How to distinguish between IEnumerable and IQueryable and in which cases each is used?

IEnumerable and IQueryable are interfaces for working with collections, but with different behaviors and purposes:

• IEnumerable<T> is used for iterating over collections in memory. Data queries are executed immediately, and all LINQ operations are processed on the client side (in memory).

• IQueryable<T> extends IEnumerable and allows building queries that can be translated into expressions, such as SQL queries. Query execution is deferred until iteration (lazy execution), and operations are performed on the data source side (e.g., database).

When to use:

• IEnumerable — if data is already loaded into memory or working with local collections.
• IQueryable — if you need to build a query to a remote source (database) and optimize the selection by performing filtering and sorting on the server.

Example:

IQueryable<User> usersQuery = dbContext.Users.Where(u => u.Age > 18);
// The database query will be formulated and executed when iterating over usersQuery

IEnumerable<User> usersList = usersQuery.ToList();
// Data is loaded into memory, and further operations will be performed locally