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Comment by scarface74 | Hacker News Reader

LINQ itself just creates expression trees. The expression tree is then translated into code by another provider (for lack of a better term). The code generated can either operated on an in memory collection or generate any other type of query.

var seniors = customers.Where(c => c.age > 65)

Could either represent C# IL, a MongoQuery, a SQL query, an ElasticSearch query, etc. depending on what type of variable “customer” is.

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Just to be a little pedantic and to inform those that might not be familiar, LINQ only creates an expression tree when the type of the source object is IQueryable. If the type of the source object is IEnumerable only (such as an in-memory list), it will not create an expression tree and instead will use the Enumerable static class methods passing in the lambdas as delegates (aka function pointers). In the latter case, there is no "code generated", it will directly enumerate the IEnumerable and do things like an "if" statement inside a "foreach" loop in the case of "where", for example.

Also, you don't need a better term than "provider" as that's what they're called. :-) The IQueryable interface includes a property called Provider that returns an IQueryProvider which can operate on an expression tree.

> LINQ only creates an expression tree when the type of the source object is IQueryable

To be further pedantic: it creates an expression when the method accepts an Expression<T>, which IQueryable happens to do.

Also, in my experience knowing when you will be working with an IQueryable and knowing when you will be working with a realized IEnumerable is a very important thing to be learned when working with Linq and datastores. You can easily load full tables into memory if you aren't careful.

This times 100. Ended up having a week of intermittent outages when a Dev on my team created a single helper method which took in an Func<> not an Expression<Func<>>. Having the same interface for both is imo dangerous

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