.NET Language-Integrated Query defines a set of general purpose standard query operators that allow traversal, filter, and projection operations to be expressed in a direct yet declarative way in any .NET-based programming language. The standard query operators allow queries to be applied to any IEnumerable<T>-based information source.
Remember your anonymous function syntax that you learned about in the client-side course? It was nice, clean syntax to pass a function to another function, such as forEach() or map(), or filter() on an array.
// Given this JavaScript array of numbers
const numbers = [9, -59, 23, 71, -74, 13, 52, 44, 2]
/*
Use filter() method to build a new array of numbers that
match two conditions. Then chain the sort() method to order
them ascending
*/
let smallPositiveNumbers = numbers.filter(n => n < 40 && n > 0).sort((f, s) => f - s)Luckily for you, you can use lambdas in C#, and the syntax is almost identical using LINQ.
// Give this C# List of numbers
List<int> numbers = new List<int>(){ 9, -59, 23, 71, -74, 13, 52, 44, 2 };
/*
Use the IEnumerable Where() method to build a new array of
numbers that match two conditions. Then chain the OrderBy()
method to order them ascending
*/
var smallPositiveNumbers = numbers.Where(n => n < 40 && n > 0).OrderBy(n => n);
/*
Use All() to see if every item in the collection passes the
provided conditions.
*/
var allBetweenLarge = numbers.All(n => n > -100 && n < 400); // true
var allBetweenSmall = numbers.All(n => n > -5 && n < 39); // falseHere's some more examples of how JavaScript Array methods and LINQ are similar:
// JavaScript
let sorted = numbers.sort();
// LINQ
IEnumerable<int> sorted = numbers.OrderBy(n => n);
IEnumerable<int> sorted = numbers.OrderByDescending(n => n);// JavaScript
const sampleNumbers = [18, 9, 5, 6, 84, 2, 5, 13];
const allAreEven = sampleNumbers.every(number => number % 2 == 0 );
// LINQ
List<int> sampleNumbers = new List<int> {18, 9, 5, 6, 84, 2, 5, 13};
bool areAllEven = sampleNumbers.All(number => number % 2 == 0);// JavaScript
var onlyEvens = sampleNumbers.filter(number => number % 2 == 0);
// LINQ
IEnumerable<int> onlyEvens = sampleNumbers.Where(number => number % 2 == 0);// JavaScript
onlyEvens.forEach(evenValue => { console.log(evenValue + " is an even value"); });
// LINQ
onlyEvens.ToList().ForEach(evenValue => Console.WriteLine(evenValue + " is an even value"));// JavaScript
var sampleNumbersSquared = sampleNumbers.map(number => number * number);
// LINQ
IEnumerable<int> sampleNumbersSquared = sampleNumbers.Select(number => number * number);// JavaScript
const small = sampleNumber.find(n => n < 5);
// LINQ
int small = sampleNumbers.First(n => n < 5);
// ...or...
int small = sampleNumbers.FirstOrDefault(n => n < 5);using System;
using System.Collections.Generic;
using System.Linq;
namespace linqExample
{
public class Person
{
public string FirstName {get; set;}
public string LastName {get; set;}
public DateTime DOB {get; set;}
public List<string> NickNames {get; set;}
}
internal class ReportEntry
{
public string FullName { get; set; }
public DateTime DOB { get; set; }
public int NickNameCount { get; set; }
}
class Program
{
static void Main(string[] args)
{
List<Person> people = new List<Person>(){
new Person(){
FirstName = "Ed", LastName = "Reed", DOB = new DateTime(1970, 04, 10),
NickNames = new List<string>(){
"Ed", "Goose", "Beast"
}
},
new Person(){
FirstName = "Dwayne", LastName = "Johnson", DOB = new DateTime(1966, 10, 11),
NickNames = new List<string>(){
"The Rock"
}
},
new Person(){
FirstName = "Bob", LastName = "Smith", DOB = new DateTime(1944, 12, 25),
NickNames = new List<string>(){
"Robby", "Smitty", "Bud", "The Tiger"
}
},
};
//Use LINQ to generate a report a report that includes the pesron's name, their DOB and the number of nicknames they have
// For example: Bob Smith (12/25/44) has 4 Nickname(s)
List<ReportEntry> nickNameReport = people.Select(p =>
new ReportEntry {
FullName = $"{p.FirstName} {p.LastName}",
DOB = p.DOB,
NickNameCount = p.NickNames.Count
}
).ToList();
foreach (ReportEntry re in nickNameReport)
{
Console.WriteLine($"{re.FullName} ({re.DOB.ToShortDateString()}) has {re.NickNameCount} Nickname(s)");
}
}
}
}Up til now we've been using what is refereed to as the LINQ "method" syntax. This means we've been calling methods on our collections to do what we need. This is by far the most common approach to using LINQ, but it's not the only one. C# also provides us the so-called "query" syntax. You will sometimes see it used and, once in a while (as we shall see), it's a more readable syntax.
using System;
using System.Linq;
using System.Collections.Generic;
public class Program
{
public static void Main() {
/*
Start with a collection that is of type IEnumerable, which
List is and initialize it with some values. This is the
class sizes for a selection of NSS cohorts.
