IEnumerable<T> represents a forward-only cursor of T. .NET 3.5 added extension methods that included the LINQ standard query operators like Where and First, with any operators that require predicates or anonymous functions taking Func<T>. IQueryable<T> implements the same LINQ standard query operators, but accepts Expression<Func<T>> for predicates and anonymous functions. Expression<T> is a compiled expression … Read more
The pros; composability: callers can add filters callers can add paging callers can add sorting etc The cons; non-testability: Your repository is no longer properly unit testable; you can’t rely on a: it working, b: what it does; the caller could add a non-translatable function (i.e. no TSQL mapping; breaks at runtime) the caller could … Read more
Marc Gravell’s answer is very complete, but I thought I’d add something about this from the user’s point of view, as well… The main difference, from a user’s perspective, is that, when you use IQueryable<T> (with a provider that supports things correctly), you can save a lot of resources. For example, if you’re working against … Read more
I think it is very very hard, if impossible, to mock Entity Framework behaviour. First and foremost because it would require profound knowledge of all peculiarities and edge cases where linq-to-entites differs from linq-to-objects. As you say: the real challenge is finding them. Let me point out three main areas without claiming to be even … Read more
First of all, IQueryable<T> extends the IEnumerable<T> interface, so anything you can do with a “plain” IEnumerable<T>, you can also do with an IQueryable<T>. IEnumerable<T> just has a GetEnumerator() method that returns an Enumerator<T> for which you can call its MoveNext() method to iterate through a sequence of T. What IQueryable<T> has that IEnumerable<T> doesn’t … Read more
Yes, both will give you deferred execution. The difference is that IQueryable<T> is the interface that allows LINQ-to-SQL (LINQ.-to-anything really) to work. So if you further refine your query on an IQueryable<T>, that query will be executed in the database, if possible. For the IEnumerable<T> case, it will be LINQ-to-object, meaning that all objects matching … Read more
Just stumbled into this oldie… To do this without the dynamic LINQ library, you just need the code as below. This covers most common scenarios including nested properties. To get it working with IEnumerable<T> you could add some wrapper methods that go via AsQueryable – but the code below is the core Expression logic needed. … Read more