Csharp/CSharp Tutorial/LINQ/where
Содержание
- 1 A Query Using the Standard Dot Notation Syntax
- 2 Assign the loop variable to another variable declared inside the statement block
- 3 Display Products With Unknown Price
- 4 Filter by Make and Speed
- 5 Filtered: prints the name of each element of an integer array that is less than 5
- 6 Filter string by its length
- 7 Get upper case chars
- 8 Ice Creams with price less than 10
- 9 LINQ Method Syntax
- 10 LINQ query to get strings starting with s
- 11 prints strings where each element has the second letter "i".
- 12 Query by string length with Linq
- 13 Query for filtering numbers
- 14 Query string value by String.StartsWith
- 15 Query Using the Query Expression Syntax
- 16 Query with an Exception
- 17 Search Customers by first name
- 18 Simple method based query
- 19 To remove all vowels from a string.
- 20 Two where clauses
- 21 Use && in where clause
- 22 Use string method in where clause
- 23 Use where to filer object list
- 24 Using where clause in a while loop
- 25 where clause
- 26 Where clause with string method and return IEnumerable object
- 27 Where Prototype
A Query Using the Standard Dot Notation Syntax
<source lang="csharp">using System; using System.Linq; using System.Collections; using System.Collections.Generic; public class MainClass {
public static void Main() {
string[] names = { "A1", "B123", "C123123", "E", "W" };
IEnumerable<string> sequence = names
.Where(n => n.Length < 6)
.Select(n => n);
foreach (string name in sequence) {
Console.WriteLine("{0}", name);
}
}
}</source>
Assign the loop variable to another variable declared inside the statement block
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; public class MainClass {
public static void Main() {
IEnumerable<char> vowels = "aeiou";
IEnumerator<char> rator = vowels.GetEnumerator();
IEnumerable<char> query = "Not what you might expect";
foreach (char vowel in "aeiou") {
char temp = vowel;
query = query.Where(c => c != temp);
}
}
}</source>
Display Products With Unknown Price
<source lang="csharp">using System; using System.Collections.Generic; using System.ruponentModel; using System.Linq;
class ProductWithNullablePrice
{
public string Name { get; private set; }
public decimal? Price { get; private set; }
public ProductWithNullablePrice(string name, decimal price)
{
Name = name;
Price = price;
}
ProductWithNullablePrice()
{
}
public static List<ProductWithNullablePrice> GetSampleProducts()
{
return new List<ProductWithNullablePrice>
{
new ProductWithNullablePrice { Name="C", Price= 9.99m },
new ProductWithNullablePrice { Name="A", Price= 4.99m },
new ProductWithNullablePrice { Name="F", Price= 3.99m },
new ProductWithNullablePrice { Name="S", Price=null}
};
}
public override string ToString()
{
return string.Format("{0}: {1}", Name, Price);
}
}
class MainClass
{
static void Main()
{
List<ProductWithNullablePrice> products = ProductWithNullablePrice.GetSampleProducts();
foreach (ProductWithNullablePrice product in products.Where(p => p.Price == null))
{
Console.WriteLine(product.Name);
}
}
}</source>
Filter by Make and Speed
<source lang="csharp">using System; using System.Collections.Generic; using System.Text; using System.Xml.Linq; using System.Linq;
class Car
{
public string PetName;
public string Color;
public int Speed;
public string Make;
public override string ToString()
{
return string.Format("Make={0}, Color={1}, Speed={2}, PetName={3}",Make, Color, Speed, PetName);
}
}
class Program
{
static void Main(string[] args)
{
Car[] myCars = new []{
new Car{ PetName = "A", Color = "Silver", Speed = 100, Make = "BMW"},
new Car{ PetName = "B", Color = "Black", Speed = 55, Make = "VW"},
new Car{ PetName = "C", Color = "White", Speed = 43, Make = "Ford"}
};
var a = from c in myCars
where c.Make == "BMW" && c.Speed >= 100
select c;
foreach (Car c in a)
{
Console.WriteLine(c.ToString());
}
}
}</source>
Filtered: prints the name of each element of an integer array that is less than 5
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text; public class MainClass {
public static void Main() {
int[] numbers = { 5, 4, 1, 3};
string[] digits = { "zero", "one", "two", "three"};
var lowNums =
from n in numbers
where n < 5
select digits[n];
Console.WriteLine("Numbers < 5:");
foreach (var num in lowNums) {
Console.WriteLine(num);
}
}
}</source>
Filter string by its length
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; class LinqDemo {
static void Main() {
string[] names = { "Tom", "Dick", "Harry" };
IEnumerable<string> filteredNames = names.Where(n => n.Length >= 4);
foreach (string name in filteredNames) Console.Write(name + "|");
}
}</source>
Get upper case chars
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Linq.Expressions; using System.Xml.Linq;
class Program
{
static void Main(string[] args)
{
String aString = "this is a test";
IEnumerable<char> query = from ch in aString where Char.IsUpper(ch) select ch;
} }</source>
Ice Creams with price less than 10
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Linq.Expressions; using System.Xml.Linq; class Program {
