Csharp/C Sharp/LINQ/where

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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>


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>


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 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>


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 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>


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>


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>
Источник — «http://www.nfex.ru/index.php?title=Csharp/C_Sharp/LINQ/where&oldid=505»