LINQ
LINQ
- Language Integrated Query (LINQ)
- Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language.
- With LINQ, a query is a first-class language construct, just like classes, methods, events.
- You write queries against strongly typed collections of objects by using language keywords and familiar operators.
- The LINQ family of technologies provides a consistent query experience for objects (LINQ to Objects), relational databases (LINQ to SQL), and XML (LINQ to XML).
- By using query syntax, you can perform filtering, ordering, and grouping operations on data sources with a minimum of code.
- You use the same basic query expression patterns to query and transform data in SQL databases, ADO.NET Datasets, XML documents and streams, and .NET collections.
- LINQ in C#
- You can write LINQ queries in C# for SQL Server databases, XML documents, ADO.NET Datasets, and any collection of objects that supports IEnumerable or the generic IEnumerable
interface. - LINQ support is also provided by third parties for many Web services and other database implementations.
- You can write LINQ queries in C# for SQL Server databases, XML documents, ADO.NET Datasets, and any collection of objects that supports IEnumerable or the generic IEnumerable
Code
- Program.cs
using System;
using System.Collections.Generic;
using System.Linq;
namespace LINQRPG
{
public enum ClassType
{
Knight,
Archer,
Mage
}
public class Player
{
public ClassType ClassType { get; set; }
public int Level { get; set; }
public int Hp { get; set; }
public int Attack { get; set; }
public List<int> item { get; set; } = new List<int>();
}
class Program
{
static List<Player> _players = new List<Player>();
static void Main(string[] args)
{
Random rand = new Random();
for(int i=0; i<100; i++)
{
ClassType type = ClassType.Knight;
switch (rand.Next(0, 3))
{
case 0:
type = ClassType.Knight;
break;
case 1:
type = ClassType.Archer;
break;
case 2:
type = ClassType.Mage;
break;
}
Player player = new Player
{
ClassType = type,
Level = rand.Next(0, 100),
Hp = rand.Next(100, 1000),
Attack = rand.Next(5, 50)
};
for (int j = 0; j < 5; j++)
player.item.Add(rand.Next(0, 100));
_players.Add(player);
}
// 1. C# /////////////////////////////////////////
{
List<Player> players = GetHightLevelKnights();
foreach (Player p in players)
Console.WriteLine($"{p.Level} {p.Hp}");
}
//////////////////////////////////////////////////
Console.WriteLine();
// 2. LINQ ///////////////////////////////////////
{
var players =
from p in _players
where p.ClassType == ClassType.Knight && p.Level >= 50
orderby p.Level
select p;
foreach (Player p in players)
Console.WriteLine($"{p.Level} {p.Hp}");
}
{
var playerItems =
from p in _players
from i in p.item
where i < 30
select new { p, i };
var li = playerItems.ToList();
}
{
var playerByLevel =
from p in _players
group p by p.Level into g
orderby g.Key
select new { g.Key, Players = g };
}
{
List<int> levels = new List<int>() { 1, 5, 10 };
var playerLevels =
from p in _players
join l in levels
on p.Level equals l
select p;
}
{
var players =
from p in _players
where p.ClassType == ClassType.Knight && p.Level >= 50
orderby p.Level ascending
select p;
var players2 = _players
.Where(p => p.ClassType == ClassType.Knight && p.Level >= 50)
.OrderBy(p => p.Level)
.Select(p => p);
}
//////////////////////////////////////////////////
}
static public List<Player> GetHightLevelKnights()
{
List<Player> players = new List<Player>();
foreach(Player player in _players)
{
if (player.ClassType != ClassType.Knight)
continue;
if (player.Level < 50)
continue;
players.Add(player);
}
players.Sort((lhs, rhs) => { return lhs.Level - rhs.Level; });
return players;
}
}
}LINQ Syntax
- from … in
- seems like foreach
from p in _players- where
- seems like filter
- search data wich is true in condition
where p.ClassType == ClassType.Knight && p.Level >= 50
.Where(p => p.ClassType == ClassType.Knight && p.Level >= 50)- orderby
- seems like sorting
- default is ascending
- You can change ascending or descending
orderby p.Level
.OrderBy(p => p.Level)- select
- make the result
- You can make ananymous object to select
select p;
select new { p, i };
.Select(p => p);- group … into
- make a group
group p by p.Level into g- join … in … on
- join two tables with condition
join l in levels
on p.Level equals lResult
// 1. C# /////////////////////////////////////////
{
List<Player> players = GetHightLevelKnights();
foreach (Player p in players)
Console.WriteLine($"{p.Level} {p.Hp}");
}
//////////////////////////////////////////////////
Console.WriteLine();
// 2. LINQ ///////////////////////////////////////
{
var players =
from p in _players
where p.ClassType == ClassType.Knight && p.Level >= 50
orderby p.Level
select p;
foreach (Player p in players)
Console.WriteLine($"{p.Level} {p.Hp}");
}
//////////////////////////////////////////////////
{
var playerItems =
from p in _players
from i in p.item
where i < 30
select new { p, i };
var li = playerItems.ToList();
}
{
var playerByLevel =
from p in _players
group p by p.Level into g
orderby g.Key
select new { g.Key, Players = g };
}
{
List<int> levels = new List<int>() { 1, 5, 10 };
var playerLevels =
from p in _players
join l in levels
on p.Level equals l
select p;
}
{
var players =
from p in _players
where p.ClassType == ClassType.Knight && p.Level >= 50
orderby p.Level ascending
select p;
var players2 = _players
.Where(p => p.ClassType == ClassType.Knight && p.Level >= 50)
.OrderBy(p => p.Level)
.Select(p => p);
}