*/
List<int> cohortStudentCount = new List<int>()
{
25, 12, 28, 22, 11, 25, 27, 24, 19
};
/*
Now we need to determine which cohorts fell within the range
of acceptable size - between 20 and 26 students. Also, sort
the new enumerable collection in ascending order.
*/
IEnumerable<int> idealSizes = from count in cohortStudentCount
where count < 27 && count > 19
orderby count ascending
select count;
// Display each number that was the acceptable size
foreach (int c in idealSizes)
{
Console.WriteLine($"{c}");
}
}
}using System;
using System.Collections.Generic;
using System.Linq;
namespace linqGroupBy
{
public class SalesReportEntry
{
public string ReportNeighborhood { get; set; }
public double ReportTotalSales { get; set; }
}
public class Kid {
public string FullName {get; set;}
public string Neighborhood {get; set;}
public double Sales {get; set;}
}
class Program
{
static void Main(string[] args)
{
List<Kid> kids = new List<Kid>(){
new Kid(){
FullName = "Billy Smith", Neighborhood = "Nolensville", Sales = 67.16
},
new Kid(){
FullName = "Jason Sync", Neighborhood = "North Nashville", Sales = 13.16
},
new Kid(){
FullName = "Kyle Edwards", Neighborhood = "Nolensville", Sales = 117.10
},
new Kid(){
FullName = "Avery Barkley", Neighborhood = "The Nations", Sales = 97.16
},
new Kid(){
FullName = "Audrey Ellington", Neighborhood = "Nolensville", Sales = 57.18
},
new Kid(){
FullName = "Juanita Voss", Neighborhood = "North Nashville", Sales = 147.12
},
new Kid(){
FullName = "Scott Avett", Neighborhood = "North Nashville", Sales = 56.11
}
};
List<SalesReportEntry> salesReport = (from kid in kids
//dealing with kids list
group kid by kid.Neighborhood into neighborhoodGroup
//now dealing with neighborhoodGroup list
select new SalesReportEntry {
ReportNeighborhood = neighborhoodGroup.Key,
ReportTotalSales = neighborhoodGroup.Sum(kidObj => kidObj.Sales)
}).OrderByDescending(sr => sr.ReportTotalSales).ToList();
foreach(SalesReportEntry entry in salesReport)
{
Console.WriteLine($"{entry.ReportNeighborhood}, {entry.ReportTotalSales}");
}
}
}
}mkdir -p ~/workspace/csharp/exercises/linq && cd $_
dotnet new console
Given the collections of items shown below, use LINQ to build the requested collection, and then Console.WriteLine() each item in that resulting collection.
// Find the words in the collection that start with the letter 'L'
List<string> fruits = new List<string>() {"Lemon", "Apple", "Orange", "Lime", "Watermelon", "Loganberry"};
IEnumerable<string> LFruits = from fruit in fruits ...// Which of the following numbers are multiples of 4 or 6
List<int> numbers = new List<int>()
{
15, 8, 21, 24, 32, 13, 30, 12, 7, 54, 48, 4, 49, 96
};
IEnumerable<int> fourSixMultiples = numbers.Where();// Order these student names alphabetically, in descending order (Z to A)
List<string> names = new List<string>()
{
"Heather", "James", "Xavier", "Michelle", "Brian", "Nina",
"Kathleen", "Sophia", "Amir", "Douglas", "Zarley", "Beatrice",
"Theodora", "William", "Svetlana", "Charisse", "Yolanda",
"Gregorio", "Jean-Paul", "Evangelina", "Viktor", "Jacqueline",
"Francisco", "Tre"
};
List<string> descend = ...// Build a collection of these numbers sorted in ascending order
List<int> numbers = new List<int>()
{
15, 8, 21, 24, 32, 13, 30, 12, 7, 54, 48, 4, 49, 96
};// Output how many numbers are in this list
List<int> numbers = new List<int>()
{
15, 8, 21, 24, 32, 13, 30, 12, 7, 54, 48, 4, 49, 96
};// How much money have we made?
List<double> purchases = new List<double>()
{
2340.29, 745.31, 21.76, 34.03, 4786.45, 879.45, 9442.85, 2454.63, 45.65
};// What is our most expensive product?
