The Language Integrated Query (LINQ) technology set provides a concise, symmetrical, and strongly typed manner to access a variety of data stores. First a review of the key C# programming constructs that enable LINQ.
LINQ-Specific Programming Constructs
At a high level, LINQ can be understood as a strongly typed query language embedded directly into the grammar of C#.
Implicit Typing of Local Variables
The var keyword of C# allows you to define a local variable without explicitly specifying the underlying data type. The variable is still strongly typed as the compiler will determine the correct data type based on the initial assignment. Many LINQ queries will return a sequence of data types which are not known until compile time.
Object and Collection Initialization Syntax
Object initialization syntax allows you to create a class or structure variable and set any number of its public properties in one fell swoop. This results in a compact and easy on the eyes syntax to prepare objects. C# also allows you to use a similar syntax to initialize collections of objects. This snippet uses collection initialization syntax to fill a List<T> of Rectangle objects:
List<Rectangle> myListOfRects = new List<Rectangle>
{
new Rectangle {TopLeft = new Point { X = 10, Y = 10},
BottomRight = new Point { X = 200, Y = 200}},
new Rectangle {TopLeft = new Point { X = 2, Y = 2 },
BottomRight = new Point { X = 100, Y = 100}},
new Rectangle {TopLeft = new Point { X = 5, Y = 5 },
BottomRight = new Point { X = 90, Y = 75}}
};When combined with implicit typing of local variables, this also allows you to declare an anonymous type which is useful when creating a LINQ projection.
Lambda Expressions
=> is the C# lambda operator. This operator allows you to build a lambda expression, which can be used any time you invoke a method that requires a strongly typed delegate as an argument. Lambdas greatly simplify how you work with delegates in that they reduce the amount of code you must author by hand. A lambda expression can be broken down into the following usage:
( ArgumentsToProcess ) => { StatementsToProcessThem }
List<int> list = new List<int>();
list.AddRange(new int[] { 20, 1, 4, 8, 9, 44 });
List<int> evenNumbers = list.FindAll(i => (i % 2) == 0);
Article notes
What is the unabbreviated form of LINQ?
Language Integrated Query
What provides a concise, symmetrical, and strongly typed manner to access a variety of data stores in C#?
LINQ
What can be understood as a strongly typed query language embedded directly into the grammar of C#?
LINQ
What keyword in C# allows you to define a local variable without explicitly specifying the underlying data type?
var
Some functionality of the LINQ to Objects API can only be accessed by calling extension methods of what class?
Enumerable
What type defines the various extension methods that the LINQ query operators such as from, in, where, orderby, select are shorthand for?
Enumerable
When LINQ Queries are compiled, the C# compiler translates all C# LINQ operators into calls on methods of what class?
Enumerable
What generic delegate type have "a great many" of the methods of Enumerable been prototyped to take as arguments (LINQ)?
Func<>
What term refers to the act of applying LINQ queries to arrays and collections?
LINQ to Objects
What term refers to the act of using LINQ to manipulate and query XML documents?
LINQ to XML
What term refers to an aspect of LINQ that lets you use LINQ queries with the ADO.NET EF Core API?
LINQ to Entities
What term refers to parallel processing of data returned from a LINQ query?
Parallel LINQ (PLINQ)
The words from, in, where, orderby, and select are examples of what in C#?
LINQ query operators
What specific syntax of LINQ is this?
IEnumerable<string> subset =
from game in currentVideoGames
where game.Contains(" ")
orderby game
select game;
LINQ query expression syntax
What specific syntax of LINQ is this?
IEnumerable<string> subset =
currentVideoGames.Where(g => g.Contains(" ")).OrderBy(g => g).Select(g => g);
Extension method syntax
What will you always want to make use of when capturing the results of a LINQ query in C#?
Implicit typing (the var keyword)
What is the type of the return value (the generic interface that it implements) of a LINQ query (most of the time)?
IEnumerable<T>
What is it called that describes how LINQ query expressions that return a sequence do not actually evaluate until you iterate over the resulting sequence?
Deferred execution
What is a thing to note about a LINQ statement that selects a single element (such as by using First()/FirstOrDefault()/Single()/SingleOrDefault()) regarding execution?
The query is executed immediately
What is the difference between First()/FirstOrDefault() and Single()/SingleOrDefault()?
Single()/SingleOrDefault() throws an exception if more than one element is returned from the query
What are the three operators that every LINQ query expression is built from?
from, in, select
What is the general template for a LINQ query expression using the operators from, in, and select?
var result = from item in container select item;
What is the general template for a LINQ query expression using the three basic operators and also where to obtain a specific subset?
var result = from item in container where BooleanExpression select item;
What is the extension method that can be used with the result of a LINQ query to remove duplicate entries?
Distinct()
What is one (or a couple) example of an extension method that can be used with LINQ queries to perform an aggregation operation on the result set?
Count()
Also: Max(), Min(), Average(), Sum()