In LINQ to Object context ,can i update an object in memory.
I mean i can create new type like
var query =from e in empList
select new {id=e.id,salary=e.salary * (10/100) };
can i update an object?
views:
92answers:
4This is correct, the foreach loop is designed to operated over an Enumerable... you can even use a for loopLINQ was designed to QUERY collections, not iterate over them
masenkablast
2010-04-01 14:44:21
He can't modify the objects created in his example. Anonymous types are immutable.
Kevin Babcock
2010-04-01 14:50:02
I don't think he meant to use an anonymous type at all
masenkablast
2010-04-01 14:50:48
+2
A:
Anonymous types are immutable. If the type is not anonymous, you can dump the collection to a List and modify that:
(from e in empList)
.ToList()
.ForEach(e => {
e.salary = 999;
})
;
Notice I'm using block syntax in the lambda.
ifwdev
2010-04-01 14:43:18
However, this will be slower than a `foreach` loop. (Because of the `ToList` call)
SLaks
2010-04-01 14:46:47
And, all things being equal, a lambda expression will be slower (to invoke, though not to execute) than normal procedural code. Lambdas are a good idea when they make the code more readable or concise, but I don't see how this is particularly more readable or concise than a simple `foreach` statement.
Adam Robinson
2010-04-01 14:53:06
I use LINQPad for a lot of misc tasks involving hundreds of thousands of records. In my experience the ToList() call is negligible compared to other bottlenecks.
ifwdev
2010-04-01 14:53:24
.AsParallel().ForAll(x => { e.salary = 999; });Is NOT slower than a foreach loop in .NET 4 btw
ifwdev
2010-04-01 14:55:56
@ifwdev: That's certainly not true as an absolute statement. Execute that on a single-core processor and you're needlessly splitting the list into multiple threads to no advantage.
Adam Robinson
2010-04-01 14:58:42
Why would it split into multiple threads if there is only one core available?But yes, you are right, it's not true as an absolute statement. In the scenarios where there is any significant performance overhead from calling ToList, I'd think AsParallel would make up for it though.
ifwdev
2010-04-01 15:16:20
+2
A:
That depends on what you're asking.
If you're asking: Can I use a LINQ statement to update data on the elements of a collection? Then the answer is simply no; LINQ is a query language, not a data modification language. Just use a foreach loop on the original collection as you would have done prior to .NET 3.5/C# 3.0. Nothing to see here.
If you're asking Can I update the values on the type I created in the query in my question?, then the answer is "sortof". The type you're creating by using new { ... } is an anonymous type, which makes it immutable (read only). There's no clean way to do what you're after, but you could do something like:
var query = from e in empList
select new { id = e.id, salary = e.salary / 10.0, record = e }
Doing this will give you a reference to the original object via the record property, which you can change values on. Realize, though, that the values on your anonymous type (id and salary, in this case) will not reflect any changes made to the referenced object in record.
foreach(var element in empList)
{
element.record.salary = 100.0;
}
This will update the salary property on the original item, but element.salary will still reflect the original value that was calculated in the query, not the new salary value / 10.0.
Adam Robinson
2010-04-01 14:45:59
A:
Sure, that will work just fine.
Note, however, that the object you are create, while looking like and having the same properties as an Emp, will be a distinct object type.
To elaborate, if you were to say:
var query =from e in empList
select e;
You are creating a collection of Emp objects. When you say:
var query =from e in empList
select new {id=e.id,salary=e.salary * (10/100) };
You are creating a collection of anonomous objects which have the same form as Emp objects. You can assign values which may be different from the original empList value, while you are creating it, but once that statement is complete, thosee values cannot be changed again. And nothing here will affect the values in empList.
James Curran
2010-04-01 14:47:37