EF - Entity Framework Core

 ✅ 1. What is Entity Framework Core? (Interview-ready definition)

EF Core is Microsoft’s Object-Relational Mapper (ORM) that lets you interact with a database using .NET classes instead of SQL queries.
It handles:

• Database Connection & Commands

• Mapping classes ↔ tables

• CRUD operations

• Migrations

• Change Tracking

• LINQ queries

Interview one-liner:

EF Core is an ORM that allows developers to work with databases using C# objects, eliminating most SQL and improving productivity while maintaining performance.

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✅ 2. EF Core Workflow (Very Important for Interviews)

Model (C# classes)
        ↓
DbContext (Mapping + Configuration)
        ↓
Database (Migrations)
        ↓
LINQ Queries (CRUD)

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✅ 3. Hands-On Practical Example (Minimal, Perfect for Learning)

Let’s create a simple Employee Management app with EF Core.

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🟦 Step 1: Install EF Core Packages

Run in terminal:

dotnet add package Microsoft.EntityFrameworkCore
dotnet add package Microsoft.EntityFrameworkCore.SqlServer
dotnet add package Microsoft.EntityFrameworkCore.Tools

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🟦 Step 2: Create Model Class

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Department { get; set; }
    public decimal Salary { get; set; }
}

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🟦 Step 3: Create DbContext

using Microsoft.EntityFrameworkCore;

public class AppDbContext : DbContext
{
    public DbSet<Employee> Employees { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
    {
        options.UseSqlServer("Server=.;Database=EFCoreDemo;Trusted_Connection=True;");
    }
}

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🟦 Step 4: Create Database Using Migrations

dotnet ef migrations add InitialCreate
dotnet ef database update

This creates your database + Employees table.

In interviews, they often ask:
What are migrations?
Answer:
Migrations help you evolve your database schema over time while keeping data safe.

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🟦 Step 5: Perform CRUD Operations

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▶ Insert Data

using var context = new AppDbContext();

var emp = new Employee
{
    Name = "John",
    Department = "IT",
    Salary = 50000
};

context.Employees.Add(emp);
context.SaveChanges();

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▶ Read Data (LINQ)

var employees = context.Employees.ToList();

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▶ Update

var emp = context.Employees.First();
emp.Salary = 60000;
context.SaveChanges();

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▶ Delete

var emp = context.Employees.First();
context.Employees.Remove(emp);
context.SaveChanges();

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✅ 4. Frequently Asked EF Core Interview Questions (with answers)

1. What is DbContext?

DbContext is the main class that manages database connections, CRUD operations, and mapping between classes and tables.

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2. What is DbSet?

DbSet represents a table. It lets you query and save instances of a model.

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3. What is Change Tracking?

EF Core automatically tracks object changes and updates only modified fields.

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4. What are Migrations?

Migrations manage schema changes without dropping the database.

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5. What is Lazy Loading, Eager Loading, Explicit Loading?

• Eager Loading: Includes related data immediately using .Include()

• Lazy Loading: Related data is loaded automatically when accessed

• Explicit Loading: Manually load related data using Entry(entity).Collection().Load()

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6. How to use LINQ with EF Core?

Example:

var itEmployees = context.Employees
                         .Where(e => e.Department == "IT")
                         .ToList();

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✅ 5. Relationship Example (Highly asked in interviews)

Example: One-to-Many

Employee → Projects
One employee can have many projects.

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Models:

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

    public List<Project> Projects { get; set; }
}

public class Project
{
    public int Id { get; set; }
    public string Title { get; set; }

    public int EmployeeId { get; set; }
    public Employee Employee { get; set; }
}

DbContext Mapping:

public DbSet<Employee> Employees { get; set; }
public DbSet<Project> Projects { get; set; }

Query with include:

var employees = context.Employees
                       .Include(e => e.Projects)
                       .ToList();