What type of optimizations does LINQ perform at the compiler level?

Question

Now that LINQ is such an integral part of .NET, are their optimizations at the compiler level that would use the optimal path to get results?

For example, imagine you had an array of integers and wanted to get the lowest value. You could do this without LINQ using a foreach, but it's certainly easier to use the Min function in LINQ. Once this goes to the compiler with LINQ would you have been better off to skip LINQ altogether or does it essentially just convert it to something similar to a foreach?

Answer 1

The C# compiler doesn't do much at all - it just calls the methods you tell it to, basically.

You could argue that removing unnecessary Select calls is an optimization:

from x in collection
where x.Condition
select x

is compiled to collection.Where(x => x.Condition) instead of collection.Where(x => x.Condition).Select(x => x) as the compiler recognises the identity transformation as being redundant. (A degenerate query of the form from x in collection select x is immune to this optimization, however, to allow LINQ providers to make sure that any query goes through at least one of their methods.)

The LINQ to Objects Min method just does a foreach, yes. Various LINQ to Objects methods do perform optimization. For example, Count() will check whether the data source implements ICollection or ICollection<T> and use the Count property if so. As madgnome points out in a comment, I wrote a bit more about this in a blog post a while ago.

Of course, other LINQ providers can perform their own optimizations.