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Top Interview Questions and Answers on C# ( 2025 )
Here are some common C# interview questions along with their answers to help you prepare:
Basic Questions
1. What is C#?
- Answer: C# is a modern, object-oriented programming language developed by Microsoft as part of its .NET initiative. It's used for building a variety of applications, including web, mobile, desktop, and gaming.
2. What are the main features of C#?
- Answer: Key features of C# include:
- Strongly typed language
- Object-oriented
- Robust memory management by using automatic garbage collection
- Support for asynchronous programming
- Rich Class Libraries
- Language Integrated Query (LINQ) for data access
3. What is the difference between value types and reference types?
- Answer: Value types store the actual data and are allocated on the stack (e.g., `int`, `float`, `struct`). Reference types store a reference to the memory location where the data is stored and are allocated on the heap (e.g., `class`, `string`, `array`).
Intermediate Questions
4. What is a delegate in C#?
- Answer: A delegate is a type that defines a method signature and can hold references to methods with a matching signature. Delegates are used to implement event handling and the callback methods.
5. What are the different types of collections in C#?
- Answer: C# provides several types of collections:
- Arrays: Fixed-size data structures.
- List<T>: A dynamic array or list of items.
- Dictionary<TKey, TValue>: A collection of key/value pairs.
- HashSet<T>: A collection of unique elements.
- Queue<T>: A first-in, first-out collection.
- Stack<T>: A last-in, first-out collection.
6. Explain the concept of inheritance in C#.
- Answer: Inheritance is an object-oriented programming principle that allows a class (child class) to inherit properties and methods from another class (parent class). It promotes code reusability and establishes a natural hierarchy between classes.
Advanced Questions
7. What is the difference between `abstract class` and `interface`?
- