Csharp/CSharp Tutorial/LINQ/FirstOrDefault

FirstOrDefault: get the first or the default

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
    public static void Main() {
        int[] numbers = { };
        int firstNumOrDefault = numbers.FirstOrDefault();
        Console.WriteLine(firstNumOrDefault);
    }
}

FirstOrDefault returns null when an Element Is Found

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.FirstOrDefault();
        Console.WriteLine(name == null ? "NULL" : name);
    }
}

FirstOrDefault with a Not Found Element

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.Take(0).FirstOrDefault();
        Console.WriteLine(name == null ? "NULL" : name);
    }
}

FirstOrDefault with Condition

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
    public static void Main() {
        List<Product> products = GetProductList();
        Product product789 = products.FirstOrDefault(p => p.Id == 789);
        Console.WriteLine("Product 789 exists: {0}", product789 != null);
    }
    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;
    }
}

FirstOrDefault with string 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.FirstOrDefault(p => p.StartsWith("B"));
        Console.WriteLine(name == null ? "NULL" : name);
    }
}

Get a customer in Africa with FirstOrDefault

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

    class Customer
    {
        public string ID { get; set; }
        public string City { get; set; }
        public string Country { get; set; }
        public string Region { get; set; }
        public decimal Sales { get; set; }
    }
    class Program
    {
        static void Main(string[] args)
        {
            List<Customer> customers = new List<Customer> {
              new Customer { ID="O", City="Cairo", Country="Egypt", Region="Africa", Sales=6000 },
              new Customer { ID="P", City="Tehran", Country="Iran", Region="Asia", Sales=7000 },
              new Customer { ID="Q", City="London", Country="UK", Region="Europe", Sales=8000 },
              new Customer { ID="R", City="Beijing", Country="China", Region="Asia", Sales=9000 },
              new Customer { ID="T", City="Lima", Country="Peru", Region="South America", Sales=2002 }
           };
            var queryResults = from c in customers select new { c.City, c.Country, c.Region };
            Console.WriteLine(queryResults.FirstOrDefault(c => c.Region == "Africa"));
        }
    }

Using FirstOrDefault

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, 3, 5, 7, 9 };
            var query = numbers.FirstOrDefault(n => n % 2 == 0);
            Console.WriteLine("The first even element in the sequence");
            Console.Write(query);
            Console.WriteLine("The last odd element in the sequence");
            query = numbers.LastOrDefault(n => n % 2 == 1);
            Console.Write(query);
   }
}