.NET Framework LINQ SelectMany (flat map)
Example
Enumerable.Select returns an output element for every input element.
Whereas Enumerable.SelectMany produces a variable number of output elements for each input element. This means that the output sequence may contain more or fewer elements than were in the input sequence.
Lambda expressions passed to Enumerable.Select must return a single item. Lambda expressions passed to Enumerable.SelectMany must produce a child sequence. This child sequence may contain a varying number of elements for each element in the input sequence.
Example
class Invoice
{
public int Id { get; set; }
}
class Customer
{
public Invoice[] Invoices {get;set;}
}
var customers = new[] {
new Customer {
Invoices = new[] {
new Invoice {Id=1},
new Invoice {Id=2},
}
},
new Customer {
Invoices = new[] {
new Invoice {Id=3},
new Invoice {Id=4},
}
},
new Customer {
Invoices = new[] {
new Invoice {Id=5},
new Invoice {Id=6},
}
}
};
var allInvoicesFromAllCustomers = customers.SelectMany(c => c.Invoices);
Console.WriteLine(
string.Join(",", allInvoicesFromAllCustomers.Select(i => i.Id).ToArray()));
Output:
1,2,3,4,5,6
Enumerable.SelectMany can also be achieved with a syntax-based query using two consecutive from clauses:
var allInvoicesFromAllCustomers
= from customer in customers
from invoice in customer.Invoices
select invoice;
