Csharp/CSharp Tutorial/LINQ/First

First operator

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);
    }
}

First with string method

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);
    }
}

Retrieving all strings in an array whose length matches that of the shortest string

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());
    }
}

Use First, Last with expression

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);
   }
}

Use First to return the first matching element as a Product

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;
    }
}