Csharp/CSharp Tutorial/LINQ/First
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>
Retrieving 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>
