C#: What is the difference between IEnumerable and IQueryable in C#?

IEnumerable — in-memory collection. IQueryable — remote data source query.

IEnumerable:

• In-memory collection.
• LINQ to Objects.
• Executes locally.
• All data loaded first.

1IEnumerable<int> numbers = new List<int> {1, 2, 3, 4, 5};
2
3// Filtering happens in memory
4var result = numbers.Where(x => x > 2);
5// All data already in memory

IQueryable:

• Remote data source (database).
• LINQ to SQL/Entity Framework.
• Executes remotely.
• Only requested data loaded.

1IQueryable<User> users = dbContext.Users;
2
3// Filtering happens in database
4var result = users.Where(u => u.Age > 18);
5// SQL: SELECT * FROM Users WHERE Age > 18

Key differences:

• IEnumerable — in-memory, local execution.
• IQueryable — remote, deferred execution.
• IEnumerable — loads all data.
• IQueryable — filters at source.

Performance:

• IEnumerable — good for small collections.
• IQueryable — good for databases.