IQueryable vs IEnumerable in C# — The Practical, No‑Nonsense Guide

IQueryable vs IEnumerable in C#

If you’ve ever worked with LINQ, Entity Framework, or any data-heavy .NET application, you’ve definitely used IEnumerable and IQueryable. They look similar, they both support LINQ, and they both return collections — so what’s the real difference?

This guide breaks it down with definitions, execution behavior, EF Core examples, performance insights, and real-world scenarios.

What Is IEnumerable?

IEnumerable represents an in-memory collection.

It is used for LINQ to Objects.

Key characteristics

• Works in memory
• Executes on the client side
• Loads all data first, then filters
• Best for collections already in memory
• Supports deferred execution

Example

IEnumerable<int> numbers = new List<int> { 1, 2, 3, 4 };

var result = numbers.Where(n => n > 2);
// Filtering happens in memory

What Is IQueryable?

IQueryable represents a queryable data source, usually a database.

It is used for LINQ to SQL / LINQ to Entities.

Key characteristics

• Works with remote data sources (SQL, NoSQL, APIs)
• Executes on the server side
• Translates LINQ into SQL
• Fetches only the required data
• Supports expression trees

Example

IQueryable<User> users = db.Users;

var result = users.Where(u => u.IsActive);
// Filtering happens in SQL

Execution Difference (The Most Important Part)

IEnumerable

• Pulls all rows into memory
• Applies filters after loading
• Can cause massive performance issues with large datasets

IQueryable

• Converts your LINQ into SQL
• Database does the filtering
• Only returns the needed rows
• Much more efficient for large datasets

Side‑by‑Side Comparison

EF Core Example (Critical Difference)

Using IEnumerable (Bad)

var users = db.Users.ToList(); // Loads ALL users

var active = users.Where(u => u.IsActive); 
// Filtering happens in memory

Using IQueryable (Good)

var active = db.Users.Where(u => u.IsActive);
// Translates to: SELECT * FROM Users WHERE IsActive = 1

Real‑World Scenarios

Scenario 1: Fetching users from a database

Use IQueryable.

var admins = db.Users.Where(u => u.Role == "Admin");

Scenario 2: Filtering a list already in memory

Use IEnumerable.

var filtered = myList.Where(x => x.IsValid);

Scenario 3: Paging (Skip/Take)

Use IQueryable — SQL handles paging efficiently.

var page = db.Orders
             .OrderBy(o => o.Id)
             .Skip(20)
             .Take(10);

Scenario 4: Complex business logic

Use IEnumerable when logic cannot be translated to SQL.

var result = db.Users
               .AsEnumerable() // Switch to IEnumerable
               .Where(u => CustomValidation(u));

Interview‑Ready Summary

• Use IQueryable for database queries
• Use IEnumerable for in-memory operations
• IQueryable = SQL translation + server-side filtering
• IEnumerable = client-side filtering
• IQueryable is more efficient for large datasets