LINQ学习笔记
1. LINQ是什么？LINQ要解决什么问题？
a) LINQ(Language-INtegrated Query) 是 Orcas(VS2008) 提供的一套超强扩展功能，可以用一种标准、简单的方式查询和更新数据。被查询的数据可以是 .NET Framework collections、 SQL databases、ADO.NET Datasets、XML documents，以及其他任何实现 IEnumerable<T> 的数据类型。LINQ 是一种语言扩展，可以使用类似 SQL 的近自然语法或扩展方法 API 来操作这些数据源。
b) 解决的问题
i. Data != Object
ii. 统一集合数据查询模型
2. C# 3.0的新特性，优雅
a) 隐式类型变量(Implicitly typed local variables)
var关键字，编译器翻译，不能在运行期改变类型
| var a = 123;
var b = "123"; |
b) 扩展方法(Extension Method)
为既有类型添加额外的功能
(1) 必须在静态类中定义。
(2) 必须是静态方法。
(3) 第一个参数必须添加 this 关键字，且参数类型必须是目标类型。
(4) 扩展方法不能访问目标类型 private、protected 成员。
(5) 同一目标类型不能有多个名称、签名一样的扩展方法。
(6) 如果扩展方法和目标类型方法名称和参数签名相同，则优先调用目标类型方法。
| static class StringExtension
{
public static void PrintLine(this string s)
{
Console.WriteLine(s);
}
public static void PrintLine(this string s, int count)
{
for (int i = 0; i < count; i++)
{
Console.WriteLine(s);
}
}
}
|
c) 对象初始化（Object initializers）
| class MyClass
{
public MyClass()
{
}
public MyClass(int x, int y)
{
this.x = x;
this.y = y;
}
private int x;
public int X
{
get { return x; }
set { x = value; }
}
private int y;
public int Y
{
get { return y; }
set { y = value; }
}
private string s;
public string S
{
get { return s; }
set { s = value; }
}
}
Call Code:
MyClass o1 = new MyClass { X = 1, Y = 2, S = "abc" };
MyClass o2 = new MyClass { X = 1, Y = 2 };
MyClass o3 = new MyClass(1, 2) { S = "abc" }; |
d) 匿名属性(Automatic properties)
(1) 必须同时包含 get; set; 定义。(由于关联字段是编译器自动生成的，没有 set 访问器也就意味着永远没有办法为该属性赋值。同理，没有 get 也不成。)
(2) 可以为 get; set; 定义不同的访问权限。
| class MyClass
{
public int X { get; set; }
public int Y { get; set; }
public int Z { get; internal set; }
public string S { get; set; }
} |
e) 匿名类型(Anonymous types)
| var o1 = new { X = 1, Y = 2, S = "abc"};
var o2 = new { X = 1, Y = 2, S = "abc"};
Console.WriteLine(o1 == o2);
Console.WriteLine(o1.GetType() == o2.GetType()); |
匿名类型特征：
(1) 编码时没有定义类型名称。
(2) 通过对象初始化器定义属性。
(3) 引用变量只能是隐式类型或 Object。
应用环境：
(1) 局部信息存储交换。
(2) LINQ 返回信息存储。
(3) 用于数据控件绑定源。
f) Lambda expressions
.Net 1.1时代的委托
| public delegate bool NumberTester(int i);
public static bool IsOdd(int i)
{
return (i % 2) == 1;
}
public static void PrintMatchingNumbers(int from, int to, NumberTester filter)
{
for (int i = from; i <= to; ++i)
{
if (filter(i))
{
Console.WriteLine(i);
}
}
}
Call Code:
PrintMatchingNumbers(1, 10, new NumberTester(IsOdd)); |
.Net 2.0时代的匿名方法
| public delegate bool NumberTester(int i);
public static void PrintMatchingNumbers(int from, int to, NumberTester filter)
{
for (int i = from; i <= to; ++i)
{
if (filter(i))
{
Console.WriteLine(i);
}
}
}
Call Code:
PrintMatchingNumbers(1, 10, delegate(int i)
{
return (i % 2) == 1;
}); |
.Net 3.0时代的Lambda expressions（参数=> 表达式）
| public delegate bool NumberTester(int i);
public static void PrintMatchingNumbers(int from, int to, NumberTester filter)
{
for (int i = from; i <= to; ++i)
{
if (filter(i))
{
Console.WriteLine(i);
}
}
}
Call Code:
PrintMatchingNumbers(1, 10, i => (i % 2) == 1); |
3. LINQ是如何实现的
在 System.Linq 名字空间下，你会发现大量直接或间接绑定到 IEnumerable<T> 的扩展方法。也就是说 LINQ 的功能都是通过这些扩展方法实现的，而编译器负责将 LINQ 语法翻译成对这些扩展方法的调用。
4. LINQ的组成
5. LINQ开门三件事
a) 获得数据源
b) 创建查询
c) 执行查询
| // Data source.
int[] numbers = new int[7] { 0, 1, 2, 3, 4, 5, 6 };
// Query creation.
