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”)