Returning IEnumerable vs. IQueryable

Question

What is the difference between returning IQueryable vs. IEnumerable, when should one be preferred over the other?

Answer 1

The top answer is good but it doesn't mention expression trees which explain "how" the two interfaces differ. Basically, there are two identical sets of LINQ extensions. Where(), Sum(), Count(), FirstOrDefault(), etc all have two versions: one that accepts functions and one that accepts expressions.

• The IEnumerable version signature is: Where(Func predicate)

• The IQueryable version signature is: Where(Expression> predicate)

You've probably been using both of those without realizing it because both are called using identical syntax:

e.g. Where(x => x.City == "") works on both IEnumerable and IQueryable

• When using Where() on an IEnumerable collection, the compiler passes a compiled function to Where()

• When using Where() on an IQueryable collection, the compiler passes an expression tree to Where(). An expression tree is like the reflection system but for code. The compiler converts your code into a data structure that describes what your code does in a format that's easily digestible.

Why bother with this expression tree thing? I just want Where() to filter my data. The main reason is that both the EF and Linq2SQL ORMs can convert expression trees directly into SQL where your code will execute much faster.

Oh, that sounds like a free performance boost, should I use AsQueryable() all over the place in that case? No, IQueryable is only useful if the underlying data provider can do something with it. Converting something like a regular List to IQueryable will not give you any benefit.