Putting it all together:
LINQ as an Example
The Problem: SQL in Code
• Programs often connect to database servers.
• Datab...
3
Searching in Collections
• Begin with a simple array of, say,
Customers.
Customer[] customers = new Customer[30];
custom...
5
Searching in Collections:
The LINQ Way
string[] londoners =
from c in customers
where c.City == “London”
select c.Name;
...
7
LINQ: How Does It Work?
• LINQ syntax = shorthand for method
invocation.
• Syntactic sugar, using “Translation
maps”
8
S...
9
Expressions == Methods?
• Where() wants a Boolean method.
• The method acts as a filter.
• Likewise for Select(): a tran...
11
C# Delegates
• C# delegates: method pointers.
• Since C# 1.0.
class Demo {
delegate void Foo();
void Bar() { … do somet...
13
Anonymous Methods
• Nameless methods = on-the-fly
delegates:
class Demo {
delegate void Foo();
void Test() {
Foo myDele...
15
Well, Not Really.
•Where(), etc. accept delegate
methods.
• But LINQ creates lambda
expressions.
• Seamless conversion ...
17
Lambda Expressions
• Lambda expression syntax:
(argumentList) => expression
oneArgument => expression
• Arguments optio...
19
Extension Methods
class Utils {
public static firstChar(this string s)
{
return s.charAt(0);
}
}
• So far, just a simpl...
21
Extension Methods
• Static methods that seem to extend
existing types.
• Where(), Select(), etc. extend array
types in ...
23
LINQ and Relational Data
• Let’s obtain a DB-table type, and query it.
DbCustomers customers = new DbCustomers(“my.mdb”...
25
Back To Lambda
Expressions
• Lambda expressions can be
converted to anonymous methods.
• Can also be coerced to express...
27
What Are Expression Trees?
• Any valid expression is converted by the
compiler to an expression tree.
– a.k.a. the abst...
It’s Just Coercion
• So, LINQ converts into expressions that
use Where(...), Select(...), etc.
• For some classes, Where(....
31
Pre-Defined Types Only?
• But…
The projection type (e.g.,
AddressBookEntry) must be pre-
defined!
32
Ad-Hoc Types
• new...
33
Ad-Hoc Types are Nameless
• How do we store the result?
??? q = from … select new {…};
• The ad-hoc type is nameless!
3...
Summary
• LINQ adds static SQL expression
correctness to C#.
• To do this, the following features were
added to C#:
– Lamb...
What Is Happening to Languages?
• As new features are added to programming
languages, the languages evolve.
• Many of the ...
A Glimpse Into the Future:
LISP (1958)
• We have seen the power of representing
program source at runtime (expression
tree...
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linq with Exampls

