Csharp/C Sharp/LINQ/First — различия между версиями
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Текущая версия на 14:38, 26 мая 2010
Содержание
First operator
<source lang="csharp"> using System; using System.Linq; using System.Collections; using System.Collections.Generic; public class MainClass {
public static void Main() { string[] presidents = {"G", "H", "a", "H", "over", "Jack"}; string name = presidents.First(); Console.WriteLine(name); }
}
</source>
First with string method
<source lang="csharp"> using System; using System.Linq; using System.Collections; using System.Collections.Generic; public class MainClass {
public static void Main() { string[] presidents = {"G", "H", "a", "H", "over", "Jack"}; string name = presidents.First(p => p.StartsWith("H")); Console.WriteLine(name); }
}
</source>
retrieves all strings in an array whose length matches that of the shortest string
<source lang="csharp"> using System; using System.Collections.Generic; using System.Linq; public class MainClass {
public static void Main() { string[] names = { "J", "P", "G", "P" }; IEnumerable<string> outerQuery = names .Where(n => n.Length == names.OrderBy(n2 => n2.Length) .Select(n2 => n2.Length).First()); }
}
</source>
Use First, Last with expression
<source lang="csharp"> using System; using System.Collections; using System.Collections.Generic; using System.Text; using System.Linq; public class MainClass{
public static void Main(){ int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9 }; var query = numbers.First(); Console.WriteLine("The first element in the sequence"); Console.Write(query); query = numbers.Last(); Console.WriteLine("The last element in the sequence"); Console.Write(query); Console.WriteLine("The first even element in the sequence"); query = numbers.First(n => n % 2 == 0); Console.Write(query); Console.WriteLine("The last even element in the sequence"); query = numbers.Last(n => n % 2 == 0); Console.Write(query); }
}
</source>
Use First to return the first matching element as a Product
<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(); Product product12 = ( from p in products where p.Id == 2 select p) .First(); Console.Write(product12.Category); } 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 = 2 }); empTree.Add(new Product { ProductName = "C", Category = "O", UnitPrice = 112, UnitsInStock = 3, Total = 34, OrderDate = new DateTime(2005, 1, 1), Id = 3 }); empTree.Add(new Product { ProductName = "D", Category = "O", UnitPrice = 112, UnitsInStock = 0, Total = 33, OrderDate = new DateTime(2005, 1, 1), Id = 4 }); empTree.Add(new Product { ProductName = "E", Category = "O", UnitPrice = 1112, UnitsInStock = 2, Total = 32, OrderDate = new DateTime(2005, 1, 1), Id = 5 }); empTree.Add(new Product { ProductName = "F", Category = "O", UnitPrice = 11112, UnitsInStock = 0, Total = 31, OrderDate = new DateTime(2005, 1, 1), Id = 6 }); 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>