IEnumerable<int> numQuery =
from num in numbers
where (num % 2) == 0
select num;
// Query execution.
foreach (int j in numQuery)
{
Console.Write("{0,1} ", j);
}
numQuery.ToList().ForEach(n => Console.WriteLine(n));
/*
IEnumerable<int> numQuery =
numbers.Where(new Func<int,bool>(IsEven));
numbers.Where(
delegate(int n)
{
return (n % 2) == 0;
}
);
numbers.Where(n => (n % 2) == 0);
from num in numbers where (num % 2) == 0 select num;
* public static bool IsEven(int n)
{
return (n % 2) == 0;
}
* */ |
6. LINQ To Object
a) DEMO：获取String类型的方法
（贯穿C# 3.0的特性，Lambda表达式，简单表达式，Select子句，OrderBy子句，Form子句，Where子句，GroupBy子句，对结果的去重，限制结果个数等）
| MethodInfo[] methodInfos = typeof(string).GetMethods();
var query = from i in methodInfos
orderby i.Name
select i.Name;
IEnumerable<string> query = methodInfos
.OrderBy(i => i.Name)
.Select(i => i.Name);
var query = from i in methodInfos
where i.IsPublic
orderby i.Name
select i.Name;
var query = from i in methodInfos
where i.IsPublic
orderby i.Name
select new { i.Name, i.ReturnType };
var query = (from i in methodInfos
where i.IsPublic
orderby i.Name
select new { i.Name, i.ReturnType }).Distinct();
var query = (from i in methodInfos
where i.IsPublic
orderby i.Name
select new { i.Name, i.ReturnType }).Distinct().Take(1);
var query = from i in methodInfos
where i.IsPublic
orderby i.Name
group i by i.Name into g
select new {KEY = g.Key, Items = g};
foreach (var s in query)
{
Console.WriteLine(s.KEY);
foreach (var q in s.Items)
{
Console.WriteLine("\t{0}", q.GetParameters().Count());
}
}
var query = from i in methodInfos
where i.IsPublic
orderby i.Name
group i by i.Name into g
select new { KEY = g.Key, MaxParameterCount = g.Max(p=>p.GetParameters().Count()) }; |
b) DEMO：获取Int类型相对String所特有的方法； 获取Int类型和String的全部方法；获取Int类型和String类型的共有方法
（两个集合求差Except，求合Union，关联Join）
（查询结果就是集合，集合就可以查询。查询可以嵌套）
| var query = (from i in typeof(int).GetMethods()
select i.Name).Except(
from i in typeof(string).GetMethods()
select i.Name
);
var query = (from i in typeof(int).GetMethods()
select i.Name).Union(
from i in typeof(string).GetMethods()
select i.Name
);
var query = (from s in typeof(string).GetMethods()
join i in typeof(int).GetMethods()
on s.Name equals i.Name
select s.Name).Distinct();
var query = (from s in typeof(string).GetMethods()
from i in typeof(int).GetMethods()
where s.Name == i.Name
select s.Name).Distinct(); |
7. LINQ To Relation
a) LINQ To DataSet
借助于扩展方法 DataTableExtensions.AsEnumerable() 将 DataTable 转换成
EnumerableRowCollection<TRow>:IEnumerable<TRow> 来实现对 LINQ 的支持。
| using System.Data;
using System.Data.Common;
using System.Data.SqlClient;
DbProviderFactory factory = SqlClientFactory.Instance;
string connStr = ConsoleApplication1.Properties.Settings.Default.NorthwindConnectionString;
using (DbConnection conn = factory.CreateConnection())
{
conn.ConnectionString = connStr;
conn.Open();
DbCommand cmd = conn.CreateCommand();
cmd.CommandText = "select * from Customers";
DbDataAdapter ad = factory.CreateDataAdapter();
ad.SelectCommand = cmd;
DataSet ds = new DataSet();
ad.Fill(ds);
var table = ds.Tables[0];
var q = from o in table.AsEnumerable()
where o.Field<string>("City") == "London"
select new
{
Name = o.Field<string>("ContactName"),
Phone = o.Field<string>("Phone")
};
foreach (var u in q)
{
Console.WriteLine(u);
}
} |
b) LINQ To SQL (DLINQ)
i. ORM工具
添加一个 “LINQ To SQL Classes”项（扩展名为dbml） 服务管理器中拖拽
ii. DataContext
| NorthWindDataContext northwind = new NorthWindDataContext()
var customer = from c in northwind.Customers
select c;
DataContext northwind = new DataContext(connString);
Table<Customer> customers = northwind.GetTable<Customer>();
var query = from c in customers select c;
|
iii. LOG
| northwind.Log = Console.Out; |
i. 简单查询
