IEnumrable and IQueryable in c#

Introduction

If you are learning C#, LINQ, or Entity Framework Core, you will often see two interfaces:

• IEnumerable<T>

• IQueryable<T>

Both are used with LINQ, but they work differently.

The main difference is where the query runs.

What is IEnumerable in C#?

IEnumerable<T> is mainly used when the data is already available in memory.

For example :

List<int> numbers = new()
{
    10, 20, 30, 40, 50
};

IEnumerable<int> result = numbers
    .Where(x => x > 20);

Here, the numbers are already in memory, so the Where() condition runs inside the application.

Simple flow :

Data → Memory → LINQ → Result

IEnumerable is commonly used with:

• List<T>

• Arrays

• Collections

• In-memory data

What is IQueryable in C#?

IQueryable<T> is mainly used for querying external data sources, especially databases.

For example, with Entity Framework Core:

IQueryable<Product> products = dbContext.Products;

var result = await products
    .Where(x => x.Price > 1000)
    .ToListAsync();

Entity Framework Core can convert this LINQ query into SQL.

It may generate SQL similar to:

SELECT * FROM Products WHERE Price > 1000;

The filtering happens in the database instead of loading all products into memory.

Simple flow :

LINQ Query → SQL → Database → Result

IEnumerable vs IQueryable

Feature	IEnumerable	IQueryable
Works mainly with	In-memory data	Database data
Query runs	In application	Usually at database
SQL translation	No	Yes, through provider
Best for	Small/in-memory collections	Database queries
Common with	List, Array	Entity Framework Core

A Database Example

Suppose we have a Products table containing 1 million records.

We want products where:

Price > 1000

Using IQueryable

With Entity Framework Core:

IQueryable<Product> products = dbContext.Products;

var result = await products
    .Where(x => x.Price > 1000)
    .ToListAsync()

Conceptually, the process is:

Application
    ↓
LINQ Where(x => x.Price > 1000)
    ↓
Entity Framework Core
    ↓
SQL
    ↓
Database
    ↓
Only matching records
    ↓
Application

The filtering is performed by the database.

What Happens When You Switch to IEnumerable?

IQueryable<Product> products = dbContext.Products;

IEnumerable<Product> result = products
    .AsEnumerable()
    .Where(x => x.Price > 1000);

AsEnumerable() changes the LINQ operations that follow it to LINQ-to-Objects.

So the Where() after AsEnumerable() is evaluated in the application.

Conceptually:

Database
    ↓
Records are read
    ↓
Application memory
    ↓
Where(x => x.Price > 1000)
    ↓
Filtered result

This can be slow if the database has a lot of records.

If the database contains a large number of records, this can result in unnecessary data transfer and increased memory usage.

Conclusion

IEnumerable<T> is mainly used for working with data in application memory, while IQueryable<T> is used to build queries that can be executed by external data sources such as databases. Understanding where the query is executed helps you write more efficient and performant LINQ and Entity Framework Core code.

Hope this helps you!