LINQ:Language-Integrated Queries

(To be included in C# 3.0)

Technology developed by

Anders Hejlsberg& friends at Microsoft

(2005)

Presented by Tal Cohen

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LINQ• A C# language extension:

– Use SQL-like syntax in C#.

• Outline:– Examples– Understanding the witchcraft

• Delegate functions• Lambda expressions• Type inference• Anonymous types• Extension methods• Expression trees

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Searching in Collections

• Begin with a simple array of, say, Customers.

Customer[] customers = new Customer[30];

customers[0] = new Customer(…);

…

customers[29] = new Customer(…);

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Searching in Collections:The Old Way

• Find the names of all London customers:

List<string> londoners = new List<string>();

foreach (Customer c in customers) {if (c.City == “London”) {

londoners.add(c.Name);}

}

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Searching in Collections:The LINQ Way

string[] londoners =

from c in customers

where c.City == “London”

select c.Name;Declarative!

SQL-like!

No loops!

Returns a simple array!

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LINQ: How Does It Work?

• LINQ syntax = shorthand for method invocation.

• “Translation maps”

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Syntax Translation Example

string[] londoners =

from c in customers

where c.City == “London”

select c.Name;

string[] londoners =

customers.

Where(expression).

Select(expression);

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Translating Expressions

• Problem: Translating

“c.City == “London””

to an expression e, such that Where(e) is valid?

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Expressions == Methods?

• Where() wants a Boolean method.

• The method acts as a filter.

• Likewise for Select(): a translation method.

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C# Delegates

• C# delegates: method pointers.

class Demo {delegate void Foo();void Bar() { … do something … };void Test() {

Foo myDelegate = new Foo(Bar); // “pointer” to Bar()myDelegate(); // invoke

}}

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Delegates as Arguments

• Delegates can be passed as arguments.– Event handlers, jobs for threads, etc.

class Demo {void Job() { … the job to carry out … };void Test() {

Thread worker = new Thread(new ThreadStart(Job));

worker.start();}

}

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Anonymous Methods

• Nameless methods = on-the-fly delegates:

class Demo {delegate void Foo();void Test() {

Foo myDelegate = delegate() { … do something …

}; myDelegate(); // invoke

}}

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Syntax Translation Example

string[] londoners =

from c in customers

where c.City == “London”

select c.Name;

string[] londoners =

customers.

Where(delegate(Customer c) {

return c.City == “London”; }).

Select(delegate(Customer c) {

return c.Name });

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Well, Not Really.

•Where(), etc. accept delegate methods.

• But LINQ creates lambda expressions.

• Seamless conversion.

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Syntax Translation Example

string[] londoners =

from c in customers

where c.City == “London”

select c.Name;

string[] londoners =

customers.

Where(c => c.City == “London”).

Select(c => c.Name);

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Lambda Expressions

• Lambda expression syntax:

(argumentList) => expressiononeArgument => expression

• Arguments optionally typed.– Type inference mechanism.– More on that later…

Shades of ML…

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Where’s Where()?

• We invoked Where() on Customers[].

• On the resulting Customers[], we invoked Select().

• New methods for arrays!?

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Extension Methods

class Utils {

public static firstChar(this string s)

{

return s.charAt(0);

}

}

• So far, just a simple static method.

• Can be used like any other.

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Extension Methods

• But now…

Using Utils;

class Demo {

void Foo() {

string s = “Hello”;

Console.WriteLine(s.firstChar());

}

}

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Extension Methods

• Static methods that seem to extend existing types.

• Where(), Select(), etc. extend IEnumerable<T>.– Defined in System.Query.

• Applicable to one-dimensional array types.

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Query Your Own Types!

• LINQ can be applied to any type.

• Just implement Where(), Select(), etc.

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LINQ and Relational Data

• Let’s obtain a DB-table type, and query it.

DbCustomers c = new DbCustomers(“my.mdb”);

string[] londoners =

from c in customers

where c.City == “London”

select c.Name;

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This Makes No Sense!

• But… Where() applies the filter to every record.

• … on the client!

• SELECT * FROM CUSTOMERS, and filter with a simple loop!?

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Back To Lambda Expressions

• Lambda expressions can be converted to anonymous methods.

• Can also be converted to expression trees.– A run-time representation of the syntax

tree.

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Example…• Our code yields:

string[] londoners = customers.

Where(c => c.City == “London”).

Select(c => c.Name);

where “customers” is of type DbCustomers.

• No

DbCustomers.Where(delegate(Customer c)) method exists.

• However:

DbCustomers.Where(

Expression<Func<Customer,bool>> xt)

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Expression Trees

• A data type.

• Represents lambda expressions at runtime.

• Used it to generate SQL at runtime.– Guaranteed to be valid.

Relational Algebra:Joins, Projections

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Multiple Generators(Cartesian Product / Join)

OrderData[] od = from c in customers where c.City == “London” from o in c.Orders where o.OrderDate.Year == 2005 select new OrderData(c.Name, o.OrderId, o.Total);

OrderData[] od = customers.Where(c => c.City == “London”).SelectMany(c =>

c.Orders.Where(o => o.OrderDate.Year == 2005).Select(o => new OrderData(c.Name,

o.OrderId, o.Total)));

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Projections

• Using LINQ’s select:

from c in customers

where c.City == “London”

select

new AddressBookEntry(c.Name, c.Phone);

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Pre-Defined Types Only?

