IEnumerable vs IQueryable — LINQ — Language Integrated Query

9. IEnumerable vs IQueryable

Intermediate★ Must knowLesson 9 of 12

	`IEnumerable<T>`	`IQueryable<T>`
Namespace	System.Collections.Generic	System.Linq
Lambdas compiled to	Delegates (`Func<T,bool>`)	Expression trees (`Expression<Func<T,bool>>`)
Execution	In memory (LINQ to Objects)	Translated by a provider (SQL) and run remotely
Filtering big tables	Loads everything, filters in RAM	`WHERE` runs in the database
Use	Collections already in memory	EF Core `DbSet`s, remote sources

csharp

IQueryable<Order> q = db.Orders.Where(o => o.Total > 1000);      // nothing sent yet
q = q.OrderBy(o => o.CreatedAt).Take(20);                         // still composing
Console.WriteLine(q.ToQueryString());                             // see the SQL (EF Core 5+)
var list = await q.ToListAsync();                                 // SELECT TOP(20) ... WHERE Total > 1000 ORDER BY ...

IEnumerable<Order> bad = db.Orders;                               // as IEnumerable
var slow = bad.Where(o => o.Total > 1000).ToList();               // SELECT * FROM Orders, then filter in memory!