What distinguishes the interfaces IEnumerable and IQueryable in usage and purpose?

IEnumerable and IQueryable are interfaces for working with collections in C#, but they differ in how queries are executed and their purpose:

• IEnumerable<T> is used for iterating over collections in memory. Data queries are executed on the client side, meaning all data is first loaded into memory and then filtered or processed. Suitable for working with local collections.

• IQueryable<T> extends IEnumerable and allows forming queries that can be translated into expressions (e.g., SQL queries) and executed on the data source side (e.g., databases). This enables optimizing data retrieval by loading only the necessary data.

Example:

IQueryable<User> usersQuery = dbContext.Users.Where(u => u.Age > 18); // Query to the database
List<User> usersList = usersQuery.ToList(); // Execute query and load data

IEnumerable<User> usersEnumerable = usersList.Where(u => u.Name.StartsWith("A")); // Filtering in memory

Thus, IQueryable is better suited for building queries to remote data sources, while IEnumerable is used for working with already loaded collections.