Linq不经常使用但必须记住的用法

class Program
{
static void Main(string[] args)
{
string split = new String('-', 30);
Console.WriteLine("匿名方法");
Console.WriteLine(split);
Func<int, int, int> func = delegate(int a, int b)
{
return a + b;
};
Console.WriteLine(func(1, 2));
Console.WriteLine("");
Console.WriteLine("OfType用法");
Console.WriteLine(split);
var arr = new object[] { 1232, "1231", 1231, "asdac" };
foreach (var value in arr.OfType<string>())
{
Console.WriteLine(value);
}
Console.WriteLine("");
Console.WriteLine("AsEnumerable用法");
Console.WriteLine(split);
Console.WriteLine("如果某个集合对象本身实现了Where方法，则可以obj.AsEnumerable().Where()方式进行调用,在某些场合下，这非常有用!");
Console.WriteLine("");
Console.WriteLine("ToLookup用法");
Console.WriteLine(split);
var lookups = (new[]{
new {id=1,name="name1"},
new {id=1,name="name2"},
new {id=2,name="name3"},
}).ToLookup(o => o.id, o => o.name);
foreach (var lookup in lookups)
{
Console.WriteLine("\b\b" + lookup.Key);
foreach (var value in lookup)
Console.WriteLine(value);
}
Console.WriteLine("");
Console.WriteLine("柯里化(Currying)表达式");
Console.WriteLine(split);
Func<int, Func<int, int>> curry1 = x => y => x + y;
Console.WriteLine(curry1(3)(4));
Console.WriteLine("理解:x=>y=>x+y 即在第一个匿名函数里面再封装一个匿名函数,在第二个匿名函数里，x可理解为闭包");
Console.WriteLine("");
Console.WriteLine("匿名类型的相等性");
Console.WriteLine(split);
var eq1 = new { id = 1, name = "name" };
var eq2 = new { id = 1, name = "name" };
var eq3 = new { name = "name", id = 1 };
Console.WriteLine("eq1.Equals(eq2)=" + eq1.Equals(eq2));
Console.WriteLine("eq1.Equals(eq3)=" + eq1.Equals(eq3));
Console.WriteLine("eq1 == eq2 =" + (eq1 == eq2));
Console.WriteLine("");
Console.WriteLine("次要排序");
Console.WriteLine(split);
var orderObj = new[]{
new {lastname="Kimmel",first="Paul",age=41},
new {lastname="Swanson",first="Dena",age=26},
new {lastname="Swanson",first="Joe",age=4},
new {lastname="Kimmel",first="Noah",age=11}
};
var sordid = from order in orderObj
orderby order.lastname, order.age
select order;
foreach (var o in sordid)
{
Console.WriteLine("lastname:{0},first:{1},age:{2}", o.lastname, o.first, o.age);
}
Console.WriteLine("");
Console.WriteLine("Intersect扩展方法");
Console.WriteLine(split);
var intersect1 = new int[] { 1, 2, 32, 3, 4, 5, 65 };
var intersect2 = new int[] { 1, 2, 3, 345, 6, 5, 65 };
Array.ForEach<int>(intersect1.Intersect(intersect2).ToArray(), e => Console.WriteLine(e + ","));
Console.WriteLine("");
Console.WriteLine("Union扩展方法");
Console.WriteLine(split);
var union1 = new int[] { 1, 2, 32, 3, 4, 5, 65 };
var union2 = new int[] { 1, 2, 3, 345, 6, 5, 65 };
Array.ForEach<int>(union1.Union(union2).ToArray(), e => Console.WriteLine(e + ","));
Console.WriteLine("");
Console.WriteLine("Except扩展方法");
Console.WriteLine(split);
var except1 = new int[] { 1, 2, 32, 3, 4, 5, 65 };
var except2 = new int[] { 1, 2, 3, 345, 6, 5, 65 };
Array.ForEach<int>(except1.Except(except2).ToArray(), e => Console.WriteLine(e + ","));
Console.WriteLine("");
Console.WriteLine("Group多字段分组");
Console.WriteLine(split);
var groups = (new[]{
new {name="name1",age=18},
new {name="name2",age=18},
new {name="name4",age=19}
}).GroupBy(o=>new{o.age,o.name});
foreach (var group in groups)
{
Console.WriteLine("key.age:{0},key.name:{1},count:{2}", group.Key.age, group.Key.name, group.Count());
}
Console.WriteLine("");
Console.WriteLine("实现左外联接");
Console.WriteLine(split);
var customers = new Customer[]{
new Customer(){ CompanyName="company1", ID=1},
new Customer(){ CompanyName="company2", ID=2},
new Customer(){ CompanyName="company3", ID=3},
new Customer(){ CompanyName="company4", ID=4},
new Customer(){ CompanyName="company5", ID=5},
};
var orders = new Order[]{
new Order(){ CompanyId=1, ID=1, ItemDescription="this is order 1"},
new Order(){ CompanyId=1,ID =2,ItemDescription="this is order 2"}
};

var joinGroup = from customer in customers
join order in orders on customer.ID equals order.CompanyId
into orderCollection
from item in orderCollection.DefaultIfEmpty(new Order())
select new { customer, item };
foreach (var g in joinGroup)
{
Console.WriteLine("companyname:{0},companyid:{1},itemdescription:{2}", g.customer.CompanyName, g.customer.ID, g.item.ItemDescription);
}
4000

Console.Read();
}
public class Customer
{
public int ID;
public string CompanyName;
}
public class Order
{
public int ID;
public int CompanyId;
public string ItemDescription;
}
}