day2

Day 2 Demos — EF Core, DI, Validation & Error Handling

Runnable samples

This folder now contains two finished reference samples you can run and debug:

SQLite version

cd demos/day2/TechConf.Sqlite.Api
dotnet run

Useful URLs and files:

• API root: http://localhost:5100
• Scalar UI: http://localhost:5100/scalar/v1
• OpenAPI JSON: http://localhost:5100/openapi/v1.json
• HTTP requests: TechConf.Sqlite.Api/requests.http
• Migrations: TechConf.Sqlite.Api/Data/Migrations/

PostgreSQL version

If you're using the dev container, run the following commands inside the dev container terminal. The dev container already installs dotnet-ef and the latest stable Aspire.Cli.

Start PostgreSQL in Docker with the latest image:

docker run --rm -d \
  --name techconf-day2-postgres \
  -e POSTGRES_USER=postgres \
  -e POSTGRES_PASSWORD=techconf \
  -e POSTGRES_DB=techconfdb \
  -p 5432:5432 \
  postgres:latest

Connection details:

• Host: localhost
• Port: 5432
• Database: techconfdb
• Username: postgres
• Password: techconf

Then run the sample:

cd demos/day2/TechConf.Postgres.Api
dotnet run

Useful URLs and files:

• API root: http://localhost:5200
• Scalar UI: http://localhost:5200/scalar/v1
• OpenAPI JSON: http://localhost:5200/openapi/v1.json
• HTTP requests: TechConf.Postgres.Api/requests.http
• Migrations: TechConf.Postgres.Api/Data/Migrations/

Both samples automatically apply migrations and seed demo data on startup. If you want to stop the PostgreSQL container later, run:

docker stop techconf-day2-postgres

Demo 1: EF Core Setup

1. Add NuGet packages: Npgsql.EntityFrameworkCore.PostgreSQL, Microsoft.EntityFrameworkCore.Design
2. Create AppDbContext with DbSet<Event>
3. Show Fluent API configuration in IEntityTypeConfiguration<Event>
4. Register with AddDbContext and connection string
5. Create migration: dotnet ef migrations add InitialCreate
6. Apply migration: dotnet ef database update
7. Query with LINQ: db.Events.ToListAsync()

Key teaching moments:

• DbContext lifecycle (scoped by default)
• Fluent API vs Data Annotations
• Migration files and what they contain (dotnet tools)

Demo 2: Entity Relationships

1. Add Session and Speaker entities
2. Configure one-to-many (Event → Sessions)
3. Configure many-to-many (Sessions ↔ Speakers)
4. Show Include() and ThenInclude() for eager loading
5. Demonstrate N+1 problem with and without Include

Demo 3: Dependency Injection

1. Show lifetime differences: Transient vs Scoped vs Singleton
2. Create IEventRepository and EventRepository
3. Register with AddScoped
4. Inject into Minimal API endpoint parameters
5. Show what happens with wrong lifetime (Singleton DbContext!)

Demo 4: Validation

1. Show .NET 10 built-in validation: AddValidation() with [Required], [StringLength]
2. Create a FluentValidation validator with complex rules
3. Build a ValidationFilter<T> endpoint filter
4. Show validation error response format

Demo 5: Problem Details

1. Add AddProblemDetails(), UseExceptionHandler(), UseStatusCodePages()
2. Throw an unhandled exception — show automatic 500 ProblemDetails
3. Create NotFoundException custom exception
4. Build IExceptionHandler with pattern matching
5. Show the ProblemDetails JSON response