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linq with Exampls

  1. 1. Putting it all together: LINQ as an Example The Problem: SQL in Code • Programs often connect to database servers. • Database servers only “speak” SQL. • Programs have to construct SQL strings. • PHP example: if (some_condition()) { $q = mysql_query(“select name from user were id = $id”) ... } • When will the problem be detected? 2
  2. 2. 3 Searching in Collections • Begin with a simple array of, say, Customers. Customer[] customers = new Customer[30]; customers[0] = new Customer(…); … customers[29] = new Customer(…); 4 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); } }
  3. 3. 5 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! Searching in Collections: The LINQ Way • LINQ is a C# feature – Introduced in C# 3.0. • LINQ = “Language INtegrated Query” • So far, this is just list comprehension added to C#. • What did it take to add list comprehension to the language?
  4. 4. 7 LINQ: How Does It Work? • LINQ syntax = shorthand for method invocation. • Syntactic sugar, using “Translation maps” 8 Syntax Translation Example string[] londoners = from c in customers where c.City == “London” select c.Name; string[] londoners = customers. Where(expression). Select(expression);
  5. 5. 9 Expressions == Methods? • Where() wants a Boolean method. • The method acts as a filter. • Likewise for Select(): a translation method. 10 Translating Expressions • Problem: Translating “c.City == “London”” to a boolean expression e, such that Where(e) is valid?
  6. 6. 11 C# Delegates • C# delegates: method pointers. • Since C# 1.0. class Demo { delegate void Foo(); void Bar() { … do something … }; void Test() { Foo myDelegate = new Foo(Bar); // “pointer” to Bar() myDelegate(); // invoke } } 12 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(); } }
  7. 7. 13 Anonymous Methods • Nameless methods = on-the-fly delegates: class Demo { delegate void Foo(); void Test() { Foo myDelegate = delegate() { … do something … }; myDelegate(); // invoke } } 14 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 });
  8. 8. 15 Well, Not Really. •Where(), etc. accept delegate methods. • But LINQ creates lambda expressions. • Seamless conversion via coercion. 16 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);
  9. 9. 17 Lambda Expressions • Lambda expression syntax: (argumentList) => expression oneArgument => expression • Arguments optionally typed. – Type inference mechanism. – More on that later… Shades of ML… 18 Where’s Where()? • We invoked Where() on Customers[]. • On the resulting Customers[], we invoked Select(). • New methods for arrays!?
  10. 10. 19 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. 20 Extension Methods • But now… Using Utils; class Demo { void Foo() { string s = “Hello”; Console.WriteLine(s.firstChar()); } }
  11. 11. 21 Extension Methods • Static methods that seem to extend existing types. • Where(), Select(), etc. extend array types in this manner. 22 Query Your Own Types! • LINQ can be applied to any type, not just built-in arrays and lists. • Just implement Where(), Select(), etc.
  12. 12. 23 LINQ and Relational Data • Let’s obtain a DB-table type, and query it. DbCustomers customers = new DbCustomers(“my.mdb”); string[] londoners = from c in customers where c.City == “London” select c.Name; 24 This Makes No Sense! • But… Where() applies the filter to every record. • … on the client! • SELECT * FROM CUSTOMERS, and filter with a simple loop!?
  13. 13. 25 Back To Lambda Expressions • Lambda expressions can be converted to anonymous methods. • Can also be coerced to expression trees. – A run-time representation of the syntax tree. 26 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)
  14. 14. 27 What Are Expression Trees? • Any valid expression is converted by the compiler to an expression tree. – a.k.a. the abstract syntax tree of the expression. – Normal part of the compilation process, in any language! • Examples: 5 + 3 * 2 c.city == “London” + 5 * 3 2 == . (dot) “London” c city 28 Expression Trees • Normally, expression trees only exist at compile-time. • In C#, the compiler can create a run-time representation of the expression tree. – The language has a data type for expression trees. – Represents lambda expressions at runtime. • Used for generating SQL at runtime. – Guaranteed to be syntactically valid, since it was created from a valid C# expression.
  15. 15. It’s Just Coercion • So, LINQ converts into expressions that use Where(...), Select(...), etc. • For some classes, Where(...) and Select(...) accept delegates; for other classes, they accept expression trees. • Lambda expressions can be coerced into either. 29 30 Projections • Using LINQ’s select: from c in customers where c.City == “London” select new AddressBookEntry(c.Name, c.Phone);
  16. 16. 31 Pre-Defined Types Only? • But… The projection type (e.g., AddressBookEntry) must be pre- defined! 32 Ad-Hoc Types • new { [name1 =] expr1,…, [ namen =] exprn} • Type implied by types of exprs. • Example: from c in customers where 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.
  17. 17. 33 Ad-Hoc Types are Nameless • How do we store the result? ??? q = from … select new {…}; • The ad-hoc type is nameless! 34 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. – No auto-typing for fields or formal parameters.
  18. 18. Summary • LINQ adds static SQL expression correctness to C#. • To do this, the following features were added to C#: – Lambda expressions. – Extension methods. – Expression types. – List comprehension. – Anonymous data types. – Type inference. 35 There’s More • There are several LINQ features we did not present here, such as: – Grouping (“GROUP BY” in SQL) – Joins (selecting from multiple tables) – ... • These require even more language features, such as closures. 36
  19. 19. What Is Happening to Languages? • As new features are added to programming languages, the languages evolve. • Many of the features come from research or experimental languages. • Note how many of the new C# features discussed here come from functional languages like ML, Haskell or LISP: – Lambda expressions, expression types, list comprehension, anonymous data types, type inference... 37 “Confessions of a Used Programming Language Salesman” • An 2007 essay by Eric Meijer (Microsoft). • Discusses how features from functional languages slowly creep into “mainstream” languages. • “Functional programming has finally reached the masses, except that it is called Visual Basic 9 instead of Haskell 98”. 38
  20. 20. A Glimpse Into the Future: LISP (1958) • We have seen the power of representing program source at runtime (expression trees). • In LISP, program source can be represented at runtime, but also generated at runtime (or compile-time). – Source code itself is a data structure (a list). • LISP macros are light-years ahead of C/C++ macros. 39 A Glimpse Into the Future: LISP (1958) • 50 years later, LISP features are slowly re-appearing in mainstream languages. – e.g., garbage collection, aspect-oriented programming, and more. • Conclusions: – a. Learn from history. – b. Know LISP, Haskell, etc: once you really understand them, it will give you serious advantages over ignorant software engineers (even if you never use these languages in practice). 40

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