Introduction
One of the most common .NET concept is:
“What is the difference between IQueryable and IEnumerable?”
Even developers with many years of experience struggle to explain this clearly.
In this article, I’ll explain the difference in simple language, with real examples and an interview & clarity perspective.
What is IEnumerable?
IEnumerable is used to work with data in memory.
- It fetches all data first
- Filtering happens after data is loaded
- Works well for small collections
Example
IEnumerable<Employee> employees = db.Employees.ToList();
var result = employees.Where(e => e.Salary > 50000);What happens internally?
- Data is fetched from database
- Stored in memory
- Filtering (
Where) is applied in application memory
Note: If the table has 1 lakh records, all are loaded first.
What is IQueryable?
IQueryable is used to query data directly from the database.
- Filtering happens at database level
- Only required data is fetched
- Best for large datasets
Example
IQueryable<Employee> employees = db.Employees;
var result = employees.Where(e => e.Salary > 50000);
What happens internally?
- Query is built
- SQL is generated
- Database returns filtered records only
Key Difference (Friendly Table)
| Feature | IEnumerable | IQueryable |
|---|---|---|
| Execution | In memory | Database |
| Performance | Slower for large data | Faster |
| SQL Generation | ❌ No | ✅ Yes |
| Use case | Small collections | Large datasets |
| Deferred Execution | Partial | Full |
Deferred Execution Explained Simply
Both support deferred execution, but:
- IEnumerable: Deferred until
.ToList()but after data fetch - IQueryable: Deferred until execution, SQL created at runtime
Example
var query = db.Employees.Where(e => e.Age > 30);
// No DB call yet
var list = query.ToList();
// DB call happens here
When to Use What?
* Use IQueryable when:
- Data comes from database
- Large tables
- Performance matters
- Filtering & paging required
* Use IEnumerable when:
- Data already in memory
- Small collections
- Business logic operations
Common Interview Trap:
Question:
“Can we convert IQueryable to IEnumerable?”
Answer:
Yes, by calling:
.ToList() or .AsEnumerable()
But once converted, database optimization is lost.
One-Line Interview Answer (Golden)
IQueryable executes queries at database level and generates SQL, whereas IEnumerable executes queries in memory after data is loaded.
Conclusion
Understanding IQueryable vs IEnumerable is not just about syntax —
it’s about performance, scalability, and clean architecture.