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C# Interview Questions for Experienced Developers (3+ Years)

This document compiles common interview questions and topics for C# developers with 3+ years of experience. It focuses on mid-level to senior topics including advanced OOP, LINQ, async programming, Entity Framework, ASP.NET Core, design patterns, SOLID principles, dependency injection, unit testing, performance optimization, and security.

Advanced OOP Concepts

• What are partial classes in C#?
  Partial classes allow splitting the definition of a class across multiple files. They are useful for separating auto-generated code from user code.

• Difference between String and StringBuilder.
  String is immutable; each modification creates a new string. StringBuilder is mutable and efficient for multiple concatenations.

• Const vs Readonly.
  Const values are compile-time constants. Readonly values can be set at runtime (in constructor) and are instance-specific.

• What is Reflection in C#?
  Reflection allows inspecting and manipulating types, methods, and assemblies at runtime using System.Reflection namespace.

• Method Overloading vs Overriding.
  Overloading: Same method name, different parameters. Overriding: Subclass provides specific implementation of inherited method.

• Equality Operator (==) vs Equals() Method.
  == compares references for reference types, values for value types. Equals() compares values.

• What are Indexers in C#?
  Indexers allow objects to be indexed like arrays using this[] syntax.

• Late Binding vs Early Binding.
  Early binding: Method calls resolved at compile-time. Late binding: Resolved at runtime, using dynamic or reflection.

• Properties in C#.
  Properties provide controlled access to private fields using get/set accessors.

• Boxing and Unboxing.
  Boxing: Converting value type to object (reference type). Unboxing: Converting object back to value type.

LINQ (Language Integrated Query)

• What is LINQ?
  LINQ allows querying data from various sources (collections, databases, XML) using C# syntax.

• Deferred Execution in LINQ.
  Queries are not executed until enumerated, allowing optimization.

• Difference between IEnumerable and IQueryable.
  IEnumerable for in-memory collections. IQueryable for database queries, supports expression trees.

• Common LINQ Operators.
  Where, Select, GroupBy, Join, OrderBy, etc.

• LINQ to SQL vs LINQ to Entities.
  LINQ to SQL for SQL Server. LINQ to Entities for Entity Framework.

Async Programming

• What is async/await?
  Async methods run asynchronously without blocking the calling thread. Await suspends execution until the task completes.

• Difference between Task and Task.
  Task for void-returning operations. Task for operations returning a value.

• ConfigureAwait(false).
  Prevents capturing the synchronization context, improving performance in library code.

• Handling Exceptions in Async Code.
  Use try/catch around await or AggregateException for Task.Wait().

• Deadlocks in Async Code.
  Occur when blocking on async code in UI threads. Avoid .Wait() or .Result on UI threads.

Entity Framework

• What is Entity Framework?
  ORM for .NET, maps objects to database tables.

• Code First vs Database First.
  Code First: Define model in code, generate DB. Database First: Generate model from existing DB.

• Lazy Loading vs Eager Loading.
  Lazy: Load related data on access. Eager: Load with main query using Include().

• Migrations in EF.
  Track and apply schema changes to DB.

• DbContext and DbSet.
  DbContext manages DB connection. DbSet represents entity collections.

• Change Tracking.
  EF tracks changes to entities for automatic updates.

ASP.NET Core

• What is ASP.NET Core?
  Cross-platform, open-source framework for building web apps.

• Middleware in ASP.NET Core.
  Components that handle requests/responses in the pipeline.

• Dependency Injection in ASP.NET Core.
  Built-in DI container for managing services.

• Razor Pages vs MVC.
  Razor Pages: Page-focused. MVC: Controller-based.

• Authentication vs Authorization.
  Authentication: Verify identity. Authorization: Check permissions.

• Kestrel vs IIS.
  Kestrel: Cross-platform web server. IIS: Windows-specific.

Design Patterns

• Singleton Pattern.
  Ensures single instance of a class.

• Factory Pattern.
  Creates objects without specifying exact classes.

• Observer Pattern.
  Notifies multiple objects of state changes.

• Repository Pattern.
  Abstracts data access logic.

• Dependency Injection Pattern.
  Injects dependencies rather than creating them.

SOLID Principles

• Single Responsibility Principle (SRP).
  A class should have one reason to change.

• Open/Closed Principle (OCP).
  Open for extension, closed for modification.

• Liskov Substitution Principle (LSP).
  Subtypes should be substitutable for base types.

• Interface Segregation Principle (ISP).
  Clients should not depend on unused interfaces.

• Dependency Inversion Principle (DIP).
  Depend on abstractions, not concretions.

Dependency Injection

• What is DI?
  Technique for achieving loose coupling by injecting dependencies.

• Constructor Injection vs Property Injection.
  Constructor: Required dependencies. Property: Optional.

• DI Containers.
  Frameworks like Autofac, Ninject for managing DI.

• Service Lifetimes.
  Transient, Scoped, Singleton.

Unit Testing

• What is Unit Testing?
  Testing individual units of code in isolation.

• Mocking Frameworks.
  Moq, NSubstitute for creating test doubles.

• Arrange-Act-Assert.
  Pattern for structuring tests.

• Test-Driven Development (TDD).
  Write tests before code.

• Code Coverage.
  Measure of code tested by unit tests.

Performance Optimization

• String Interning.
  Reuse string instances to save memory.

• Object Pooling.
  Reuse expensive objects.

• Asynchronous Programming.
  Avoid blocking threads.

• Memory Leaks.
  Unmanaged resources, event handlers.

• Profiling Tools.
  Visual Studio Profiler, dotTrace.

Security

• SQL Injection Prevention.
  Use parameterized queries.

• Cross-Site Scripting (XSS).
  Encode output, validate input.

• Authentication Methods.
  JWT, OAuth, Windows Auth.

• Encryption.
  Use HTTPS, encrypt sensitive data.

• Input Validation.
  Validate and sanitize user input.

This list is comprehensive but concise. Use it for preparation and practice explaining concepts with examples.