ReadingNotes

LINQ & Delegates

Language Integrated Query (LINQ)

Overview of LINQ:

LINQ integrates query capabilities directly into the C# language.
Traditional queries are expressed as strings without type checking at compile time or IntelliSense support.
LINQ introduces a query as a first-class language construct, making it declarative and easier to work with.
Query expressions are written in a declarative query syntax and can be used to perform filtering, ordering, and grouping operations on various data sources.

Query Expression Overview:

Query expressions can be used to query and transform data from any LINQ-enabled data source, including SQL databases, XML documents, web services, and .NET collections.
Query expressions use familiar C# language constructs and are strongly typed.
Queries are not executed until iterated over, for example, in a foreach statement.
Query expressions can be expressed in two forms: query syntax and method syntax. Query syntax is often more readable and concise.

Enabling LINQ Querying:

For in-memory data, LINQ to Objects is used if the data implements IEnumerable<T>.
If enumeration is not suitable, LINQ standard query operator methods can be defined in the type or extended for custom implementations with deferred execution.
For remote data sources, implementing the IQueryable<T> interface is the best option.

IQueryable LINQ Providers:

LINQ providers that implement IQueryable can vary in complexity.
Less complex providers might interface with a single method of a Web service and have a closed type system.
Medium complexity providers might target data sources with partially expressive query languages and have a richer type system.
Complex providers, like LINQ to SQL, can translate complete LINQ queries to an expressive query language like SQL and have an open type system.

LINQ provides a powerful and flexible way to query and manipulate data in C#, unifying different data sources under a common query syntax. It simplifies the process of querying and transforms data, leading to more concise and readable code.

Introduction to LINQ Queries (C#)

The article provides an “Introduction to LINQ Queries (C#).” LINQ, or Language Integrated Query, offers a unified approach for querying and manipulating data across different data sources in C#. Traditionally, developers needed to learn distinct query languages for various data formats like SQL for databases and XQuery for XML.
LINQ simplifies this by introducing a consistent model based on querying objects. All LINQ query operations consist of three parts: obtaining the data source, creating the query, and executing the query. The data source must support the generic IEnumerable interface or its derived interfaces to be queried with LINQ.
The query specifies what data to retrieve and can include filtering, sorting, and shaping operations. C# provides new query syntax to make writing queries more straightforward. The actual execution of the query is deferred until the query variable is iterated over, often in a foreach statement, referred to as deferred execution.
The article emphasizes that LINQ works with different data sources, such as arrays, XML documents, SQL databases, and more, as long as they can be represented as queryable types. LINQ to SQL is mentioned as an example of LINQ working with object-relational mapping to query databases.
The concept of forcing immediate execution of queries is discussed, along with methods like ToList() or ToArray() to cache the results. The article concludes by highlighting that LINQ provides a powerful and flexible way to work with data in C# using a consistent and expressive syntax for querying various data sources.

Standard Query Operators Overview (C#)

The article “Standard Query Operators Overview (C#)” provides a summary of the standard query operators in LINQ, which enable querying and manipulating data in C#. The standard query operators consist of methods that operate on sequences represented by objects implementing IEnumerable or IQueryable. These operators offer a range of query capabilities, including filtering, projection, aggregation, sorting, and more.

The article highlights that there are two sets of standard query operators: one for in-memory collections (IEnumerable) and another for database queries (IQueryable). Operators returning singleton values execute immediately, while those returning sequences defer query execution and return an enumerable object.

Developers can chain query methods together, allowing for complex queries to be constructed. The article also discusses the query expression syntax, which provides dedicated C# language keywords for some operators, making query construction more convenient.

Furthermore, the article explains that developers can extend the standard query operators with domain-specific methods or even replace them with custom implementations to cater to specific requirements.

Examples of common query operations are provided, including obtaining a data source, filtering, ordering, grouping, joining, and selecting (projections).

Overall, the article serves as a concise overview of how standard query operators form the foundation of LINQ, offering a powerful and flexible approach for querying data in C#.

Walkthrough: Writing Queries in C# (LINQ)

The article “Walkthrough: Writing Queries in C# (LINQ)” provides a step-by-step demonstration of how to write LINQ query expressions using C# language features. The walkthrough covers the following main sections:

Create a C# Project: The article explains how to create a new C# console application project in Visual Studio, ensuring that it has the necessary references to System.Core.dll and a using directive for the System.Linq namespace.
Create an in-Memory Data Source: A data source is set up using a simple list of Student objects. Each Student record contains a first name, last name, and an array of integers representing their test scores. The article demonstrates the use of object and collection initializers to create and populate the list of students.
Create the Query: A basic LINQ query is created to retrieve all students whose score on the first test is greater than 90. The query uses the “from…where…select” syntax to filter the data based on the specified condition.
Execute the Query: The query is executed using a foreach loop, and the results are displayed in the console window.
Additional Filters and Ordering: The article demonstrates how to add multiple filter conditions to the query using logical operators (AND and OR) and how to order the results based on specific fields.
Grouping the Results: Grouping is introduced in the query using the “group…by” clause, which groups students by the first letter of their last name. The results are displayed accordingly.
Introducing Identifiers with “let”: The “let” keyword is used to introduce an identifier for an expression result within the query. It allows the calculation of total scores for each student and provides a cleaner way to write complex expressions.
Using Method Syntax in a Query Expression: The article explains that some query operations can only be expressed using method syntax. It demonstrates how to use method syntax to calculate the average score of the class.
Transforming the Select Clause: The select clause is used to transform or project the query results into a sequence of strings. The example provided retrieves first names of students with the last name “Garcia” and displays them.
Working with Anonymous Types: An anonymous type is introduced in the select statement to create a sequence of Students whose total score is greater than the class average, along with their Student ID.

The article concludes by mentioning additional LINQ providers (e.g., LINQ to SQL, LINQ to DataSet, LINQ to XML, and LINQ to Objects) and encourages readers to explore them further.

Overall, the walkthrough provides a comprehensive introduction to LINQ query expressions in C#, starting from project creation to more advanced query techniques.