Csharp/CSharp Tutorial/LINQ/FirstOrDefault
Содержание
FirstOrDefault: get the first or the default
<source lang="csharp">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);
}
}</source>
FirstOrDefault returns null when an Element Is Found
<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.FirstOrDefault();
Console.WriteLine(name == null ? "NULL" : name);
}
}</source>
FirstOrDefault with a Not Found Element
<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.Take(0).FirstOrDefault();
Console.WriteLine(name == null ? "NULL" : name);
}
}</source>
FirstOrDefault with Condition
<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 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;
}
}</source>
FirstOrDefault with string 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.FirstOrDefault(p => p.StartsWith("B"));
Console.WriteLine(name == null ? "NULL" : name);
}
}</source>
Get a customer in Africa with FirstOrDefault
<source lang="csharp">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"));
}
}</source>
Using FirstOrDefault
<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, 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);
}
}</source>
