Linq - 'Saving' OrderBy operation (c#)
Assume I have generic list L
of some type in c#. Then, using linq, call OrderBy()
on it, passing开发者_StackOverflow in a lambda expression.
If I then re-assign the L
, the previous order operation will obviously be lost.
Is there any way I can 'save' the lambda expression I used on the list before i reassigned it, and re-apply it?
Use a Func delegate to store your ordering then pass that to the OrderBy method:
Func<int, int> orderFunc = i => i; // func for ordering
var list = Enumerable.Range(1,10).OrderByDescending(i => i); // 10, 9 ... 1
var newList = list.OrderBy(orderFunc); // 1, 2 ... 10
As another example consider a Person
class:
public class Person
{
public int Id { get; set; }
public string Name { get; set; }
}
Now you want to preserve a sort order that sorts by the Name
property. In this case the Func
operates on a Person
type (T
) and the TResult
will be a string since Name
is a string and is what you are sorting by.
Func<Person, string> nameOrder = p => p.Name;
var list = new List<Person>
{
new Person { Id = 1, Name = "ABC" },
new Person { Id = 2, Name = "DEF" },
new Person { Id = 3, Name = "GHI" },
};
// descending order by name
foreach (var p in list.OrderByDescending(nameOrder))
Console.WriteLine(p.Id + ":" + p.Name);
// 3:GHI
// 2:DEF
// 1:ABC
// re-assinging the list
list = new List<Person>
{
new Person { Id = 23, Name = "Foo" },
new Person { Id = 14, Name = "Buzz" },
new Person { Id = 50, Name = "Bar" },
};
// reusing the order function (ascending by name in this case)
foreach (var p in list.OrderBy(nameOrder))
Console.WriteLine(p.Id + ":" + p.Name);
// 50:Bar
// 14:Buzz
// 23:Foo
EDIT: be sure to add ToList()
after the OrderBy
calls if you need a List<T>
since the LINQ methods will return an IEnumerable<T>
.
Calling ToList()
or ToArray()
on your IEnumerable<T>
will cause it to be immediately evaluated. You can then assign the resulting list or array to "save" your ordered list.
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