Top 50 C# Interview Questions and Answers

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C# interview Questions and Answers

Table of Contents

Beginner Level Questions

  1. What is C#?
    • Answer: C# is a modern, object-oriented programming language developed by Microsoft as part of its .NET initiative. It is widely used for developing Windows applications, web applications, and games.
  2. What are the features of C#?
    • Answer: Key features include object-oriented programming, type safety, interoperability, scalability, versioning support, and garbage collection.
  3. What is the difference between ref and out keywords in C#?
    • Answer:
      • ref: Requires the variable to be initialized before passing.
      • out: Does not require initialization before passing; must be assigned a value in the method.
  4. What are value types and reference types in C#?
    • Answer:
      • Value types: Store data directly (e.g., int, float).
      • Reference types: Store references to data (e.g., objects, strings).
  5. What is the difference between == and .Equals() in C#?
    • Answer:
      • == compares references for reference types and values for value types.
      • .Equals() checks for value equality and can be overridden for custom logic.
  6. What is a nullable type in C#?
    • Answer: A nullable type allows variables to hold null as a valid value. Syntax: int? num = null;.
  7. What is the purpose of the using statement?
    • Answer: It ensures the proper disposal of objects that consume unmanaged resources, like file handles.
  8. What is a delegate in C#?
    • Answer: A delegate is a type-safe pointer to a method.
  9. Explain the difference between an abstract class and an interface.
    • Answer:
      • Abstract class: Can have implementations and fields; supports inheritance.
      • Interface: Purely declarative; all members must be implemented.
  10. What is LINQ in C#?
    • Answer: LINQ (Language Integrated Query) is a feature for querying collections using a syntax similar to SQL.

Intermediate Level Questions

  1. What is the difference between IEnumerable and IQueryable?
    • Answer:
      • IEnumerable: Used for in-memory collection iteration.
      • IQueryable: Supports querying data from out-of-memory sources (like databases).
  2. What are access modifiers in C#?
    • Answer: Defines access levels:
      • public, private, protected, internal, protected internal, private protected.
  3. What is boxing and unboxing in C#?
    • Answer:
      • Boxing: Converting a value type to a reference type.
      • Unboxing: Extracting the value type from an object.
  4. Explain async and await in C#.
    • Answer: Used for asynchronous programming to prevent blocking operations and enhance responsiveness.
  5. What is Task in C#?
    • Answer: Represents an asynchronous operation. It is part of the Task-based Asynchronous Pattern (TAP).
  6. What is yield keyword in C#?
    • Answer: It is used to produce a value sequence one element at a time in an iterator method.
  7. What is the difference between readonly and const?
    • Answer:
      • readonly: Can be initialized at runtime or in the constructor.
      • const: Must be initialized at compile-time.
  8. Explain polymorphism in C#.
    • Answer: Polymorphism enables methods to have different implementations, achieved using method overriding or overloading.
  9. What is the purpose of partial classes in C#?
    • Answer: Allows splitting a single class definition across multiple files.
  10. What is extension method in C#?
    • Answer: A way to add new methods to existing types without modifying their source.

Advanced Level Questions

  1. What is dependency injection in C#?
    • Answer: A design pattern used to inject dependencies into a class at runtime, enhancing testability and flexibility.
  2. Explain Garbage Collection in C#.
    • Answer: Automatic memory management that reclaims unused objects to free up resources.
  3. What is the difference between abstract and virtual methods?
    • Answer:
      • Abstract: No implementation; must be overridden.
      • Virtual: Can have an implementation but can be overridden.
  4. What is the difference between Dispose() and Finalize()?
    • Answer:
      • Dispose(): Explicitly releases resources.
      • Finalize(): Called by the garbage collector before reclaiming objects.
  5. Explain delegates and events in C#.
    • Answer:
      • Delegates: Method references.
      • Events: A specialized delegate used for broadcasting notifications.
  6. What is ThreadPool in C#?
    • Answer: A pool of worker threads for managing parallel execution.
  7. What is a sealed class?
    • Answer: Prevents inheritance.
  8. What are generics in C#?
    • Answer: Enable type safety while allowing classes, interfaces, and methods to operate with any data type.
  9. Explain memory management in C#.
    • Answer: Managed by the CLR through garbage collection, which cleans up unused objects.
  10. What is the difference between Struct and Class?
    • Answer:
      • Struct: Value type; stored on the stack.
      • Class: Reference type; stored on the heap.

Scenario-Based Questions

  1. How to implement Singleton in C#?
    • Answer: Use a private constructor and a static instance property.
  2. What are Attributes in C#?
    • Answer: Metadata added to code elements for additional information, such as [Obsolete].
  3. What is dynamic type in C#?
    • Answer: Allows variables to bypass compile-time type checking.
  4. How does async-await improve performance?
    • Answer: By freeing up threads during I/O-bound operations.
  5. What is Deadlock, and how to avoid it?
    • Answer: A situation where threads wait indefinitely for each other. Avoid by using proper locking mechanisms like Monitor.
  6. What is the purpose of Thread.Join()?
    • Answer: Blocks a calling thread until the specified thread finishes execution.
  7. How does exception handling work in C#?
    • Answer: Using try, catch, finally, and throw statements.
  8. What is the difference between Thread and Task?
    • Answer:
      • Thread: Lower-level concept for concurrency.
      • Task: High-level abstraction for managing asynchronous code.
  9. What is a WeakReference in C#?
    • Answer: Holds a reference to an object while still allowing it to be garbage-collected.
  10. How do you secure data in C# applications?
    • Answer: Use encryption libraries such as System.Security.Cryptography.

Practical and Conceptual Questions

  1. Explain covariance and contravariance in C#.
  2. How does the lock keyword prevent race conditions?
  3. What is the async return type for methods?
  4. What is the difference between is and as operators?
  5. How do extension methods work internally?
  6. What is reflection in C#?
  7. What are async streams in C#?
  8. What is the difference between yield and return?
  9. How do you implement custom serialization?
  10. How do you handle cross-cutting concerns in .NET?

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