List<double> prices = new List<double>()
{
879.45, 9442.85, 2454.63, 45.65, 2340.29, 34.03, 4786.45, 745.31, 21.76
};List<int> wheresSquaredo = new List<int>()
{
66, 12, 8, 27, 82, 34, 7, 50, 19, 46, 81, 23, 30, 4, 68, 14
};
/*
Store each number in the following List until a perfect square
is detected.
Expected output is { 66, 12, 8, 27, 82, 34, 7, 50, 19, 46 }
Ref: https://msdn.microsoft.com/en-us/library/system.math.sqrt(v=vs.110).aspx
*/// Build a collection of customers who are millionaires
public class Customer
{
public string Name { get; set; }
public double Balance { get; set; }
public string Bank { get; set; }
}
public class Program
{
public static void Main() {
List<Customer> customers = new List<Customer>() {
new Customer(){ Name="Bob Lesman", Balance=80345.66, Bank="FTB"},
new Customer(){ Name="Joe Landy", Balance=9284756.21, Bank="WF"},
new Customer(){ Name="Meg Ford", Balance=487233.01, Bank="BOA"},
new Customer(){ Name="Peg Vale", Balance=7001449.92, Bank="BOA"},
new Customer(){ Name="Mike Johnson", Balance=790872.12, Bank="WF"},
new Customer(){ Name="Les Paul", Balance=8374892.54, Bank="WF"},
new Customer(){ Name="Sid Crosby", Balance=957436.39, Bank="FTB"},
new Customer(){ Name="Sarah Ng", Balance=56562389.85, Bank="FTB"},
new Customer(){ Name="Tina Fey", Balance=1000000.00, Bank="CITI"},
new Customer(){ Name="Sid Brown", Balance=49582.68, Bank="CITI"}
};
}
}/*
Given the same customer set, display how many millionaires per bank.
Ref: https://stackoverflow.com/questions/7325278/group-by-in-linq
Example Output:
WF 2
BOA 1
FTB 1
CITI 1
*/As a light introduction to working with relational databases, this example works with two collections of data - banks and customers - that are related through the Bank attribute on the customer. In that attribute, we store the abbreviation for a bank. However, we want to get the full name of the bank when we produce our output.
This is called joining the collections together.
Read the Cross Join example to get started.
NOTE: You might also find this page on the Microsoft Docs site helpful.
/*
TASK:
As in the previous exercise, you're going to output the millionaires,
but you will also display the full name of the bank. You also need
to sort the millionaires' names, ascending by their LAST name.
Example output:
Tina Fey at Citibank
Joe Landy at Wells Fargo
Sarah Ng at First Tennessee
Les Paul at Wells Fargo
Peg Vale at Bank of America
*/
// Define a bank
public class Bank
{
public string Symbol { get; set; }
public string Name { get; set; }
}
// Define a customer
public class Customer
{
public string Name { get; set; }
public double Balance { get; set; }
public string Bank { get; set; }
}
public class Program
{
public static void Main() {
// Create some banks and store in a List
List<Bank> banks = new List<Bank>() {
new Bank(){ Name="First Tennessee", Symbol="FTB"},
new Bank(){ Name="Wells Fargo", Symbol="WF"},
new Bank(){ Name="Bank of America", Symbol="BOA"},
new Bank(){ Name="Citibank", Symbol="CITI"},
};
// Create some customers and store in a List
List<Customer> customers = new List<Customer>() {
new Customer(){ Name="Bob Lesman", Balance=80345.66, Bank="FTB"},
new Customer(){ Name="Joe Landy", Balance=9284756.21, Bank="WF"},
new Customer(){ Name="Meg Ford", Balance=487233.01, Bank="BOA"},
new Customer(){ Name="Peg Vale", Balance=7001449.92, Bank="BOA"},
new Customer(){ Name="Mike Johnson", Balance=790872.12, Bank="WF"},
new Customer(){ Name="Les Paul", Balance=8374892.54, Bank="WF"},
new Customer(){ Name="Sid Crosby", Balance=957436.39, Bank="FTB"},
new Customer(){ Name="Sarah Ng", Balance=56562389.85, Bank="FTB"},
new Customer(){ Name="Tina Fey", Balance=1000000.00, Bank="CITI"},
new Customer(){ Name="Sid Brown", Balance=49582.68, Bank="CITI"}
};
/*
You will need to use the `Where()`
and `Select()` methods to generate
instances of the following class.
public class ReportItem
{
public string CustomerName { get; set; }
public string BankName { get; set; }
}
*/
List<ReportItem> millionaireReport = ...
foreach (var item in millionaireReport)
{
Console.WriteLine($"{item.CustomerName} at {item.BankName}");
}
}
}