C sharp is an object-oriented programming language developed by Microsoft. C# is used with the .NET framework for creating websites, applications, and games. C# has a varied reasons for being popular:

• Comparatively easier: Starting with C# is termed comparatively easier than other programming languages
• Wider use of development: Using C#, you are prone to create web applications or gaming apps. C# has some fantastic features like automatic garbage collection, interfaces, etc. which help build better applications.
• Larger target audience: Collaboration with Microsoft provides an edge to the applications created using C# since it would have a wider target audience.

Since C# is such a widely used programming language, a plethora of big and small organizations base their products using it. So, prepare yourself with basic and advanced level C# questions to ace the interviews. 

C# Interview Questions for Freshers

1. How is C# different from C?

You would always know C being the procedural language while C# is a more object-oriented language. The biggest difference is that C# supports automatic garbage collection by Common Language Runtime (CLR) while C does not. C# primarily needs a .NET framework to execute while C is a platform-agnostic language. 

2. What is C# and the .NET Framework?

Answer:
C# is a modern, object-oriented language developed by Microsoft. The .NET Framework (and .NET Core/.NET 5+) provides runtime and libraries for building cross‑platform apps. 

3. What are value types vs. reference types?

• Value types (e.g., int, struct) store data directly; assignment creates a copy.
• Reference types (e.g., class, string) store references to data on the heap; assignment copies the reference. 

Example:

a = 5, b = a; 
b = 10;
// a == 5

class Person { public string Name; }
var p1 = new Person { Name = "Alice" };
var p2 = p1;
p2.Name = "Bob";
// p1.Name == "Bob"

4. Difference between string and StringBuilder?

• string is immutable → concatenations create new objects.
• StringBuilder is mutable → efficient for repetitive string operations. 

s = "";
for(int i=0;i<1000;i++) s += "X";  // poor performance

var sb = new StringBuilder();
for(int i=0;i<1000;i++) sb.Append("X");
string result = sb.ToString();   // efficient

5. What are access modifiers?

• public, private, protected, internal, protected internal, private protected.

They control visibility and encapsulation of class members. 

6. Explain class vs. struct.

• class: reference type, uses heap, inheritance allowed.
• struct: value type, lightweight, no inheritance. 

7. What is the difference between the Abstract class vs. interface?

Let’s dig into the differences between an abstract class and an interface:

• • Abstract classes are classes that cannot be instantiated ie. that cannot create an object. The interface is like an abstract class because all the methods inside the interface are abstract methods.
  • Surprisingly, abstract classes can have both abstract and non-abstract methods but all the methods of an interface are abstract methods.
  • Since abstract classes can have both abstract and non-abstract methods, we need to use the Abstract keyword to declare abstract methods. But in the interface, there is no such need.

  An abstract class has constructors while an interface encompasses none. 

  Ex.

  Abstract class:

  public abstract class Country{
  public abstract void City();
  }

  Interface:

  public interface Books{
  void Codingbooks();
  }

8. Explain LINQ and difference between IEnumerable vs. IQueryable.

• LINQ enables integrated querying on collections.
• IEnumerable: in-memory, immediate execution.
• IQueryable: deferred execution, suited for remote data sources (e.g., database).

GenericList<float> list1 = new GenericList<float>();
GenericList<Features> list2 = new GenericList<Features>();
GenericList<Struct> list3 = new GenericList<Struct>();

void Method(ref int refArgument)
{
   refArgument = refArgument + 10;
}
int number = 1;
Method(ref number);
Console.WriteLine(number);
// Output: 11

public static string GetNextFeature(ref int id)  
{  
   string returnText = "Next-" + id.ToString();  
   id += 1;  
   return returnText;  
}  
public static string GetNextFeature(out int id)  
{  
   id = 1;  
   string returnText = "Next-" + id.ToString();  
   return returnText;  
}   

12. Async/await vs. threads/tasks?

• Task is a high-level abstraction for asynchronous operations; Thread is a low-level OS thread.
• async and await enable non-blocking, I/O-bound operations

13. What are delegates and events?

• Delegate: a type-safe function pointer; supports multicast.
• Event: built on delegates to implement publisher-subscriber patterns.

delegate void Notify(string msg);
class Publisher {
  public event Notify OnChange;
  public void Raise() => OnChange?.Invoke("Changed");
}

14. Explain exception handling with using, Dispose, and Finalize.

• using ensures IDisposable.Dispose() is called.
• Dispose() is manually invoked cleanup.
• Finalize() is a GC‑invoked destructor. 

15. What’s the CLR and how does it manage memory?

• CLR (Common Language Runtime) executes C# code, handles GC, JIT compilation, and type safety. 