static void Main(string[] args)
{
List<Icecream> icecreamsList = new List<Icecream>
{
new Icecream {Name="A", Price=10.5 },
new Icecream {Name="B", Price=9.80 },
new Icecream {Name="C", Price=7.5 }
};
IEnumerable<Icecream> i = from ice in icecreamsList where ice.Price < 10 select ice;
foreach (Icecream ice in i)
{
Console.WriteLine("{0} is {1}", ice.Name, ice.Price);
}
}
} public class Icecream {
public string Name { get; set; }
public double Price { get; set; }
}</source>
LINQ Method Syntax
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text;
class Program{
static void Main(string[] args)
{
string[] names = { "Zheng", "Smith"};
var queryResults = names.Where(n => n.StartsWith("S"));
foreach (var item in queryResults)
{
Console.WriteLine(item);
}
}
}</source>
LINQ query to get strings starting with s
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text;
class Program
{
static void Main(string[] args)
{
string[] names = { "Alonso", "Zheng", "Smith" };
var queryResults =
from n in names
where n.StartsWith("S")
select n;
foreach (var item in queryResults)
{
Console.WriteLine(item);
}
}
}</source>
prints strings where each element has the second letter "i".
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text; public class MainClass {
public static void Main() {
string[] digits = { "ziro", "one", "two", "three"};
var reversedIDigits = (
from d in digits
where d[1] == "i"
select d)
.Reverse();
foreach (var d in reversedIDigits) {
Console.WriteLine(d);
}
}
}</source>
Query by string length with Linq
<source lang="csharp">using System;
using System.Linq;
class Program
{
static void Main(string[] args)
{
string[] colors = {"Red", "Orange", "Yellow", "Green"};
var colorQuery = from color in colors
where color.Length <= 5
orderby color
select color;
foreach (string s in colorQuery)
Console.WriteLine(s);
Console.WriteLine("\nPress any key to continue.");
}
}</source>
Query for filtering numbers
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text;
class Program
{
static void Main(string[] args)
{
Random generator = new Random(0);
int[] numbers = new int[1000];
for (int i = 0; i < 1000; i++)
{
numbers[i] = generator.Next();
}
var queryResults = from n in numbers where n < 100 select n;
foreach (var item in queryResults)
{
Console.WriteLine(item);
}
}
}</source>
Query string value by String.StartsWith
<source lang="csharp">using System; using System.Linq; using System.Collections; using System.Collections.Generic; public class MainClass {
public static void Main() {
string[] presidents = {"Ad", "Ar", "Bu", "B", "C", "C"};
string president = presidents.Where(p => p.StartsWith("Ad")).First();
Console.WriteLine(president);
}
}</source>
Query Using the Query Expression Syntax
<source lang="csharp">using System; using System.Linq; using System.Collections; using System.Collections.Generic; public class MainClass {
public static void Main() {
string[] names = { "A123123", "B123", "C123123", "E123", "W" };
IEnumerable<string> sequence = from n in names
where n.Length < 6
select n;
foreach (string name in sequence) {
Console.WriteLine("{0}", name);
}
}
}</source>
Query with an Exception
<source lang="csharp">using System; using System.Linq; using System.Collections; using System.Collections.Generic; public class MainClass {
public static void Main() {
string[] presidents = {"AAAA", "aaaa", "bacDert", "B1234", "Carter"};
IEnumerable<string> items = presidents.Where(s => Char.IsLower(s[4]));
Console.WriteLine("After the query.");
foreach (string item in items)
Console.WriteLine(item);
}
}</source>
Search Customers by first name
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text;
public class Customer
{
public string FirstName { get; set; }
public string LastName { get; set; }
public string EmailAddress { get; set; }
public override string ToString()
{
return string.Format("{0} {1}\nEmail: {2}",FirstName, LastName, EmailAddress);
}
}
public class Tester
{
static void Main()
{
List<Customer> customers = new List<Customer>{
new Customer { FirstName = "A",
LastName = "B",
EmailAddress = "o@a.ru"},
new Customer { FirstName = "C",
LastName = "H",
EmailAddress = "k@a.ru" },
new Customer { FirstName = "D",
LastName = "C",
EmailAddress = "d@a.ru" },
new Customer { FirstName = "J",
LastName = "G",
EmailAddress = "j@a.ru" },
new Customer { FirstName = "L",
LastName = "H",
EmailAddress = "l@a.ru" }
};
IEnumerable<Customer> result = from customer in customers where customer.FirstName == "D" select customer;
List<Customer> cachedResult = result.ToList<Customer>();
foreach (Customer customer in cachedResult)
Console.WriteLine(customer.ToString());
customers[3].FirstName = "Donna";
foreach (Customer customer in cachedResult)
Console.WriteLine(customer.ToString());
}
}</source>
Simple method based query
<source lang="csharp">using System; using System.Linq;
class Program
{
static void Main(string[] args)
{
string[] names = {"Jesse", "Donald", "Mary"};
var dNames = names.Where(n => n.StartsWith("D"));
foreach (string foundName in dNames)
{
Console.WriteLine("Found: " + foundName);
}
}
}</source>
To remove all vowels from a string.