| using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
var query = from c in northwind.Customers
orderby c.ContactName
select c;
foreach (var c in query)
{
Console.WriteLine("{0} {1}", c.ContactName, c.ContactTitle);
}
}
using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
string city = "London";
var query = from c in northwind.Customers
where c.City == city
orderby c.ContactName
select c;
foreach (var c in query)
{
Console.WriteLine("{0} {1}", c.ContactName, c.City);
}
}
using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
string city = "London";
var query = from c in northwind.Customers
where c.City == city
orderby c.ContactName
select c;
city = "Paris";
foreach (var c in query)
{
Console.WriteLine("{0} {1}", c.ContactName, c.City);
}
}
|
ii. One to One /One to Many / Many to Many
| var p = (from c in northwind.Categories
where c.CategoryName == "Meat/Poultry"
select c).First().Products;
var c = (from p in northwind.Products
where p.CategoryID == 2
select p).First().Category;
var q = from c in northwind.Categories
from p in northwind.Products
where c.CategoryName == "Meat/Poultry" && c.CategoryID == p.CategoryID
select new { c.CategoryName, p.ProductName };
var q = from c in northwind.Categories
where c.CategoryName == "Meat/Poultry"
join p in northwind.Products
on c.CategoryID equals p.CategoryID
select new { c.CategoryName, p.ProductName }; |
iii. 增删改
| using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
Product p = new Product { ProductName = "MyProduct" };
northwind.Products.Add(p);
northwind.SubmitChanges();
Console.WriteLine(p.ProductID);
}
using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
Product product = (from p in northwind.Products
where p.ProductName == "MyProduct"
select p).First();
northwind.Products.Remove(product);
northwind.SubmitChanges();
}
using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
var query = (from p in northwind.Products
where p.ProductID == 5
select p).First();
query.QuantityPerUnit = "48 boxes";
northwind.SubmitChanges();
} |
iv. 事务
| using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Connection.Open();
northwind.Transaction = northwind.Connection.BeginTransaction();
try
{
Product p = new Product { ProductName = "MyProduct2" };
northwind.Products.Add(p);
northwind.SubmitChanges();
northwind.Transaction.Commit();
}
catch(Exception ex)
{
northwind.Transaction.Rollback();
Console.WriteLine(ex.Message);
}
} |
v. 分页
| using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
int pageIndex = 4;
int pageSize = 10;
int skipCount = (pageIndex - 1) * pageSize;
var query = from c in northwind.Customers
orderby c.ContactName
select c;
Console.WriteLine("Page {0}", pageSize);
foreach (var c in query.Skip(skipCount).Take(pageSize))
{
Console.WriteLine("Name:{0} Phone:{1}", c.ContactName, c.Phone);
}
}
SELECT TOP 10 [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName],
[t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode],
[t0].[Country], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
WHERE NOT (EXISTS(
SELECT NULL AS [EMPTY]
FROM (
SELECT TOP 30 [t1].[CustomerID]
FROM [dbo].[Customers] AS [t1]
ORDER BY [t1].[ContactName]
) AS [t2]
WHERE [t0].[CustomerID] = [t2].[CustomerID]
))
ORDER BY [t0].[ContactName]
-- Context: SqlProvider(Sql2000) Model: AttributedMetaModel Build: 3.5.20706.1
SELECT TOP 10 [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName],
[t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode],
[t1].[Country], [t1].[Phone], [t1].[Fax]
FROM (
SELECT ROW_NUMBER() OVER (ORDER BY [t0].[ContactName]) AS [ROW_NUMBER], [t0]
.[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0]
.[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].
