Chapter 15 - Introduce to LINQ pot

Chapter 15. Introduction to LINQ Hoang Anh Viet VietHa@it-hut.edu.vn Hanoi University of Technology 1 Microsoft Objectives “LINQ allows you to build strongly typed query expressions, which can be applied to a number of LINQ targets to manipulate “data” in the broadest sense of the word. Here, you will learn about LINQ to Objects, which allows you to apply LINQ expressions to containers of data (arrays, collections, custom types). This information will serve you well when we examine how to apply LINQ expressions to relational databases (via LINQ to ADO) and XML documents.” 2 Microsoft Roadmap 15.1 Understanding the Role of LINQ 15.2 A First Look at LINQ Query Expressions 15.3 LINQ and Generic Collections 15.4 LINQ and Nongeneric Collections 15.5 The Internal Representation of LINQ Query Operators 15.6 Investigating the C# LINQ Query Operators 3 Microsoft 15.1 The Role of LINQ  The LINQ API is an attempt to provide a consistent, symmetrical manner in which programmers can obtain and manipulate “data” in the broad sense of the term  LINQ Expressions Are Strongly Typed and Extendable  The Core LINQ Assemblies  System.Core.dll  System.Data.Linq.dll  System.Data.DataSetExtensions.dll  System.Xml.Linq.dll 4 Microsoft The Role of LINQ  Advantages of LINQ  For now:  Dynamically-created string queries  Doesn't provide compile-time checking  Native code and non-native code is mixed  With LINQ  Static typing  Compile-time syntax checking  IntelliSense  Native code 5 Microsoft Roadmap 15.1 Understanding the Role of LINQ 15.2 A First Look at LINQ Query Expressions 15.3 LINQ and Generic Collections 15.4 LINQ and Nongeneric Collections 15.5 The Internal Representation of LINQ Query Operators 15.6 Investigating the C# LINQ Query Operators 6 Microsoft 15.2 A First Look at LINQ Query Expressions  LINQ and Implicitly Typed Local Variables  Type of the variable induced from expression  Must include initializer  Can’t be null. Why not?  What happens if “var” is a class in scope?  Works in for loops 7 Microsoft LINQ Query Expressions  LINQ and Extension Methods  The Problem: How to add a method to a type you don’t control and can’t subtype?  Example: How to add Standard Query Operators to IEnumerable<T>, without deriving a new interface?  Solution: Extension methods aren’t really members of the target type, but the syntax makes them look that way  Compile-time type safety is preserved 8 Microsoft LINQ Query Expressions  LINQ and Extension Methods  Not for properties, events, operators  Currently under consideration  Equivalent to calling the static method  Difference from actually extending the class?  How could we use this for Polynomials?  Disadvantages?  Implicitness  Security 9 Microsoft LINQ Query Expressions  Creating Extension Methods  Define a Static Class  Define a static method whose first argument is an object of type T  [System.Runtime.CompilerServices.Extension] attribute on first parameter tells CLR the method is an Extension  C# binds the attribute to keyword “this”  Create a T object, and call the method as if it were a member of the object  When the compiler fails to find the method among T’s members, it searches for an extension method in scope with matching name and signature 10 [...]... Roadmap 15. 1 Understanding the Role of LINQ 15. 2 A First Look at LINQ Query Expressions 15. 3 LINQ and Generic Collections 15. 4 LINQ and Nongeneric Collections 15. 5 The Internal Representation of LINQ Query Operators 15. 6 Investigating the C# LINQ Query Operators Microsoft 33 15. 6 Investigating the C# LINQ Query Operators Operators Meaning in Life from, in Used to define the backbone for any LINQ expression,... a strongly typed container Microsoft 16 Roadmap 15. 1 Understanding the Role of LINQ 15. 2 A First Look at LINQ Query Expressions 15. 3 LINQ and Generic Collections 15. 4 LINQ and Nongeneric Collections 15. 5 The Internal Representation of LINQ Query Operators 15. 6 Investigating the C# LINQ Query Operators Microsoft 17 15. 3 LINQ and Generic Collections  LINQ query expressions can also manipulate data within... First Look at LINQ Query Expressions 15. 3 LINQ and Generic Collections 15. 4 LINQ and Nongeneric Collections 15. 5 The Internal Representation of LINQ Query Operators 15. 6 Investigating the C# LINQ Query Operators Microsoft 24 15. 5 The Internal Representation of LINQ Query Operators    Introduced to the process of building query expressions using various C# query operators (such as from, in, where, orderby,... IEnumerator IComparable IComparer 19 Applying a LINQ Expression  grabbing items from the List where the Speed property is greater than 55 Microsoft 20 Roadmap 15. 1 Understanding the Role of LINQ 15. 2 A First Look at LINQ Query Expressions 15. 3 LINQ and Generic Collections 15. 4 LINQ and Nongeneric Collections 15. 5 The Internal Representation of LINQ Query Operators 15. 6... the same LINQ query multiple times to the same container and obtain the latest and greatest results Microsoft 15 The Role of Immediate Execution    To evaluate a LINQ expression from outside the confines of foreach logic, you are able to call any number of extension methods defined by the Enumerable type Extension methods:ToArray, ToDictionary(), and ToList() Capture a LINQ query... the C# LINQ Query Operators Microsoft 21 15. 4 LINQ and Nongeneric Collections  It is possible to iterate over data contained within nongeneric collections using the generic Enumerable.OfType() method Microsoft 22 Nongeneric Collections  Use the previously defined Car type and import the System.Collections namespace Microsoft 23 Roadmap 15. 1 Understanding the Role of LINQ 15. 2 A First Look at LINQ. .. you to extract a subset of data from a fitting container where Used to define a restriction for which items to extract from a container select Used to select a sequence from the container Performs joins based on specified key Remember, join, on, equals, into these “joins” do not need to have anything to do with data in a relational database orderby, ascending, descending Allows the resulting subset to. ..Replacing LINQ Extension Methods  Extension Methods have lower priority of resolution than type members  So if a type has a member with same signature as a LINQ extension method, the type’s member will be called, and there is no ambiguity  This is how DLINQ and XLINQ replace the default LINQ implementations of the Standard Query Operators Microsoft 11 Lambda Expressions... where, orderby, and select) C# compiler actually translates these tokens into calls on various methods of the System .Linq. Enumerable type Many methods require a generic delegate of type Func Microsoft 25 Query Expressions  Adds querying to language  Important for interaction with DB  Built-in data structures  Leave query planning to data structure designer  Implemented using above functionality... Expression  Lambda expressions  Can be converted to delegate type  if parameters and body match   Microsoft Participate in type inference If expression body, get expression trees 13 Lambda Expression  Lambda expressions  Type inference  Microsoft If call Select(customers, c => c.Name);  T, S mapped to appropriate types 14 The Role of Differed Execution  LINQ query expressions are not actually evaluated . Role of LINQ 15. 2 A First Look at LINQ Query Expressions 15. 3 LINQ and Generic Collections 15. 4 LINQ and Nongeneric Collections 15. 5 The Internal Representation of LINQ Query Operators 15. 6 Investigating. container 16 Microsoft Roadmap 15. 1 Understanding the Role of LINQ 15. 2 A First Look at LINQ Query Expressions 15. 3 LINQ and Generic Collections 15. 4 LINQ and Nongeneric Collections 15. 5 The Internal Representation of LINQ. Nongeneric Collections 15. 5 The Internal Representation of LINQ Query Operators 15. 6 Investigating the C# LINQ Query Operators 6 Microsoft 15. 2 A First Look at LINQ Query Expressions  LINQ and Implicitly