What is EF Core?

The ORM that lets you query SQL with C#. No SQL scripts. 4 mins.

Detailed Guide

Your First "Aha" Moment: You already wrote SQL. EF Core just lets you write it in C#.

1. The Problem EF Core Solves

Without ORM:

var cmd = new SqlCommand("SELECT * FROM Users WHERE Age > @age", conn);
cmd.Parameters.AddWithValue("@age", 18);
var reader = cmd.ExecuteReader(); // 10 lines to map to List

With EF Core:

var users = await _db.Users.Where(u => u.Age > 18).ToListAsync();

Same SQL runs on database. But you wrote C#. IntelliSense, compile-time check, no string SQL.

❌ ADO.NET/Dapper

You write SQL. You map results.
Rename column = app crashes at runtime.

✅ EF Core

You write LINQ. EF maps it.
Rename property = compile error. Safe.

2. 3 Core Pieces You Must Know

EF Core Concept	Real World Meaning
`DbContext`	Your database session. Unit of Work. Tracks all changes.
`DbSet<User>`	Your table. Query it with LINQ.
`SaveChanges()`	Commits all INSERT/UPDATE/DELETE in 1 batch.

3. Code First in 60 Seconds

Step 1: Install

dotnet add package Microsoft.EntityFrameworkCore.SqlServer

Step 2: Model

public class User {
    public int Id { get; set; }
    public string Name { get; set; }
}

Step 3: DbContext

public class AppDbContext : DbContext {
    public DbSet Users { get; set; }
    public AppDbContext(DbContextOptions opt) : base(opt) { }
}

Step 4: Migration = Creates Table

Add-Migration Init
Update-Database

Stop Here. Think. EF Core didn't remove SQL. It just moved it. You still need to know SQL to debug slow queries.
Next lesson: DbContext lifetime. Get it wrong = memory leak + ₹50k Azure bill.

Career-Killer Mistake: Using EF Core for reports with 20 joins.
EF Core LINQ → SQL is 95% good. For complex GroupBy + Window Functions, write raw SQL or use Dapper. Right tool for right job.

1. The Interview Theory: What, Why, How

What is EF Core? Lightweight, extensible, cross-platform ORM from Microsoft. Maps C# objects to relational databases. Translates LINQ to SQL.

Why does it exist? Productivity. Change Tracking. Migrations. LINQ safety. You ship CRUD 10x faster vs ADO.NET. Trade-off: 5-10% perf cost vs Dapper.

How does it work? 1. You query DbSet with LINQ. 2. EF Core expression tree → SQL. 3. SQL runs on DB. 4. Results materialized to C# objects. 5. ChangeTracker watches objects. 6. SaveChanges() generates SQL diff.

2. The #1 Interview Question: "EF Core vs Dapper?"

Your Answer: "EF Core = Full ORM. ChangeTracker, Migrations, LINQ, lazy loading. I use for 90% CRUD, admin panels, fast dev. Dapper = Micro-ORM. Maps SQL to objects only. No tracking. I use for read-heavy APIs, complex reports, 10k RPS. Example: Zomato uses EF Core for restaurant onboarding dashboard, Dapper for homepage restaurant search API."

This answer gets you hired. "EF Core is easier" gets you rejected.

3. Code First vs DB First: The CTO View

Decision	Code First	DB First
Source of Truth	C# classes	SQL Database
Command	`Add-Migration`	`Scaffold-DbContext`
Greenfield	Yes. Full control	No
Legacy DB	Painful	Perfect. 500 tables = 1 command
DBA Team	Hates you. They lose control	Loves you. They own schema

4. EF6 vs EF Core: Why Rewrite?

EF6 =.NET Framework, Windows only, EDMX designer, heavy. No DI.
EF Core =.NET Core, cross-platform, DI-first, no EDMX, batching on SaveChanges. EF Core 7+ is faster than EF6.
Migrate when: You need Linux/Docker, or new features like JSON columns, bulk ops. Stay EF6 when: 1000-table EDMX, zero budget.

5. ChangeTracker - The Hidden Engine

var user = await _db.Users.FindAsync(1); // State = Unchanged
user.Name = "New"; // State = Modified
_db.Users.Add(new User()); // State = Added
await _db.SaveChanges(); // Generates UPDATE + INSERT in 1 roundtrip

Theory: ChangeTracker is a graph of all entities. On SaveChanges, it diffs current vs original values. This is why EF Core can be slower than Dapper. It's doing work.

Career-Killer Mistake #2: _db.Users.ToList() on 1M rows.
Theory: ChangeTracker will track 1M objects. 1 object ~ 100 bytes overhead = 100MB RAM + slow SaveChanges.
Fix: Use .AsNoTracking() for read-only. _db.Users.AsNoTracking().ToList(). Memory drops 90%.

What's Next = Your First Production Bug: You know WHAT EF Core is. Next lesson: DbContext lifetime. Use AddScoped not AddSingleton. Get it wrong = Cannot access disposed object in prod at 3 AM. Click Next. Don't skip.

Quick Check 🧠

Ready for DbContext? You know WHAT EF Core is. Next: DbContext & DbSet teaches you DI lifetime, OnModelCreating, and the bug that crashes prod at 3 AM. Let's go →

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