[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
) AS [t1]
WHERE [t1].[ROW_NUMBER] > @p0
ORDER BY [t1].[ContactName]
-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [30]
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.20706.1 |
vi. ExecuteCommand、ExecuteQuery
| using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
northwind.ExecuteCommand(
"Update Employees Set Country = {0} where title = {1}",
"USA",
"Sales Manager"
);
}
using (NorthWindDataContext northwind = new NorthWindDataContext())
{
northwind.Log = Console.Out;
var q = northwind.ExecuteQuery<Employee>("select * from [Employees]");
foreach (var e in q)
{
Console.WriteLine(e.LastName);
}
} |
8. LINQ To XML
a) 查询
| var doc = XDocument.Load("NorthWindMapping.xml");
var query = from t in doc.Elements("Database").Elements("Table")
where t.Attribute("Name").Value.StartsWith("C")
select t;
foreach (var i in query)
{
Console.WriteLine(i.Attribute("Name").Value);
}
// 从XML字符串生成XDocument对象
var doc = XDocument.Parse(
@"<people>
<person>
<id>1</id>
<name>user1</name>
<age>1</age>
</person>
</people>");
|
b) 创建
| var doc = new XDocument(
new XDeclaration("1.0", "utf-8", "yes"),
new XComment("test xml"),
new XElement("Customers",
new XElement("Customer",
new XAttribute("title", "CEO"),
new XElement("id", 1),
new XElement("name", "张三"),
new XElement("age", 36),
new XElement("email", new XCData("<a href=\"mailto:[email protected]\">邮件</a>"))
),
new XElement("Customer",
new XElement("id", 2),
new XElement("name", "李四"),
new XElement("age", 28)
)
)
); |
c) 更改
| var doc = XDocument.Load("test.xml");
var query = from c in doc.Elements("Customers").Elements("Customer")
where c.Attribute("title") != null && c.Attribute("title").Value == "CEO"
select c;
foreach (var e in query)
{
e.SetAttributeValue("title", "CEO,CTO");
}
var query2 = from c in doc.Elements("Customers").Elements("Customer")
where Convert.ToInt16(c.Element("id").Value) == 2
select c;
foreach (var e in query2)
{
e.Remove();
}
doc.Save("test.xml");
|
9. LINQ的性能
| using System.Data;
using System.Data.Linq;
using System.Data.SqlClient;
using System.Diagnostics;
static public string UsingSqlReader()
{
Stopwatch watch = new Stopwatch();
watch.Start();
using (SqlConnection conn = new SqlConnection(connString))
{
if (conn.State != System.Data.ConnectionState.Open)
conn.Open();
for (int i = 1; i <= 1000; i++)
{
SqlCommand cmd = new SqlCommand(
"SELECT ProductID,ProductName FROM Products WHERE ProductID=" + i.ToString(), conn);
cmd.CommandType = CommandType.Text;
using (SqlDataReader reader = cmd.ExecuteReader())
{
if (reader.Read())
{
Product p = new Product();
p.ProductID = (int)reader["ProductID"];
p.ProductName = (string)reader["ProductName"];
}
}
}
}
watch.Stop();
return "[UsingSqlReader] total time: " + watch.Elapsed.ToString();
}
static public string UsingLinqToSQL()
{
Stopwatch watch = new Stopwatch();
watch.Start();
NorthWindDataContext northwind = new NorthWindDataContext();
for (int i = 1; i <= 1000; i++)
{
Product product = northwind.Products.SingleOrDefault(p => p.ProductID == i);
}
watch.Stop();
return "[UsingLinqToSQL] total time: " + watch.Elapsed.ToString();
}
static public string UsingOptimizeLinqToSQL()
{
Stopwatch watch = new Stopwatch();
watch.Start();
using (SqlConnection conn = new SqlConnection(connString))
{
if (conn.State != System.Data.ConnectionState.Open)
conn.Open();
NorthWindDataContext northwind = new NorthWindDataContext(conn);
for (int i = 1; i <= 1000; i++)
{
Product product = northwind.Products.SingleOrDefault(p => p.ProductID == i);
}
}
watch.Stop();
return "[UsingOptimizedLinqToSQL] total time: " + watch.Elapsed.ToString();
}
static public string UsingCompiledLinqToSQL()
{
Stopwatch watch = new Stopwatch();
watch.Start();
var query = CompiledQuery.Compile(
(NorthWindDataContext northwind, int index)
=> northwind.Products.SingleOrDefault(p => p.ProductID == index));
using (SqlConnection conn = new SqlConnection(connString))
{
if (conn.State != System.Data.ConnectionState.Open)
conn.Open();
NorthWindDataContext northwind = new NorthWindDataContext(conn);
for (int i = 1; i <= 1000; i++)
{
Product p = query(northwind, i);
}
}
watch.Stop();
return "[UsingCompiledLinqToSQL] total time: " + watch.Elapsed.ToString();
}
Console.WriteLine(UsingSqlReader());
Console.WriteLine(UsingLinqToSQL());
Console.WriteLine(UsingOptimizeLinqToSQL());
Console.WriteLine(UsingCompiledLinqToSQL());
Console.ReadLine(); |