I have a collection of objects of one type that I'd like to convert to a different type. This can be done easily with foreach, but I'd like to figure out how to use Linq's aggregate function to do it.
The problem is all the Aggregate examples use types line string or int, which support the '+' operator. I'd like to have the accumulator type be a list, which doesn't support '+' semantics.
Here's a quick example:
public class DestinationType
{
public DestinationType(int A, int B, int C) { ... }
}
var set = from item in context.Items
select new { item.A, item.B, item.C };
var newSet = set.Aggregate( new List<DestinationType>(),
(list, item) => list.Add(new DestinationType(item.A, item.B, item.C)) );
The problem is that List<>.Add returns void. The return type of the second parameter to Aggregate needs to be a List.
If I had a list type that supported '+' type semantics I could just make the second parameter
list + item
However I can't find any collection type that supports this kind of thing.
Seems like this should be easily possible in Linq. Is there a way? Also, if I'm missing an entirely easier way, I'd love to learn about that too. Thanks!
Assuming this is LINQ to Objects, try...
var newSet = set.Aggregate(new List<DestinationType>(),
(list, item) =>
{
list.Add(new DestinationType(item.A, item.B, item.C));
return list;
});
I think a call to Select combined with ToList() might be what you need here. For example:
context.Items
.Select(item => new DestinationType(item.A, item.B, item.C))
.ToList();
You can just apply select
here.
var newSet = set.Select(item => new DestinationType(...)).ToList();
Aggregate
(generally known as fold
or reduce
) is used to combine elements together where select
applies a function to each element.
Ex:
Let f
be a unary function, then
[a, b, c].
select
(f)
equals [f(a), f(b), f(c)]
.
Let f
be a binary function, then [a, b, c].
aggregate
(f, init)
equals f(a, f(b, f(c, init)))
.
The way you chose in your example is uncommon in C# but often used in functional programming where (linked) lists are transformed into new lists instead of changing an existing collection:
reversed = fold (\list element -> element:list) [] [1..10]
If you really want to do this computation with aggregate
, use Dustin's solution or better implement a linked-list-based type for immutable collections (you can even give this type an operator +
).
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