16. Explain dependency injection.

• Dependency Injection (DI) is a design pattern that helps achieve Inversion of Control (IoC) between classes and their dependencies.Instead of a class creating its own dependencies, they are “injected” from the outside—making code more flexible, testable, and loosely coupled.
• Real-World Analogy :- Think of a restaurant chef who doesn’t buy ingredients (dependency) themselves—they are provided by a supplier (injector). This allows the chef to cook different meals (functionality) without worrying about sourcing.

17. Explain Generics and When to Use Them

Generics in C# allow you to define type-safe data structures without committing to actual data types until the class or method is instantiated.

Benefits:

• Type safety: Catch errors at compile-time.

• Reusability: Write once, use for any data type.

• Performance: Avoids boxing/unboxing.

Examples:

// Generic class
public class DataStore<T>
{
    public T Data { get; set; }
}

var intStore = new DataStore<int> { Data = 100 };
var stringStore = new DataStore<string> { Data = "Hello" };

Use Cases:

• List<T>, Dictionary<TKey, TValue>

• Custom repositories

• Reusable utilities

18. What is Reflection and Its Use Cases?

Reflection allows you to inspect metadata, types, methods, and properties at runtime.

Common Uses:

• Dynamic object creation

• Loading plugins dynamically

• Building frameworks and serializers

Example:

Type type = typeof(string);
Console.WriteLine(type.FullName);
foreach (var method in type.GetMethods())
{
    Console.WriteLine(method.Name);
}

Use Cases:

• Test frameworks like NUnit

• ORMs like Entity Framework

• Serialization libraries

19. Advanced Threading: Deadlocks, Task.WhenAll vs WhenAny, Cancellation Tokens

Deadlocks: Occurs when two threads wait indefinitely for each other to release resources.

Example:

lock(obj1)
{
    lock(obj2) { }
}
// Another thread
lock(obj2)
{
    lock(obj1) { }
}

Avoid by:

• Locking in consistent order

• Using Monitor.TryEnter

Task.WhenAll: Waits for all tasks to complete.

await Task.WhenAll(task1, task2);

Task.WhenAny: Continues once any task finishes.

await Task.WhenAny(task1, task2);

Cancellation Token: Gracefully cancel a running task.

var cts = new CancellationTokenSource();
Task.Run(() => {
    while (!cts.Token.IsCancellationRequested)
    {
        // do work
    }
}, cts.Token);

20. File Handling, Serialization, and Encryption

File Handling:

using (FileStream fs = new FileStream("data.txt", FileMode.Create))
{
    byte[] info = new UTF8Encoding(true).GetBytes("Hello World");
    fs.Write(info, 0, info.Length);
}

Serialization:

// JSON
var json = JsonSerializer.Serialize(obj);
var obj2 = JsonSerializer.Deserialize<MyClass>(json);

Encryption: Use Aes or Rijndael for symmetric encryption.

using Aes aes = Aes.Create();
aes.Key = ...; // provide key
aes.IV = ...;  // provide IV

21. Performance and Optimization: Boxing/Unboxing, Memory Usage

Boxing: Converting value type to object (heap). Unboxing: Extracting value type from object.

int num = 123;
object obj = num; // boxing
int num2 = (int)obj; // unboxing

Problems:

• Memory overhead

• Performance hit

String Immutability: Repeated += operations create multiple string instances. Use StringBuilder instead.

Thread Safety:

• Avoid shared mutable state

• Use lock, ConcurrentDictionary, ImmutableList

22. Design Patterns: Builder vs Abstract Factory

Builder Pattern:

• Separates object construction from its representation.

• Useful when object requires many optional parameters.

var builder = new CarBuilder();
builder.SetEngine("V8").SetWheels(4);
var car = builder.Build();

Abstract Factory Pattern:

• Creates families of related objects without specifying their concrete classes.

interface ICarFactory { ICar CreateCar(); IEngine CreateEngine(); }
class SportsCarFactory : ICarFactory {...}

23. Concurrency in Real-World Scenarios

Locks and Monitors: Prevent simultaneous access to shared resources.

lock(someObject)
{
    // critical section
}

async/await: For asynchronous programming.

public async Task LoadDataAsync()
{
    var data = await GetDataFromApi();
}

TransactionScope: Ensures ACID compliance.

using (var scope = new TransactionScope())
{
    // db operations
    scope.Complete();
}

Best Practices:

• Avoid blocking threads

• Prefer async/await for I/O-bound operations

• Use SemaphoreSlim for controlling concurrency

24. What is the difference between late binding and early binding in c#? 

int num = 23; // 23 will assigned to num
Object Obj = num; // Boxing

int num = 23;         // value type is int and assigned value 23
Object Obj = num;    // Boxing
int i = (int)Obj;    // Unboxing

Conclusion

In this blog, we explored advanced C# interview questions covering essential concepts like Generics, Reflection, Threading, File Handling, Performance Optimization, Design Patterns, and Real-world Concurrency. Whether you’re preparing for your first job or leveling up to senior roles, mastering these topics will give you a significant edge in technical interviews.

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