¿Cómo se usa LINQ para encontrar el duplicado de una propiedad específica?

Question

Customer customerOne = new Customer("John", "Doe"); 
Customer customerTwo = new Customer("Super", "Man"); 
Customer customerThree = new Customer("Crazy", "Guy"); 
Customer customerFour = new Customer("Jane", "Doe"); 
Customer customerFive = new Customer("Bat", "Man"); 

List<Customer> customers = new List<Customer>(); 
customers.Add(customerOne); 
customers.Add(customerTwo); 
customers.Add(customerThree); 
customers.Add(customerFour); 
customers.Add(customerFive); 

¿Qué consulta LINQ devolvería un enumerable para todos los clientes con el mismo apellido?

Los resultados deben incluir una instancia de: John Doe, Jane Doe, Super Man, y el hombre del palo

Answer 1

var result = from c in customers 
       join c2 in customers on c.LastName equals c2.LastName 
       where c != c2 
       select c; 

Answer 2

var groups = customers.GroupBy(c => c.LastName).Where(g => g.Skip(1).Any()); 
foreach(var group in groups) { 
    Console.WriteLine(group.Key); 
    foreach(var customer in group) { 
     Console.WriteLine("\t" + customer.FirstName); 
    } 
} 

Salida:

Doe 
     John 
     Jane 
Man 
     Super 
     Bat 

Usted puede aplanar la secuencia resultante de los grupos en una secuencia con

var list = groups.SelectMany(g => g); 

Answer 3

customers.GroupBy(c => c.LastName).Where(g => g.Skip(1).Any()).SelectMany(c => c) 

o con la sintaxis de LINQ:

 var q = from c in customers 
       group c by c.LastName into g 
       where g.Skip(1).Any() 
       from c in g 
       select c;