1 linq introduction. outline goals of linq anatomy of a linq query more expression examples linq to...

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LinQ Introduction

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Outline

Goals of LinQ

Anatomy of a LinQ query

More expression examples

LinQ to Objects

LinQ to XML

LinQ to SQL

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Goals of LinQ

Integrate data queries into .Net languages

Before C# 3.0 you could use delegates, extension methods, anonyous methods and Visitor Pattern to make something similar to LinQ.

But the syntax is messy and key points, eg. selection criteria, are not easy to read.

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Goals of LinQ

Provide a standardized way to query data

Challenges:

Different data types

Different data representations (xml, sql, objects)

Data organization

Hierarchical – xml (and object)

Relational – sql

Before LinQ you should use different api’s for accessing databases, objects and xml

LinQ provides one single way (nearly) to access it all

LinQ works on collections that implements IEnumerable<T>

(the .Net language must therefore support generics)

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Anatomy of a LinQ query

An example:string[] characters = { "Donald", "Mickey", "Goofy", "Minnie",

"Daisy", "Scrooge" };

IEnumerable<string> query = from c in characters where c.StartsWith("M")!=true orderby c descending select c;

foreach(string s in query)Console.WriteLine(s);

The collection is here a simple string array

But the same query will run for more complex objects, SQL, XML etc.

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Demo

With objects

After demo, note the following:

Intellisense

Static type checking

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Query Expressions

Types of expressions

Filtering

e.g. Where

Projections

e.g. Select

Joining

e.g. Join

Partitioning

e.g Skip and Take

Ordering

e.g OrderBy

Aggregation

e.g. Count and Sum

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Providers

The same expresions works on different kinds of data

This is done by accessing a provider

A LinQ provider is a gateway to a querable type.

There are several builtin providers

Objects

SQL

XML

Active Directory

PLINQ (Parallel processing)

Etc.

And many custom providers too:

LinQ to Amazon

LinQ to Twitter

Etc.

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How are people in Aalborg ?

Twitter demo

Get LinqToTwitter here: http://linqtotwitter.codeplex.com/

var twitterCtx = new TwitterContext();.. var queryResults = from search in twitterCtx.Search where search.Type == SearchType.Search && search.Attitude == Attitude.Positive && search.GeoCode ==

"57.028811,9.917771,25km" select search;

foreach (SearchEntry entry in srch.Results) Console.WriteLine(entry.Text);

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Deferred Execution

Normally the query is not executed before the result is needed

This is when only lazy operators (where, orderby...) are used.

When busy operators are used, the query is executed immediately (count, average)

var adults = from p in personList where p.Age > 18 orderby p.Age select (p.FirstName + " " + p.LastName); Console.WriteLine(adults.Count()); personList.Add(new Person { FirstName = “Ib", LastName = “Madsen", Age = 35 }); foreach (var p in adults) { Console.WriteLine(p.ToString()); }

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Composed Queries

A composed query is a query that uses another query.

In behind LinQ will make new query that is optimized for the given data store (objects, sql, xml...)

var adults = from p in personList where p.Age > 18 orderby p.Age select (p.FirstName + " " + p.LastName);..... var query = from p in adults where p.StartsWith("B") select p; foreach (var p in query) Console.WriteLine(p);

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Encapsulate Query

It is not possible directly to return an anonymous type

And it wouldn’t be nice either Therefore it not is possible to return a query if is declared as var

The nice way here is to declare the query as an IEnumerable<type> and return that.

If the query is a join or a projection etc. then make a class that maps the output from ‘select’ and return a collection of objects of that class

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Collections of objectsWe already seen how to access a collection of objectsthis is called LinQ to objects

LinQ to objects is a good alternative to foreach and other iterations

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Custom providers

And we have seen use of a custom provider to access a webservice.

If it is a plain webservice that returns a collection, then we could have accessed that with LinQ to objects

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In the next part, we will see how to access XML and SQLServer.

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LinQ to XML

Uses the System.Xml.Linq namespace

Is somewhat different from other xml api’s

The XElement class is the key class.

