If You Can’t Answer These 40 C# Questions, You’re Not Ready for 2025 Interviews
5/24/2025
Whether you're aiming for a junior .NET role or a senior software engineer position, one thing is clear: C# interviews in 2025 are tougher, deeper, and more practical than ever. The industry expects you to not only understand the syntax but also demonstrate architectural thinking, performance awareness, and fluency in modern .NET practices.
Below are 40 C# interview questions—each with a concise, polished answer to help you solidify your understanding and walk into interviews confidently.
🔹 Fundamentals & Language Features
What is the difference between ref, out, and in parameters?
- ref passes a variable by reference and must be initialized before passing. out also passes by reference but must be assigned inside the method. in is read-only and guarantees the method will not modify the value.
What is boxing and unboxing in C#?
- Boxing is converting a value type to an object type. Unboxing extracts the value type from the object. It involves performance costs due to heap allocation.
What are nullable reference types and how do you enable them?
- Nullable reference types allow string? to express a possibly-null reference. Enable via #nullable enableor in the .csproj using <Nullable>enable</Nullable>.
What is the difference between == and .Equals()?
- == can be overloaded and may compare by reference or value. .Equals() is an instance method used to compare values and should be overridden for custom types.
How do value types differ from reference types?
- Value types store data directly and are allocated on the stack. Reference types store references to objects on the heap.
What is a struct in C# and when should you use it?
- A struct is a value type ideal for small, immutable data objects. Use when performance matters and you don’t need inheritance.
What is the dynamic keyword and when is it appropriate to use?
- dynamic bypasses compile-time type checking. Useful when working with COM objects, dynamic JSON, or reflection-heavy scenarios.
How does C# handle memory management?
- C# uses automatic memory management via the .NET garbage collector, which reclaims memory used by unreachable objects.
Explain the difference between const, readonly, and static.
- const values are compile-time constants. readonly values can be assigned at runtime (in constructors). static means the member belongs to the type, not an instance.
What are extension methods? How are they useful?
- Extension methods add functionality to existing types without modifying them. They’re declared in static classes with this in the first parameter.
🔸 OOP & Design Patterns
What is the SOLID principle and why is it important?
- SOLID stands for Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. These principles promote maintainable, scalable software.
What is dependency injection and how is it implemented in .NET?
- Dependency Injection (DI) is a pattern to decouple object creation. In .NET, it's typically configured in Program.csvia builder.Services.AddScoped<T>().
What is polymorphism? How does C# support it?
- Polymorphism allows objects to be treated as instances of their base type. C# supports it through method overriding and interfaces.
What’s the difference between abstraction and encapsulation?
- Abstraction hides implementation details. Encapsulation restricts access using access modifiers. Both improve code clarity and security.
When would you use the strategy pattern vs. the factory pattern?
- Use strategy to select algorithms at runtime. Use factory to delegate object creation.
What is the difference between interface and abstract class?
- Interfaces define contracts without implementation. Abstract classes can include base implementation and fields.
Explain the singleton pattern. How can it be implemented thread-safely?
- A singleton ensures a class has one instance. Use Lazy<T> or a static readonly field with a private constructor for thread-safe implementation.
What is the Repository Pattern and how does it improve testability?
- It abstracts data access, decoupling logic from persistence. Repositories make it easier to mock data access in unit tests.
🔧 Asynchronous Programming & Performance
What is the difference between Task, async, and await?
- Task represents an asynchronous operation. async marks methods that use await, which yields execution until the task completes.
What are ConfigureAwait(false) and its implications?
- It prevents resuming on the captured synchronization context. Useful in libraries to avoid deadlocks in UI apps.
What’s the difference between concurrency and parallelism in C#?
- Concurrency is managing multiple tasks at once. Parallelism runs them simultaneously. C# supports both via async/await and Parallel APIs.
How do you handle thread safety in .NET?
- Use locks (lock), Monitor, Mutex, ConcurrentDictionary, or immutable data. Avoid shared mutable state.
What are ValueTask and IAsyncEnumerable?
- ValueTask is a lightweight Task to reduce allocations. IAsyncEnumerable streams data asynchronously using await foreach.
How can you avoid deadlocks in async code?
- Avoid blocking calls like .Result or .Wait(). Use ConfigureAwait(false) and async all the way.
When should you use a SemaphoreSlim?
- Use it to limit concurrent access to a resource, especially in async code.
🔌 .NET Core / .NET 8 / Blazor
What’s new in .NET 8 and why should you upgrade?
- .NET 8 offers better performance, native AOT compilation, and updates to Blazor and Minimal APIs. It's the latest LTS release.
How does Blazor Server differ from Blazor WebAssembly?
- Blazor Server runs on the server using SignalR. Blazor WASM runs in the browser. Server has faster startup; WASM is more client-side.
What are Minimal APIs and when should you use them?
- Minimal APIs provide a lightweight syntax to define HTTP endpoints. Ideal for microservices or small APIs.
How does dependency injection work in .NET Core?
- Register services in IServiceCollection. The runtime injects them via constructor injection at runtime.
What is Middleware in ASP.NET Core?
- Middleware are components in the request pipeline. Examples: authentication, logging, CORS. Order matters.
How do you manage configuration and secrets in .NET apps?
- Use appsettings.json, environment variables, and UserSecrets for local dev. Azure Key Vault in production.
What are the lifecycle methods in a Blazor component?
- OnInitialized, OnInitializedAsync, OnParametersSet, OnAfterRender, Dispose. They control setup and cleanup.
🗄️ Data Access & EF Core
What’s the difference between EF Core and Dapper?
- EF Core is a full ORM with LINQ, change tracking. Dapper is a lightweight micro-ORM—faster, less abstraction.
How do you configure relationships and navigation properties in EF Core?
- Use Fluent API or data annotations. Define foreign keys, navigation properties, and HasMany, WithOne, etc.
How does EF Core handle change tracking?
- It tracks changes to entities in the DbContext. You can inspect or disable it for performance using AsNoTracking().
What’s the difference between FirstOrDefault, SingleOrDefault, and Find?
- FirstOrDefault: gets first match or null. SingleOrDefault: expects one match, throws if multiple. Find: fetches by primary key.
How do migrations work in EF Core?
- Use Add-Migration and Update-Database to create and apply schema changes. They generate C# code files.
How do you optimize EF Core queries for performance?
- Use .AsNoTracking(), projections (Select), explicit loading, and avoid N+1 queries with Include().
🧠 Advanced & Theoretical
What are expression trees and when would you use them?
- Expression trees represent code as data. Used in LINQ providers, query building, and dynamic expressions.
Explain how reflection works in C# and when to use it responsibly.
- Reflection allows inspecting and interacting with types at runtime. Use for dynamic loading or serialization. It’s powerful but can hurt performance and type safety.
✅ Final Thoughts
If these questions challenged you—good. These are not just interview preps; they're a roadmap for becoming a well-rounded .NET developer in 2025. Master these areas and you'll walk into interviews with confidence, whether it's a startup or an enterprise-level SaaS company.
Keep studying, keep building, and stay sharp.
You've got this.