Transcript LINQ - DidaWiki
LINQ Slides from Gang Luo, Xuting Zhao and Damien Guard
What is LINQ?
Language Integrated Query Make query a part of the language Component of .NET Framework 3.5
Query without LINQ
Objects using loops and conditions foreach (Customer c in customers) if (c.Region == "UK") ...
Databases using SQL SELECT * FROM Customers WHERE Region='UK' XML using XPath/XQuery //Customers/Customer[@Region='UK']
ADO without LINQ
SqlConnection con = new SqlConnection(...); con.Open(); SqlCommand cmd = new SqlCommand( @"SELECT * FROM Customers WHERE c.Region = @Region", con ); cmd.Parameters.AddWithValue("@Region", "UK"); DataReader dr = cmd.ExecuteReader(); while (dr.Read()) { string name = dr.GetString(dr.GetOrdinal("Name")); string phone = dr.GetString(dr.GetOrdinal("Phone")); DateTime date = dr.GetDateTime(3); } dr.Close(); con.Close();
Query with LINQ
C# var myCustomers = from c in customers where c.Region == "UK" select c;
More LINQ queries
var goodCusts = (from c in db.Customers
where c.PostCode.StartsWith("GY") orderby c.Sales descending select c).Skip(10).Take(10);
Local variable type inference
Compiler can infer types from initializer assignments var keyword indicates compiler inferred type Still strong-typed This is not like JavaScript var var a = 10; // Simple types var x = new { Blog = “attardi”, Created = DateTime.Now }; // Anonymous types Essential when using anonymous types
Anonymous Types
Object of new type generated on the fly without first defining it.
Useful for projection to select one or more fields of another structure.
The type will be dynamically generated with setters and getters to corresponding members.
Some common methods are also provided.
No other methods will be added to this type.
But that is already enough!
The object is created and initialized by Anonymous Object Initializer.
Advantages
Unified data access Single syntax to learn and remember Strongly typed Catch errors during compilation IntelliSense Prompt for syntax and attributes Bindable result sets
Architecture
LINQ to Objects
int[] nums = new int[] {0,4,2,6,3,8,3,1}; double average = nums.Take(6).Average(); var above = from n in nums where n > average select n;
LINQ to Objects
Query any IEnumerable
Many useful operators available Sum, Max, Min, Distinct, Intersect, Union Expose your own data with IEnumerable
LINQ operators
Aggregate Conversion Aggregate Average Count Max Min Sum Cast OfType ToArray ToDictionary ToList ToLookup ToSequence Ordering Partitioning OrderBy ThenBy Descending Reverse Skip SkipWhile Take TakeWhile Sets Concat Distinct Except Intersect Union and others …
Query Expression
SQL-like: from s in names where s.Length == 5 orderby select s.ToUpper(); OO-style: names.Where(s => s.Length==5) .OrderBy(s => s) .Select(s => s.ToUpper()); Where, OrderBy, and Select are operators. The arguments to these operators are Lambda Expression.
Lambda Expressions
Examples: s => s.Length == 5 Executable function Anonymous functional. Can be assigned to a delegate variable.
No need to indicate the types Can be passed to methods as parameters.
Expression Tree Efficient in-memory data representations of lambda expressions Changing the behaviors of the expressions Applying your own optimization
Function Types
Func
Methods Extension
Control not only by Lambda Expression, but also by methods extension } public static class Enumerable { public static IEnumerable
LINQ Operations
Join When there is relationship (e.g. foreign key) between two tables, no explicit join operation is needed Using dot notation to access the relationship properties, and navigate all the matching objects.
var q = from o in db.Orders where o.Customer.City
== “London” select o; To join any two data sources on any attribute, you need an explicit join operation. var query = names.Join(people, n => n, p => p.Name, (n, p) => p); The lambda expression for shaping (n, p) => p will be applied on each matching pairs.
LINQ Operations (cont.)
Group Join The lambda expression for shaping is applied on the outer element and the set of all the inner elements that matches the outer one.
Shape the result at a set level var query = names.GroupJoin(people, n => n, p => p.Name, (n, matching) => new { Name = n, Count = matching.Count() } )
LINQ Operations (cont.)
Select Many Each object in the result set may contain a collection or array Select many help decompose the structure and flatten the result var query = names.SelectMany(n => people.Where(p => n == p.Name)) All the elements could be traversed in one foreach loop.