When instanizing a XElement you can generate the whole document in the constructor

XElement doc = new XElement("Inventory", new XElement("Car", new XAttribute("ID","1000"), new XElement("Color", "Red"), new XElement("Make", "Ford")) ); <Inventory>

<Car ID="1000"> <Color>Red</Color> <Make>Ford</Make> </Car> </Inventory>

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Use LinQ to generate XML XElement personDoc =

new XElement("People", from c in personList orderby c.LastName select new XElement("Person",

new XAttribute("Age", c.Age), new XElement("FirstName", c.FirstName), new XElement("LastName", c.LastName) )); <People>

<Person Age="2"> <FirstName>Caroline</FirstName> <LastName>Bendtsen</LastName> </Person> <Person Age="67"> <FirstName>Bjarne</FirstName> <LastName>Hansen</LastName> </Person> <Person Age="13">...

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Use LinQ to search in XML

XElement doc = MakeXElementFromList(); var query = from p in doc.Elements("Person") where Convert.ToInt32(p.Attribute("Age").Value) < 40 select p; foreach (var p in query) Console.WriteLine(p.Value);

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LinQ to SQL

LinQ accesses the sql db through a datacontext class

The class can be created with a wizard in VisualStudio,where you select which tables to access

Only SQLServer is supported from Microsoft

But dbms vendors like Oracle are also providing support for LinQ. (haven’t tested it myself).

An (better?) alternative is to use the Entity Framework

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Use the wizard to create the DataContext class

Add a new item to the project using the ”LINQ to SQL classes” template.

Select the database and the tables that shall be available

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Select the tables

Note that wizard knows the carnalities

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Using LinQ to SQL

Select orders from a certain customer

NorhwindDataContext dc = new NorhwindDataContext(); var orders = from o in dc.Orders where o.CustomerID == "ALFKI" orderby o.OrderDate select o.OrderID;

foreach (var o in orders) Console.WriteLine(o);

SELECT [t0].[OrderID]FROM [dbo].[Orders] AS [t0]WHERE [t0].[CustomerID] = @p0ORDER BY [t0].[OrderDate]

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A join

Write products that the customer has brought var orders = from o in dc.Order_Details join p in dc.Products on o.ProductID equals p.ProductID where o.Order.CustomerID == "ALFKI" orderby o.Order.OrderDate select new { product = p.ProductName, orderDate = o.Order.OrderDate };foreach (var p in orders) Console.WriteLine("{0:dd-MM-yyyy}: {1}",

(DateTime)p.orderDate, p.product);{SELECT [t1].[ProductName] AS [product], [t2].[OrderDate] AS [orderDate]FROM [dbo].[Order Details] AS [t0]INNER JOIN [dbo].[Products] AS [t1] ON [t0].[ProductID] = [t1].[ProductID]INNER JOIN [dbo].[Orders] AS [t2] ON [t2].[OrderID] = [t0].[OrderID]WHERE [t2].[CustomerID] = @p0ORDER BY [t2].[OrderDate]}

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Join

Join is similar to Inner Join in SQL

That means that it is the intersection between two sequences

The inner sequence is a keyed collection, that makes it a lot faster than a subquery (or traversing in a nested loop)

Note that it uses Equals instead of == (remember the difference?)

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Group

Group transforms a sequence into a sequence of groups that contains a subsequence

var products = from p in dc.Products group p by p.Category.CategoryName;

foreach (var group in products) { Console.WriteLine("\nCategory: {0}", group.Key); foreach (var product in group) Console.WriteLine(product.ProductName);

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Use objects as keys

It is possible to use objects as keys.

