What is the difference between IEnumerable, IQueryable, and List<T> in .NET, and when should you use each in real-world applications?

Understanding the differences between IEnumerable, IQueryable, and List is crucial for writing optimized, scalable, and maintainable .NET applications — especially in scenarios involving LINQ, Entity Framework, or large datasets.

🔹 IEnumerable:
Namespace: System.Collections.Generic

Represents a forward-only cursor for in-memory collection iteration

Evaluation is deferred, but data must be in memory

🟢 Use when:

You want to iterate over already-loaded data

You don’t need query translation to SQL or remote execution

🔧 Example:

List<int> numbers = new List<int> { 1, 2, 3 };
IEnumerable<int> even = numbers.Where(x => x % 2 == 0);

🔹 IQueryable:
Namespace: System.Linq

Supports remote LINQ query translation (e.g., to SQL)

Deferred execution — query runs only when enumerated

🟢 Use when:

You’re querying a database using Entity Framework / LINQ to SQL

You want to build efficient queries that are executed server-side

🔧 Example:

IQueryable<Employee> emps = dbContext.Employees.Where(e => e.Age > 30);

🔹 List:
Namespace: System.Collections.Generic

A concrete, resizable collection stored entirely in memory

Allows indexing, adding/removing, sorting, etc.

🟢 Use when:

You want full control over a data collection

You need to modify, access, or cache data in memory

You’ve materialized data from a query (like .ToList())

🔧 Example:

var employees = dbContext.Employees.ToList(); // Now it's a List<Employee>
employees.Add(new Employee { Name = "Niraj" });

Summary Table:

In Short:

Use IQueryable for querying, IEnumerable for in-memory logic, and List when you need hands-on data manipulation.