1. What an ORM is, and EF6 vs EF Core

An ORM (object-relational mapper) maps classes to tables and objects to rows, generating SQL so you work with C# objects and LINQ instead of DataReader loops. Entity Framework is Microsoft's ORM, in two generations:

EF6 Database First with EDMX (legacy apps): Add → ADO.NET Entity Data Model → "EF Designer from database" → pick tables/procedures → Visual Studio generates ShopModel.edmx (diagram) + T4 templates that generate entity classes and a ShopEntities : DbContext. Changing the database means "Update Model from Database" in the designer. The connection string contains metadata (metadata=res://*/ShopModel.csdl|...;provider=System.Data.SqlClient;provider connection string="...").

// EF6 (System.Data.Entity) — .NET Framework MVC 5 / Web Forms
using (var db = new ShopEntities())
{
    var orders = db.Orders.Include(o => o.Customer).Where(o => o.Total > 1000).ToList();   // Include(lambda) needs System.Data.Entity
    db.Orders.Add(new Order { CustomerId = 42, Total = 4500 });
    db.SaveChanges();
}

// EF Core (Microsoft.EntityFrameworkCore) — same idea, modern API
await using var db = new ShopDbContext(options);
var orders = await db.Orders.Include(o => o.Customer).Where(o => o.Total > 1000).AsNoTracking().ToListAsync();

Migrating EF6 → EF Core: move EDMX models to code-first classes (scaffold from the database), replace System.Data.Entity namespaces, review lazy-loading assumptions (add explicit Include), re-test queries that EF Core translates differently (client evaluation is no longer silent), replace EF6-only features (EDMX stored-procedure mapping, entity splitting differences), and adopt EF Core migrations from a baseline.

Interview tip: "Code First vs Database First": Code First — classes are the source of truth and migrations create/alter the database; Database First — the existing database is the source of truth and the model is generated from it (EDMX in EF6, scaffolding in EF Core).