It can be of an anonymous type or of a defined class var products = from p in dc.Products group p by new { cname = p.Category.CategoryName, cid = p.Category.CategoryID } into productCategories orderby productCategories.Key.cid select productCategories;

foreach (var group in products){ Console.WriteLine("\nID: {0} Category: {1}", group.Key.cid,group.Key.cname); foreach(var product in group) Console.WriteLine(product.ProductName);}

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Group into

Group will end the query. Use ‘Into’ to continue the query

var products = from p in dc.Products group p by p.Category.CategoryName into productCategories orderby productCategories.Key select productCategories; foreach (var group in products) { Console.WriteLine("\nCategory: {0}", group.Key); foreach (var product in group) Console.WriteLine(product.ProductName);

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Grouping and projecting

Make a projection on the content of the group

var products = from p in dc.Products group p by p.Category into productCategories where productCategories.Count() < 8 orderby productCategories.Key.CategoryName select new { cname = productCategories.Key.CategoryName, count = productCategories.Count() }; foreach (var group in products) { Console.WriteLine("Category: {0}, Count: {1}", group.cname,group.count); }

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Nested queries

Nested queries are similar to nested SELECT in SQL.

But be careful: O(n2)

var products = from p in dc.Products where p.CategoryID == (from c in dc.Categories where c.CategoryName == "Seafood" select c).First().CategoryID select p;

foreach (var p in products){ Console.WriteLine("Category: {0}, Product: {1}", p.Category.CategoryName, p.ProductName);}

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Let keyword

Reuse expressions

var products = from p in dc.Products group p by p.Category into productCategories let upperName= productCategories.Key.CategoryName.ToUpper() where productCategories.Count() < 8 orderby upperName select new { cname = upperName, count = productCategories.Count() }; foreach (var group in products) { Console.WriteLine("Category: {0}, Count: {1}", group.cname,group.count); }

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A few words on operators

The LinQ operators are implemented with extension methods (recall the .Where method)

You can define your own operator by defining an extension method

And you can overwrite the existing operators

But be careful:

The operator should still do the same sort of things as it was originally intended for.E.g. the Where operator should still be a filter operator

If the operator is not an aggregator it should be lazy to support deferred execution.

Use ”yield” keyword when returning items from lazy operators

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Define a contrary version of Where

The extension method must be in another namespace

using System;using System.Collections.Generic;using LinQExamples;namespace MaryTheContrary //Rasmus Modsat{ public static class ExtensionMethods { public static IEnumerable<Person> Where( this IEnumerable<Person> sequence, Func<Person, bool> predicate) { foreach (Person p in sequence) { if (!predicate(p)) //Contrary yield return p; } } }}

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Use of the contrary ‘where’

The compiler differs between different where’s by the namespace. So you must use ‘using <the namespace>

using MaryTheContrary;

....

IEnumerable<Person> nonAdults = from p in personList where p.Age > 18 orderby p.Age select p; foreach(Person p in nonAdults) Console.WriteLine(“First Name: {0} Age:{1}", p.FirstName,p.Age);

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LINQPad

Utility for interactively search with LinQ

You can get it here: http://www.linqpad.net/

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Exercise 1

You can get the processes that runs on the machine withIEnumerable<Process> System.Diagnostics.Process.GetProcesses()

The Process object has several properties, e.g.

ProcessName returns the name of the process

WorkingSet64 returns the allocated memory in bytes

TotalProccessorTime return the CPU time that has been used

TODO:

Write and test a LinQ query that returns the processes that has allocated more than 50mb (1mb=10124*1024 bytes) and ordered by process name. The allocated memory should be outputted in mb.

Do the same in a foreach without LinQ.

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Exercise 2, advancedContinued from exercise 1.

You will get an exception if you try to get TotalProcessorTime on a process that you do not own, e.g. a system process

And it not possible to filter those processes out !

A workaround could be like this: IEnumerable<Process> processes = Process.GetProcesses(); foreach (Process p in processes){ try { Console.WriteLine("Process Name: {0} Time: {1}",

p.ProcessName, p.TotalProcessorTime); } catch (Exception e){ Console.WriteLine("Process Name: {0} Time: {1}", p.ProcessName, null); }}

TODO: Override “select” so it inserts null’s for values that are not available. Test it in a LinQ expression

(To get it work: Remove “Using System.Linq”, and also implement “Where”)