• But…

The projection type (e.g., AddressBookEntry) must be pre-defined!

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Ad-Hoc Types

• new { [name1 =] expr1,…, [ namen =] exprn}• Type implied by types of exprs.

• Example:

from c in customerswhere c.City == “London”select new { c.Name, c.Phone };

If name is not specified, and expr is either property or

x.property, then property’s name will be used.

Must not be null-typed.

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Ad-Hoc Types are Nameless

• How do we store the result???? q = from … select new {…};

• The ad-hoc type is nameless!

• Can’t use Object– Can’t downcast to access the properties.

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Auto-Typed Variables

• var x = 7; // x will be of type int

• var q = from … select new {…};

// q will be an array of the anonymous type

Console.WriteLine(q[0].Name);

• Local variables only.

Extra

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Relational Data in the OO World?

• “Regular” LINQ queries yields “rectangular” data.

• e.g., var od = from c in customers

where c.City == “London”from o in c.Orderswhere o.OrderDate.Year == 2005select new { c.Name, o.OrderId, o.Total };

= multiple lines per customer.

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Relational Data in the OO World?

Joe Average122324100.23

Joe Average31552390.00

Joe Average98972249.95

John Smith87436649.90

John Smith32477795.00

Miss Piggy435882234.65

Kermit345529345.21

Kermit42334095.95

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OO Data in the OO World!

• Nested queries:var od =

from c in customers

where c.City == “London”

select new {

c.Name,

Orders = from o in c.Orders

where o.OrderDate.Year == 2005

select { o.OrderId, o.Total }

};

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OO Data in the OO World!

122324100.23

31552390.00

98972249.95

Joe Average

87436649.90

32477795.00

John Smith

435882234.65Miss Piggy

345529345.21

42334095.95

Kermit

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Ad-Hoc Type Equality

• Two ad-hoc type expressions that have the same ordered set of property names and types share the same ad-hoc type.

var p1 = new { Name = “Moo”, Age = 12 };

var p2 = new { Name = “Elk”, Age = 17 };

p1 = p2;

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Expression Tree Generation

Expression<Func<int, bool>> exprLambda = x => (x & 1) == 0;

ParameterExpression xParam = Expression.Parameter(typeof(int), "x");

Expression<Func<int, bool>> exprLambda = Expression.Lambda<Func<int, bool>>( Expression.EQ(

Expression.BitAnd(xParam, Expression.Constant(1)),

Expression.Constant(0)),xParam);

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Some LINQ Examples

from m in typeof(string).getMethods

select m.Name;

CloneCompareFormatCopyToCopy

IndexOfIndexOfIndexOfInsert

SubstringSubstring

…

String[]

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Some LINQ Examples

from m in typeof(string).getMethods

where !m.IsStatic

select m.Name;

CloneCompareCopyToCopy

IndexOfIndexOfIndexOfInsert

SubstringSubstring

…

String[]

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Some LINQ Examples

from m in typeof(string).getMethods

where !m.IsStatic

orderby m.name

select m.Name;Clone

CompareCopyCopyToIndexOfIndexOfIndexOfInsert

SubstringSubstring

…

String[]

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Some LINQ Examples

from m in typeof(string).getMethods

where !m.IsStatic

orderby m.name

select m.Name

groupby m.name;Key = CloneKey = CompareKey = CopyKey = CopyToKey = IndexOfKey = InsertKey = Substring

…

KeyGroupPairs[]

Group = …Group = …Group = …Group = …Group = …Group = …Group = …

…

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Some LINQ Examples

from m in typeof(string).getMethods

where !m.IsStatic

orderby m.name

select m.Name

groupby m.name

into g

select new {

Method = g.Key,

Overloads = g.Group.Count

};

Name = CloneName = CompareName = CopyName = CopyToName = IndexOfName = InsertName = Substring

…

var[]

Overloads = 1Overloads = 2Overloads = 1Overloads = 1Overloads = 5Overloads = 3Overloads = 6

…

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Updates with LINQ?

• Future feature: updating capabilities.

from c in customers

where c.City == “Peking”

set c.City = “Beijing”; ??

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DLINQ

• ADO.NET’s next-generation.

• Among other features: a database-to-classes utility.– Class per DB table.– Property per DB column.– Foreign keys, etc. reflected in the classes.

• Result: valid table/field names in the SQL.

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DLINQ – Roll Your Own

• You can also define your own O/R mapping.

• By adding attributes (metadata) to classes/fields.

• Runtime error potential.

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Updates with DLINQ

• DLINQ-generated classes include a change-tracking / DB updating mechanism.– a-la EJBs.

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XLINQ

• Same idea, for searching in XML data.

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Extension Methods + Boxing…

delegate void Proc();static class TestExtensions{

static public void Times(this Int32 n, Proc proc){

for (int i = 1; i <= n; i++(proc();

}}

Which allows you to write:

13.Times(() => Console.Write("X"));