- Why do we need the conversion operators?
2. What is the AsEnumerable operator?
3. What is the signature of the AsEnumerable operator?
4. What are the ToArray(), ToList() operators?
5. What are the signatures of the ToArray(), ToList() operators?
6. What is the ToDictionary() operator?
7. What is the signature of the ToDictionary() operator?
8. What is the ToLookup() operator?
9. What is the signature of the ToLookup () operator?
10. What is the OfType() operator?
11. What is the signature of the OfType() operator?
12. What is the Cast() operator?
13. What is the signature of the Cast() operator?
We can implement our own operators, but sometimes we need to call standard LINQ operators, so to get opportunity to call standard operators we can call the AsEnumerable() operator:
public static IEnumerable<TSource> AsEnumerable<TSource>( this IEnumerable<TSource> source );
Example 1:
public sealed class Customers : List<Customer>
{
public Customers(IEnumerable<Customer> items) : base(items) { }
}
public static class CustomersOperators
{
public static IEnumerable<Customer> Where(this Customers items, Func<Customer, bool> filter)
{
return items;
}
}
var customers = new Customers(Data.Customers);
var specificWhereExpr = from c in customers
where c.Country == Countries.Italy
select c;
foreach (var item in specificWhereExpr)
{
Trace.WriteLine(item);
}
Trace.WriteLine("-========================================-");
var genericWhereExpr = from c in customers.AsEnumerable()
where c.Country == Countries.Italy
select c;
foreach (var item in genericWhereExpr)
{
Trace.WriteLine(item);
}
Output:
Paolo – Brescia – Italy
Marco – Torino – Italy
James – Dallas – USA
Frank – Seattle – USA
-========================================-
Paolo – Brescia – Italy
Marco – Torino – Italy
To avoid unnecessary evaluation we can use ToArray() or ToList() operators. Using those operators we can get a snapshot of a source array.
public static TSource[] ToArray<TSource>( this IEnumerable<TSource> source );
public static List<TSource> ToList<TSource>( this IEnumerable<TSource> source );
Example 2:
var productGreater30 = (from p in Data.Products
where p.Price > 30
select p).ToArray();
foreach (var item in productGreater30)
{
Trace.WriteLine(item);
}
var OrdersByCustomer = from c in Data.Customers
from o in c.Orders
join p in productGreater30 on o.IdProduct equals p.IdProduct
select new { c.Name, Total = p.Price * o.Quantity };
foreach (var item in OrdersByCustomer)
{
Trace.WriteLine(item);
}
Output:
4 – 40
5 – 50
6 – 60
{ Name = Frank, Total = 1000 }
Grigory's space 