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>