Guided course · C# Language

Module 1 · First Steps3 lessons

Module 2 · Functions and Everyday Data3 lessons

04Methods, Parameters and Small Responsibilities (included with Full Access)
05Arrays, Lists and Dictionaries: Choose the Right Collection (included with Full Access)
06Strings, Nullability and Validation Boundaries (included with Full Access)

Module 3 · Objects and Design3 lessons

07Classes, Properties and Valid State (included with Full Access)
08Values, References, Records and Equality (included with Full Access)
09Interfaces, Composition and Replaceable Boundaries (included with Full Access)

Module 4 · Reliable C# Programs3 lessons

Module 5 · Asynchronous Work and Verification3 lessons

Module 6 · Build and Defend a Complete Application3 lessons

16JSON Persistence and Safe Loading (included with Full Access)
17Capstone: Build a C# Study Planner (included with Full Access)
18Capstone Review: Edge Cases, Refactoring and Interview Defense (included with Full Access)

Module 7 · Intermediate: Object Contracts and Generic APIs3 lessons

19Inheritance and Polymorphism: Contracts You Can Substitute (included with Full Access)
20Generic Constraints and Variance: Designing Typed APIs (included with Full Access)
21Events and Delegate Lifetimes: Publish Changes Safely (included with Full Access)

Module 8 · Intermediate: Query and Data Pipelines3 lessons

22Iterators and Streaming: Yield, Enumeration and Ownership (included with Full Access)
23LINQ Grouping and Joins: Model Data Relationships (included with Full Access)
24Equality and Ordering: Reliable Keys and Deterministic Results (included with Full Access)

Module 9 · Intermediate: Robust Asynchronous Services3 lessons

25Concurrent Tasks: Results, Failures and Cancellation (included with Full Access)
26HTTP Boundaries: Reuse Clients, Validate Responses, Cancel Work (included with Full Access)
27Time and Reliability: TimeProvider, Timeouts and Repeatable Tests (included with Full Access)

Module 10 · Advanced: Concurrency and Backpressure3 lessons

28Shared State: Locks, Interlocked and Safe Invariants (included with Full Access)
29Channels: Bounded Pipelines and Completion (included with Full Access)
30Async Streams: IAsyncEnumerable and Cancellation (included with Full Access)

Module 11 · Advanced: Memory and Runtime Behavior3 lessons

31Allocation and GC: Measure Lifetimes Before Optimizing (included with Full Access)
32Span and Memory: Slices, Ownership and Async Boundaries (included with Full Access)
33Structs, Boxing and Ref Semantics: Copy Costs and Correctness (included with Full Access)

Module 12 · Advanced: Architecture and Delivery3 lessons

34Reflection, Attributes and Trimming-Aware Design (included with Full Access)
35Benchmarking and Diagnostics: Measure Claims Responsibly (included with Full Access)
36Advanced Capstone: Bounded Import Pipeline and Failure Recovery (included with Full Access)

Module 13 · Practice and Assessment1 lesson

Module 3 · Objects and Design · Lesson 9 of 37

Interfaces, Composition and Replaceable Boundaries

8 min readTechInterviewNotes Editorial TeamUpdated 13 Oct 2026Save

This lesson comes with Full Access

Its text, video and cheat sheet open with Full Access. This course's free lessons open without an account. Start with Your First C# Program: Build, Run and Understand It.

Practice test: Interfaces and Composition — Lesson Practice10 questions · 25 minutes · 1 mark each, no negative marking

Practice test10 questions · 25 minutes · 1 mark each, no negative marking. Take it