How does linq concat work under the hood?

Question

I am considering replacing code looking like this: ``` foreach (var meshCut in meshCuts0) { ComputePolygons(meshCut, polygons); } foreach (var meshCut in meshCuts1) { ComputePolygons(meshCut, polygons); } ``` With linq looking like this: ``` meshCuts0.Concat(meshCuts1).ForEach(m => ComputePolygons(m, polygons)); ``` I do not know how linq is implemented so I am not sure about the performance consequences. I would appreciate some help please. 1) Will Concat create a list copy or is it just an enumerator doing something like this: ``` public static IEnumerable Concat(IEnumerable a, IEnumerable b) { foreach (var t in a) { yield return t; } foreach (var t in b) { yield return t; } } ``` 2) Will this be the same behaviour on Mono? 3) Is there any reference explaining how linq api functions are implemented for the performance conscious? thanks! Edit Ok there is no ForEach so just assume I also define something like this: ``` public static void ForEach(this IEnumerable sour…

Accepted Answer

Firstly, your code would only work if you also introduce a `ForEach` extension method on `IEnumerable<T>`. I suggest you don't do that - see Eric Lippert's blog post for the reasons I agree with.

I suggest you write it as:

```
foreach (var meshCut in meshCuts0.Concat(meshCuts1))
{
    ComputePolygons(meshCut, polygons);
}
```

`Concat` performs fine - it just iterates over the first sequence then the second, yielding items as it goes. It doesn't buffer all the elements. There's a very slight performance penalty for the extra level of indirection you're adding, but that's about all.

I'd expect Mono to behave the same way - `Concat` is very simple. In fact, as Mono is open source, you can check it for yourself. (It may move over time, of course...)

A while ago I blogged about LINQ to Objects in detail, reimplementing the whole thing from scratch and documenting various aspects of its performance, including where there's room for improvement. See my Edulinq blog series for more details.

How does linq concat work under the hood?

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