Aggregation Standard aggregation operators: Min, Max, Sum, Average.
int totalLength=names.Sum(s => s.Length); General purpose (generic) operator: static U Aggregate
LINQ Deferred
A LINQ data source can actually implement one of two interfaces: IEnumerable CreateQuery(Expression exp); S Execute(Expression exp); Create deferred query execution plan
IQueryable
IQueryable
An expression tree will represent all the deferred queries as a whole.
Several operations could be “merged”, only one SQL query will be generated and sent to database.
Multi-level defer
Lambda Expressions Revisited
Lambda expressionscan represent either IL code or data Expression
Expression Trees
Expression tree are hierarchical trees of instructions that compose an expression Add value of expression trees Actual creating of IL is deferred until execution of query Implementation of IL creation can vary Trees can even be remoted for parallel processing
Creating IL from Expression Trees
Right before execution tree is compiled into IL Expression
Linq to SQL generates IL that runs SQL commands Linq to Objects builds IL with Sequence extensions methods
Nested defer
Nested defer var q = from c in db.Customers
where c.City
== “London” select new { c.ContactName, c.Phone } var q2 = from c in q.AsEnumerable() select new { Name = DoNameProcessing(c.ContactName), Phone = DoPhoneProcessing(C.Phone) }; What if you want the intermediate result?
string lastName = “Simpson” var persons = from p in personList where p.LastName = lastName select p; lastName = “Flanders” foreach (Person p in persons) Console.WriteLine
(“{0} {1}”, p.FirstName, p.LastName);
Deferred Execution
Advantages
Performance!
Query dependency!
Disadvantages
Divide one query into multiple ones If you iterate over the result set 100 times, the query will be executed 100 times.
Users have to be very careful
LINQ to SQL
Data Model } [Table(Name=“Customers”)] public class Customer { [Column(Id=true)] public string CustomerID; … private EntitySet
It achieves this by translating the operations into SQL statements.
LINQ to SQL
Object-relational mapping Records become strongly-typed objects Data context is the controller mechanism Facilitates update, delete & insert Translates LINQ queries behind the scenes Type, parameter and injection safe
Database mapping
Map tables & fields to classes & properties Generates partial classes with attributes Each record becomes an object Data context represents the database Utilize tables, views or stored procedures
Modifying objects
Update Set object properties Delete context.
Table .DeleteOnSubmit(object) Insert context.
Table .InsertOnSubmit(object) Commit changes back context.SubmitChanges() Transactional - all or nothing
Consistency
Every object will be tracked by LINQ the moment it is loaded from database.
The tracking mechanism monitor the manipulation on relationship properties.
Once you modify one side of the relationship, LINQ will modify the other to keep it consistent.
When an object is deleted, it could still exist in memory, but it will not cause inconsistency.
Concurrency
Optimistic concurrency Conflict checking when SubmitChanges() is called By default, transaction will abort and an exception will be thrown when a conflict is detected.
User can handle the conflict in the exception catch block.
User can set whether or not to detect the conflict when one column get updated.
Transaction/Update
When update, first check whether new object is added (by tracking mechanism) if yes, insert statement will be generated.
What does Django do here?
Modification will not hit the database until the SubmitChanges() method is called All operations will be translated into SQL statements All modifications will be encapsulated into a transaction.
Transaction/Update (cont.)
If an exception is throw during the update, all the changes will be rolled back One SubmitChanges() is actually one transaction. (pros and cons?) Users can also explicitly indicate a new transaction scope.
LINQ to XML Class Hierarchy
http://msdn.microsoft.com/en-us/library/bb308960.aspx
LINQ to XML
LINQ to XML var query = from p in people where p.CanCode
select new XElement (“Person”, new XAttribute (“Age”, p.Age), p.Name); XML to LINQ var x = new XElement (“People”, from p in people where p.CanCode
select new XElement (“Person”, new XAttribute (“Age”, p.Age), p.Name);
Performance
LINQ has more control and efficiency in O/R Mapping than NHibernate LINQ: Externl Mapping or Attribute Mapping NHibernate: Externl Mapping Because of mapping, LINQ is slower than database tools such as SqlDataReader or SqlDataAdapter In large dataset, their performance are more and more similar