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LinQ Introduction

LinQ Introduction. Outline. Goals of LinQ Anatomy of a LinQ query More expression examples LinQ to Objects LinQ to XML LinQ to SQL. Goals of LinQ. Integrate data queries into .Net languages

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LinQ Introduction

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  1. LinQ Introduction

  2. Outline • Goals of LinQ • Anatomy of a LinQ query • More expression examples • LinQ to Objects • LinQ to XML • LinQ to SQL

  3. 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.

  4. 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)

  5. 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.

  6. Demo • With objects • After demo, note the following: • Intellisense • Static type checking

  7. 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

  8. 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.

  9. How are people in Aalborg ? • Twitter demo • Get LinqToTwitter here: http://linqtotwitter.codeplex.com/ var twitterCtx = newTwitterContext(); .. 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);

  10. 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()); }

  11. 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);

  12. 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

  13. Collections of objects • We 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

  14. 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

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

  16. 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> <CarID="1000"> <Color>Red</Color> <Make>Ford</Make> </Car> </Inventory>

  17. Use LinQ to generate XML XElement personDoc =newXElement("People", from c in personList orderby c.LastName selectnewXElement("Person", newXAttribute("Age", c.Age), newXElement("FirstName", c.FirstName), newXElement("LastName", c.LastName) )); <People> <PersonAge="2"> <FirstName>Caroline</FirstName> <LastName>Bendtsen</LastName> </Person> <PersonAge="67"> <FirstName>Bjarne</FirstName> <LastName>Hansen</LastName> </Person> <PersonAge="13"> ...

  18. Use LinQ to search in XML XElement doc = MakeXElementFromList(); var query = from p in doc.Elements("Person") whereConvert.ToInt32(p.Attribute("Age").Value) < 40 select p; foreach (var p in query) Console.WriteLine(p.Value);

  19. 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

  20. 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

  21. Select the tables • Note that wizard knows the carnalities

  22. 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] = @p0 ORDER BY [t0].[OrderDate]

  23. 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 selectnew { 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] = @p0 ORDER BY [t2].[OrderDate] }

  24. 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?)

  25. Group • Group transforms a sequence into a sequence of groups that contains a subsequence var products = from p indc.Products group p byp.Category.CategoryName; foreach (var group in products) { Console.WriteLine("\nCategory: {0}", group.Key); foreach (var product in group) Console.WriteLine(product.ProductName);

  26. 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 indc.Products group p bynew { cname = p.Category.CategoryName, cid = p.Category.CategoryID } intoproductCategories orderbyproductCategories.Key.cid selectproductCategories; 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); }

  27. Group into • Group will end the query. Use ‘Into’ to continue the query var products = from p indc.Products group p byp.Category.CategoryNameintoproductCategories orderbyproductCategories.Key selectproductCategories; foreach (var group in products) { Console.WriteLine("\nCategory: {0}", group.Key); foreach (var product in group) Console.WriteLine(product.ProductName);

  28. 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 selectnew { cname = productCategories.Key.CategoryName, count = productCategories.Count() }; foreach (var group in products) { Console.WriteLine("Category: {0}, Count: {1}", group.cname,group.count); }

  29. 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); }

  30. 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 selectnew { cname = upperName, count = productCategories.Count() }; foreach (var group in products) { Console.WriteLine("Category: {0}, Count: {1}", group.cname,group.count); }

  31. 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

  32. Define a contrary version of Where • The extension method must be in another namespace using System; usingSystem.Collections.Generic; usingLinQExamples; namespaceMaryTheContrary//Rasmus Modsat { publicstaticclassExtensionMethods { publicstaticIEnumerable<Person> Where(thisIEnumerable<Person> sequence, Func<Person, bool> predicate) { foreach (Person p in sequence) { if (!predicate(p)) //Contrary yieldreturn p; } } }}

  33. Use of the contrary ‘where’ • The compiler differs between different where’s by the namespace. So you must use ‘using <the namespace> usingMaryTheContrary; .... IEnumerable<Person> nonAdults = from p inpersonList wherep.Age > 18 orderbyp.Age select p; foreach(Person p innonAdults) Console.WriteLine(“First Name: {0} Age:{1}",p.FirstName,p.Age);

  34. LINQPad • Utility for interactively search with LinQ • You can get it here: http://www.linqpad.net/

  35. 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.

  36. Exercise 2, advanced • Continued 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 LinQexpression • (To get it work: Remove “Using System.Linq”, and also implement “Where”)

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