C# Exercises

A multi-module .NET 10 solution for drilling C# concepts that come up in senior engineering interviews and production codebases — not just syntax, but the behavior that trips people up. Four independent projects, each focused on a distinct area.

Module 1: LeetCode Problems

Five algorithmic problems in C# 12, emphasizing patterns idiomatic to the language:

• LRU Cache (#146) — LinkedList<(int key, int value)> + Dictionary for O(1) get/put/evict using C# primary constructors and tuple syntax.
• Print FooBar Alternately (#1115) / Building H2O (#1117) — SemaphoreSlim-based thread synchronization and Interlocked.Increment for lock-free counters.
• Meeting Rooms (#252) — Sorted interval scan using C# 12 collection expressions.

Module 2: DI Lifetimes

An ASP.NET Core Minimal API that makes DI lifetime behavior observable rather than theoretical. A single Marker class registered three times, singleton, scoped, transient, exposes six GUIDs via GET /data. You can watch in real time that singleton IDs are stable, scoped IDs match within a request but differ across requests, and transient IDs are always unique.

The module also reproduces the captive dependency bug: a singleton injecting a scoped dependency. ValidateScopes = true and ValidateOnBuild = true are force-enabled, surfacing the startup exception immediately rather than silently corrupting state in production.

Module 3: EF Core Internals

A console app running against a 500-row SQLite database with SQL logging enabled, making invisible EF Core behavior visible:

• Tracking vs. AsNoTracking — Mutate a tracked entity, observe EntityEntry.State shift to Modified, call SaveChangesAsync, watch it return to Unchanged. Mutating a no-tracking entity: SaveChanges silently does nothing.
• N+1 problem — Load 50 orders without Include, access order.OrderLines in a loop: 51 queries. Reload with .Include(o => o.OrderLines): 1 query with LEFT JOIN.
• AsSplitQuery — Two Include chains cause cartesian explosion. .AsSplitQuery() splits into separate SELECTs. ToQueryString() prints the actual SQL.

Module 4: Domain-Driven Design

A pure domain model with EF Core persistence implementing DDD patterns in production-quality C#:

• Aggregate root (Order) — Private constructor + static Create() factory. AddLine/RemoveLine only allowed in Draft state. Confirm() / Ship() / Cancel() enforce a state machine with InvalidOperationException on invalid transitions. Collections exposed as IReadOnlyCollection<>.
• Value objects (Money, ProductSnapshot) — Inherit from abstract ValueObject with component-based equality. Money supports arithmetic operators with currency-matching guards. ProductSnapshot is a frozen copy at purchase time, acting as an anti-corruption layer across bounded contexts.
• Domain events — IDomainEvent interface wired to the aggregate root.

Tooling

CI runs dotnet build -warnaserror, dotnet format --verify-no-changes, and dotnet test on every push. Husky pre-commit hooks enforce formatting and zero compiler warnings before any commit reaches the remote.