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; public class MainClass {
public static void Main() {
IEnumerable<char> query = "Not what you might expect";
query = query.Where(c => c != "a");
query = query.Where(c => c != "e");
query = query.Where(c => c != "i");
query = query.Where(c => c != "o");
query = query.Where(c => c != "u");
foreach (char c in query) Console.Write(c);
}
}</source>
Two where clauses
<source lang="csharp">using System; using System.Collections; using System.Collections.Generic; using System.Text; using System.Linq; class Employee {
int _id;
int _idRole;
string _lastName;
string _firstName;
public int ID {
get { return _id; }
set { _id = value; }
}
public int IDRole {
get { return _idRole; }
set { _idRole = value; }
}
public string LastName {
get { return _lastName; }
set { _lastName = value; }
}
public string FirstName {
get { return _firstName; }
set { _firstName = value; }
}
} class Salary {
int _id;
int _year;
double _salary;
public int ID {
get { return _id; }
set { _id = value; }
}
public int Year {
get { return _year; }
set { _year = value; }
}
public double SalaryPaid {
get { return _salary; }
set { _salary = value; }
}
} public class MainClass {
public static void Main() {
List<Employee> people = new List<Employee> {
new Employee { ID = 1, IDRole = 1, LastName = "A", FirstName = "B"},
new Employee { ID = 2, IDRole = 2, LastName = "G", FirstName = "T"}
};
List<Salary> salaries = new List<Salary> {
new Salary { ID = 1, Year = 2004, SalaryPaid = 10000.00 },
new Salary { ID = 1, Year = 2005, SalaryPaid = 15000.00 },
new Salary { ID = 1, Year = 2005, SalaryPaid = 15000.00 }
};
IEnumerable<Salary> q = from p in people
where p.ID == 1
from s in salaries
where s.ID == p.ID
select s;
}
}</source>
Use && in where clause
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text; public class MainClass {
public static void Main() {
List<Product> products = GetProductList();
var expensiveInStockProducts =
from p in products
where p.UnitsInStock > 0 && p.UnitPrice > 3.00M
select p;
foreach (var product in expensiveInStockProducts) {
Console.WriteLine(product.ProductName);
}
}
static List<Product> GetProductList() {
List<Product> empTree = new List<Product>();
empTree.Add(new Product { ProductName = "A", Category = "O", UnitPrice = 12, UnitsInStock = 5, Total = 36, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "B", Category = "O", UnitPrice = 2, UnitsInStock = 4, Total = 35, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "C", Category = "O", UnitPrice = 112, UnitsInStock = 3, Total = 34, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "D", Category = "O", UnitPrice = 112, UnitsInStock = 0, Total = 33, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "E", Category = "O", UnitPrice = 1112, UnitsInStock = 2, Total = 32, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "F", Category = "O", UnitPrice = 11112, UnitsInStock = 0, Total = 31, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
return empTree;
}
} class Product : IComparable<Product> {
public string ProductName { get; set; }
public string Category { get; set; }
public int UnitPrice { get; set; }
public int UnitsInStock { get; set; }
public int Total { get; set; }
public DateTime OrderDate { get; set; }
public int Id { get; set; }
public override string ToString() {
return String.Format("Id: {0}, Name: {1} , Category: {3}", this.Id, this.ProductName, this.Category);
}
int IComparable<Product>.rupareTo(Product other) {
if (other == null)
return 1;
if (this.Id > other.Id)
return 1;
if (this.Id < other.Id)
return -1;
return 0;
}
}</source>
Use string method in where clause
<source lang="csharp">using System; using System.Linq; public class MainClass {
public static void Main() {
string[] greetings = { "hello world", "hello LINQ", "hello" };
var items =
from s in greetings
where s.EndsWith("LINQ")
select s;
foreach (var item in items)
Console.WriteLine(item);
}
}</source>
Use where to filer object list
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; using System.Text; public class MainClass {
