IQueryable vs IEnumerable in C# – Real Difference with Examples

Introduction

Developers frequently encounter the question “IQueryable vs IEnumerable” in C# and .NET interviews.

“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 the application loads the data.
• Works well for small collections

Example

IEnumerable<Employee> employees = db.Employees.ToList();

var result = employees.Where(e => e.Salary > 50000);

What happens internally?

1. The query fetches data from the database.
2. Stored in memory
3. Filtering (Where) is applied in application memory

Note: If the table has 1 lakh records, it loads all records into memory first.

---

What is IQueryable?

IQueryable queries 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?

1. Query is built
2. SQL is generated
3. Database returns filtered records only

Key Differences Between IQueryable vs IEnumerable

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()

However, once you convert it, you lose database-level optimization.

---

One-Line Interview Answer (IQueryable vs IEnumerable)

IQueryable executes queries at database level and generates SQL, whereas IEnumerable executes queries in memory after data is loaded.

---

Conclusion

IQueryable vs IEnumerable is not just about syntax —
it’s about performance, scalability, and clean architecture.