IEnumerable vs IQueryable in C#: Explained with Examples (Beginner’s Guide)

When working with data in modern .NET applications, developers often face a common question:

Should I use IEnumerable or IQueryable?

Both are powerful interfaces that let you query and manipulate data, but they work very differently behind the scenes.
Choosing the right one can make a big difference in your app’s performance, memory usage, and scalability.

In this guide, we’ll break everything down step by step — in simple terms — so even if you’re just starting with .NET, you’ll understand exactly how and when to use each.

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📘 What is IEnumerable?

IEnumerable is an interface that represents a sequence of objects that can be iterated (looped) over.
It belongs to the System.Collections namespace.

You can think of it as a tool to work with data already loaded into memory — such as a list or an array.

🔹 Key Points

• Works in-memory (after the data is loaded).
• Supports deferred execution for methods like Where, Select, etc.
• Cannot translate queries to SQL when used with Entity Framework.
• Perfect for small, in-memory collections like List<T> or Array.

💻 Example: Using IEnumerable

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6 };

        IEnumerable<int> evenNumbers = numbers.Where(n => n % 2 == 0);

        foreach (var num in evenNumbers)
        {
            Console.WriteLine(num);
        }
    }
}

🟢 Here, all data (numbers) is already in memory.
The filtering (n % 2 == 0) happens in-memory, not in a database.

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🌐 What is IQueryable?

IQueryable extends IEnumerable and is designed for querying remote data sources, such as databases.
It’s part of the System.Linq namespace.

In short — it lets you write LINQ queries that get translated into SQL and executed on the database server instead of in your app.

🔹 Key Points

• Supports deferred execution.
• Can translate queries into SQL (works great with Entity Framework).
• Executes filters, sorting, and joins on the database, reducing memory usage.
• Ideal for large datasets and remote queries.

💻 Example: Using IQueryable

using System;
using System.Linq;
using Microsoft.EntityFrameworkCore;

class Program
{
    static void Main()
    {
        using var context = new AppDbContext();

        IQueryable<User> adultUsers = context.Users
                                             .Where(u => u.Age >= 18);

        foreach (var user in adultUsers)
        {
            Console.WriteLine($"{user.Name} - {user.Age}");
        }
    }
}

🟢 In this example, the WHERE condition runs directly in the database.
Only the filtered records are fetched — saving time and memory.

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⚖️ IEnumerable vs IQueryable — Key Differences

Feature	IEnumerable	IQueryable
Namespace	System.Collections	System.Linq
Execution Location	In-memory	Database (remote)
Query Translation	No SQL translation	Translates to SQL
Deferred Execution	Yes	Yes
Best For	Small in-memory collections	Large datasets or databases
Performance	Slower for large data	Faster due to server-side filtering

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💡 When Should You Use Which?

✅ Use IEnumerable when:

• You’re working with in-memory data (like lists, arrays, or results already fetched).
• You want simple iterations and lightweight operations.

✅ Use IQueryable when:

• You’re working with databases or remote data sources.
• You want filtering, sorting, or paging to happen on the server for better performance.

⚠️ Pro Tip:
Avoid calling .ToList() or .ToArray() too early on an IQueryable object.
Doing so forces it to load all the data into memory — killing the performance benefits!

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🛠️ Real-World Example

Imagine you have a database with millions of users, and you want users older than 30.

❌ Using IEnumerable

var users = context.Users.ToList(); // Loads all users in memory
var result = users.Where(u => u.Age > 30);

👉 This loads every record into memory first — which is slow and wasteful.

✅ Using IQueryable

var result = context.Users.Where(u => u.Age > 30);

👉 This generates a SQL query like:

SELECT * FROM Users WHERE Age > 30

and only retrieves the needed users — much faster and more efficient.

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🧾 Summary

Feature	IEnumerable	IQueryable
Execution	In-memory	Server-side (Database)
Use Case	Small datasets	Large datasets
Performance	Slower	Faster
SQL Translation	❌ No	✅ Yes

In short:

• IEnumerable → Great for simple, local collections.
• IQueryable → Perfect for database or large dataset queries.

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🚀 Final Thoughts

Understanding the difference between IEnumerable and IQueryable is key to writing efficient and scalable .NET applications.

Next time you write a LINQ query — pause and think:

Am I working with data that’s already in memory or still in the database?

That single choice can make a huge performance difference.

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Thanks for reading! 🙌
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