.NET Core Interview Questions

🧠 C# & .NET Interview Q&A (Core Concepts)

A quick and structured guide covering commonly asked .NET, C#, LINQ, and SQL interview questions.

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1️⃣ Difference Between ref and out

• ref → Variable must be initialized before passing.

• out → Variable does not need initialization before passing.

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2️⃣ Difference Between Abstract Class and Interface

🔹 Abstract Class

• abstract keyword is used.

• Cannot be instantiated.

• Can have both abstract and non-abstract methods.

🔹 Interface

• interface keyword is used.

• Contains only abstract methods.

• Supports multiple inheritance.

🔹 Abstract Method

• Method without implementation (only signature).

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3️⃣ Types of Access Modifiers

• public → Accessible from anywhere.

• private → Accessible only within the class.

• protected → Accessible in derived classes.

• internal → Accessible within the same assembly.

• private protected → Accessible within same class or derived class in same assembly.

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4️⃣ What is Singleton Pattern?

• Ensures that only one instance of a class is created.

• Commonly used for logging, caching, configuration.

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5️⃣ What is using Keyword?

• Used to import namespaces/libraries.

• Also used to define a scope for automatic memory/resource disposal.

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6️⃣ Concept of Dependency Injection (DI)

• A design pattern based on IoC (Inversion of Control).

• Dependencies are injected from external sources instead of creating inside class.

🔹 Lifetimes in .NET Core:

• AddTransient → New instance every time.

• AddScoped → One instance per request.

• AddSingleton → Single instance throughout application lifecycle.

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7️⃣ What is LINQ?

• Language Integrated Query

• Used to query:

  • Databases (via Entity Framework)

  • In-memory collections

• Provides built-in methods for filtering, sorting, grouping.

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8️⃣ Difference Between String and StringBuilder

• String → Immutable (cannot change after creation)

• StringBuilder → Mutable (can modify without creating new object)

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9️⃣ Difference Between async and await

• async → Marks a method as asynchronous.

• await → Waits for task completion without blocking thread.

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🔟 IEnumerable vs IQueryable

• IEnumerable → Works with in-memory collections (List, Array).

• IQueryable → Works with external data sources (Database, EF).

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1️⃣1️⃣ Exception Handling in ASP.NET Core

• Global Exception Handling → UseExceptionHandler()

• try-catch → Local handling

• ILogger → Logging errors and messages

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1️⃣2️⃣ What is Entity Framework?

• ORM (Object Relational Mapper)

• Allows interaction with DB using C# objects instead of SQL

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1️⃣3️⃣ JOIN vs INNER JOIN in SQL

• No difference → Both are interchangeable.

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1️⃣4️⃣ Caching in ASP.NET

• IMemoryCache → In-memory caching

• IDistributedCache → External caching (e.g., Redis)

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1️⃣5️⃣ Security in .NET Web API

• JWT (JSON Web Token) → Authentication

• OAuth → Third-party authentication

• Role-Based Authorization

• Claims-Based Authorization

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🧠 Key Takeaways

• Focus on core fundamentals + practical usage.

• Understand real-world application of concepts like DI, caching, security.

• These topics are highly relevant for .NET interviews (L1–L3 levels).

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This acts as a quick revision sheet for interviews and daily development reference.