IEnumerable<T> vs IQueryable<T>

The key difference between IQueryable<T> and IEnumerable<T> is how queries are executed.

IQueryable<T>:

• Supports LINQ-to-SQL (or LINQ-to-any other remote data source), meaning queries are translated to SQL and executed on the database.
• For example, when filtering IQueryable<Customer> with Where(c => c.IsGold), the filtering happens at the database level, and only gold customers are selected.

When to use:

• When working with remote data sources (e.g., databases) or any provider that supports IQueryable (e.g., Entity Framework).
• When you want to build dynamic, composable queries and let the underlying provider execute them efficiently.

IEnumerable<T>:

• Represents LINQ-to-objects, meaning the entire dataset is first loaded from the database into memory. Then, any further operations (e.g., filtering) are performed in-memory.
• For example, with IEnumerable<Customer>, all customers are first retrieved from the database, and only after loading the data, filtering (like Where(c => c.IsGold)) happens in-memory.

When to use:

• When the data is already in-memory and you are working with collections such as lists or arrays.
• When you don't need to execute the query on a remote server and want to process data locally in-memory.