public static void Main() {
List<Product> products = GetProductList();
var soldOutProducts =
from p in products
where p.UnitsInStock == 0
select p;
Console.WriteLine("Sold out products:");
foreach (var product in soldOutProducts) {
Console.WriteLine("{0} is sold out!", product.ProductName);
}
}
static List<Product> GetProductList() {
List<Product> empTree = new List<Product>();
empTree.Add(new Product { ProductName = "A", Category = "O", UnitPrice = 12, UnitsInStock = 5, Total = 36, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "B", Category = "O", UnitPrice = 2, UnitsInStock = 4, Total = 35, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "C", Category = "O", UnitPrice = 112, UnitsInStock = 3, Total = 34, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "D", Category = "O", UnitPrice = 112, UnitsInStock = 0, Total = 33, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "E", Category = "O", UnitPrice = 1112, UnitsInStock = 2, Total = 32, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
empTree.Add(new Product { ProductName = "F", Category = "O", UnitPrice = 11112, UnitsInStock = 0, Total = 31, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
return empTree;
}
} class Product : IComparable<Product> {
public string ProductName { get; set; }
public string Category { get; set; }
public int UnitPrice { get; set; }
public int UnitsInStock { get; set; }
public int Total { get; set; }
public DateTime OrderDate { get; set; }
public int Id { get; set; }
public override string ToString() {
return String.Format("Id: {0}, Name: {1} , Category: {3}", this.Id, this.ProductName, this.Category);
}
int IComparable<Product>.rupareTo(Product other) {
if (other == null)
return 1;
if (this.Id > other.Id)
return 1;
if (this.Id < other.Id)
return -1;
return 0;
}
}</source>
Using where clause in a while loop
<source lang="csharp">using System; using System.Collections.Generic; using System.Linq; public class MainClass {
public static void Main() {
IEnumerable<char> vowels = "aeiou";
IEnumerator<char> rator = vowels.GetEnumerator();
IEnumerable<char> query = "Not what you might expect";
char vowel;
while (rator.MoveNext()) {
vowel = rator.Current;
query = query.Where(c => c != vowel);
}
}
}</source>
where clause
<source lang="csharp">using System; using System.Collections; using System.Collections.Generic; using System.Text; using System.Linq; using System.Reflection; class Person {
int _id;
int _idRole;
string _lastName;
string _firstName;
public int ID {
get { return _id; }
set { _id = value; }
}
public int IDRole {
get { return _idRole; }
set { _idRole = value; }
}
public string LastName {
get { return _lastName; }
set { _lastName = value; }
}
public string FirstName {
get { return _firstName; }
set { _firstName = value; }
}
} class Program {
static void Main(string[] args) {
List<Person> people = new List<Person> {
new Person { ID = 1, IDRole = 1, LastName = "A", FirstName = "B"},
new Person { ID = 2, IDRole = 2, LastName = "G", FirstName = "T"},
new Person { ID = 3, IDRole = 2, LastName = "G", FirstName = "M"},
new Person { ID = 4, IDRole = 3, LastName = "C", FirstName = "G"}
};
var query = people.Where((p, index) => p.IDRole == index);
}
}</source>
Where clause with string method and return IEnumerable object
<source lang="csharp">using System; using System.Linq; using System.Collections; using System.Collections.Generic; public class MainClass {
public static void Main() {
string[] presidents = {"AA", "A", "AAA", "B", "Ca", "C"};
IEnumerable<string> items = presidents.Where(p => p.StartsWith("A"));
foreach (string item in items)
Console.WriteLine(item);
}
}</source>
Where Prototype
<source lang="csharp">using System; using System.Linq; using System.Collections; using System.Collections.Generic; public class MainClass {
public static void Main() {
string[] presidents = {"A", "Ar", "B", "Bu", "C", "Cleveland"};
IEnumerable<string> sequence = presidents.Where(p => p.StartsWith("C"));
foreach (string s in sequence)
Console.WriteLine("{0}", s);
}
}</source>
