C# Interview Preparation

# CHAPTER 28

C# Interview Preparation #

1. Introduction #

2. Learning Objectives #

• Answer the most frequent Core C# theoretical questions.

• Explain OOP principles clearly.

• Discuss advanced topics like async/await and LINQ.

• Understand common C# white-board coding challenges.

3. Core C# & .NET Interview Questions #

1. What is the difference between Managed and Unmanaged code? *Answer:* Managed code runs inside the .NET Common Language Runtime (CLR), which handles memory management (Garbage Collection), security, and type safety. Unmanaged code runs directly on the OS hardware (like C or C++), requiring manual memory management.

2. Explain the difference between Value Types and Reference Types. *Answer:* Value Types (like int, bool, struct) hold their data directly and are stored on the Stack. Reference Types (like string, class, array) store a memory address on the Stack that points to the actual data stored on the Heap.

3. What is the difference between String and StringBuilder? *Answer:* String is immutable; every time you modify it, a new string is created in memory, which is terrible for performance in loops. StringBuilder is mutable and modifies the string buffer in-place, making it highly efficient for heavy text manipulation.

4. What is the var keyword? Is it dynamically typed? *Answer:* No, var is strongly and statically typed. It simply tells the compiler to deduce the data type based on the right side of the assignment at compile-time. Once the type is set, it cannot be changed.

4. OOP Interview Questions #

5. Name and define the 4 Pillars of OOP. *Answer:*

• Encapsulation: Hiding internal state and requiring interaction through controlled properties/methods.

• Abstraction: Hiding complex implementation details behind simple interfaces.

• Inheritance: Creating child classes that inherit fields/methods from parent classes.

• Polymorphism: Allowing a single method name to behave differently (Overriding) or treating derived objects as base objects.

6. What is the difference between an Interface and an Abstract Class? *Answer:* A class can inherit only one Abstract Class, but can implement multiple Interfaces. Abstract classes define core identity ("Is-A") and can contain state (fields). Interfaces define capabilities ("Can-Do") and cannot contain state.

7. What is Method Overloading vs Method Overriding? *Answer:* Overloading (Compile-time) is multiple methods with the same name but different parameters in the same class. Overriding (Runtime) is a child class replacing a parent's virtual method using the override keyword.

5. Advanced C# Interview Questions #

8. Explain async and await. Does it create a new thread? *Answer:* async/await enables asynchronous programming to keep threads responsive (especially UI threads or Web Server threads) during I/O operations (like database calls). It does NOT inherently create a new OS thread; it uses a state machine to release the current thread back to the Thread Pool while waiting, and resumes when the task completes.

9. What is LINQ? Explain Deferred Execution. *Answer:* Language-Integrated Query (LINQ) provides SQL-like querying capabilities directly in C#. Deferred Execution means a LINQ query is not actually executed against the data source when it is defined, but only when the data is explicitly iterated over (e.g., via a foreach loop or calling .ToList()).

10. What is Dependency Injection (DI)? *Answer:* A design pattern where an object receives its dependencies from an external source rather than creating them itself (using new). It promotes loose coupling and makes code highly testable. It is a core, built-in feature of ASP.NET Core.

6. Common Coding Challenges #

Challenge 1: String Reversal (Without built-in methods)

public static string ReverseString(string input)
{
    char[] charArray = input.ToCharArray();
    int left = 0;
    int right = charArray.Length - 1;
    
    while (left < right)
    {
        char temp = charArray[left];
        charArray[left] = charArray[right];
        charArray[right] = temp;
        left++;
        right--;
    }
    return new string(charArray);
}

Challenge 2: LINQ Filtering *Task:* Given a List<int>, return a list containing only the even numbers, sorted descending.

var results = numbers.Where(n => n % 2 == 0)
                     .OrderByDescending(n => n)
                     .ToList();

7. Common Mistakes in Interviews #

• Using ArrayList: If asked to store data dynamically, never say ArrayList. Always use List<T>. Mentioning ArrayList shows outdated knowledge.

• Confusing override with new: The new keyword hides a base method, while override actually replaces it for runtime polymorphism. Always prefer override.

• Not mentioning using blocks: If asked to read a file or connect to a database, failing to wrap StreamReader or SqlConnection in a using block is an instant red flag regarding your memory management skills.

8. Best Practices Checklist #

• Ensure you understand the difference between C# (the language), .NET (the runtime/framework), and ASP.NET Core (the web framework).

• Be prepared to discuss SOLID principles, specifically the Single Responsibility Principle and Dependency Inversion.

9. Exercises #

1. 1. Hand-write the answers to the 10 theoretical questions above.

1. 2. Open an IDE and write a C# program that implements the FizzBuzz challenge: Print numbers 1 to 100. If divisible by 3, print "Fizz". Divisible by 5, print "Buzz". Divisible by both, print "FizzBuzz".

10. MCQs with Answers #

11. Interview Questions #

• Q: Explain the Garbage Collector. Can you force it to run? (Answer: Yes, via GC.Collect(), but it is highly discouraged as it disrupts the CLR's optimized schedule).

• Q: What is the difference between an IEnumerable and an IQueryable in Entity Framework?

12. Summary #

13. Next Chapter Recommendation #