a.Age, DogsID = b.ID, DogsName = b.Name, b.PersonID };

 var list2 = from a in list join b in dogs on a.ID equals b.PersonID select new { a.ID, a.Name, a.Age };
            ap.Show(list2.ToList());

跟自连接一样

LINQ TO SQL中的join，如果带有into,可以提前对表进行过滤条件的操作，而不用等到两表进行迪卡尔积产生虚似表后再进行join on的条件过滤。

关于left join

 var list2 = from a in list
                        join b in dogs on a.ID equals b.PersonID into c
                        from o in c.DefaultIfEmpty()
                        select new
                        {
                            a.ID,
                            a.Name,
                            a.Age,
                            DogsID =o==null?: o.ID,
                            DogsName = o == null ? "" : o.Name,
                            PersonID=o==null?:o.PersonID
                        };

在数据库中就不需要这么复杂，这里list中要考虑右侧可能为空，

数据库查询就不需要。

多查询

  static void Main(string[] args)
        {
            ActionPerson ap = new ActionPerson();
            List<Person> list = ap.GetPersons();
            List<Dog> dogs = ap.GetDogs();
            List<DogClothes> dogClotheses = ap.GetDogClotheses();
            var result = from a in list
                         join b in dogs
                         on a.ID equals b.PersonID
                         join c in dogClotheses
                         on b.ID equals c.DogID
                         select new
                         {
                             a.ID ,
                             a.Name ,
                             a.Age,
                             DogID=b.ID ,
                             DogName=b.Name,
                             b.PersonID ,
                             DogClothesID=c.ID,
                             DogClothesName=c.Name ,
                             DogID2=c.DogID
                         };
            ap.Show(result.ToList ());
            Console.ReadLine();
        }
        

   var result = from a in list
                         from b in dogs
                         from c in dogClotheses
                         where a.ID == b.PersonID && b.ID == c.DogID
                         select new
                         {
                             a.ID,
                             a.Name,
                             a.Age,
                             DogID = b.ID,
                             DogName = b.Name,
                             b.PersonID,
                             DogClothesID = c.ID,
                             DogClothesName = c.Name,
                             DogID2 = c.DogID
                         };

结果上同

http://developer.51cto.com/art/200909/152189.htm

http://www.cnblogs.com/ASPNET2008/archive/2008/12/21/1358152.html