Putting it all together: LINQ as an Example. The Problem: SQL in Code Programs often connect to...
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Putting it all together: LINQ as an Example
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Transcript of Putting it all together: LINQ as an Example. The Problem: SQL in Code Programs often connect to...
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?
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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!
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?
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LINQ: How Does It Work?
• LINQ syntax = shorthand for method invocation.
• Syntactic sugar, using “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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Expressions == Methods?
• Where() wants a Boolean method.
• The method acts as a filter.
• Likewise for Select(): a translation method.
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Translating Expressions
• Problem: Translating
“c.City == “London””
to a boolean expression e, such that Where(e) is valid?
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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
}}
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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 via coercion.
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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 array types in this manner.
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Query Your Own Types!
• LINQ can be applied to any type, not just built-in arrays and lists.
• Just implement Where(), Select(), etc.
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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;
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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 coerced 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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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
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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.
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.
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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.
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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!
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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 typeConsole.WriteLine(q[0].Name);
• Local variables only.– No auto-typing for fields or formal parameters.
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.
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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.
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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...
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“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”.
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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.
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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).
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