What is the use of
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 a remote database, with many ORM systems, you have the option of fetching data from a table in two ways, one which returns
IEnumerable<T>, and one which returns an
IQueryable<T>. Say, for example, you have a Products table, and you want to get all of the products whose cost is >$25.
If you do:
IEnumerable<Product> products = myORM.GetProducts(); var productsOver25 = products.Where(p => p.Cost >= 25.00);
What happens here, is the database loads all of the products, and passes them across the wire to your program. Your program then filters the data. In essence, the database does a
SELECT * FROM Products, and returns EVERY product to you.
With the right
IQueryable<T> provider, on the other hand, you can do:
IQueryable<Product> products = myORM.GetQueryableProducts(); var productsOver25 = products.Where(p => p.Cost >= 25.00);
The code looks the same, but the difference here is that the SQL executed will be
SELECT * FROM Products WHERE Cost >= 25.
From your POV as a developer, this looks the same. However, from a performance standpoint, you may only return 2 records across the network instead of 20,000....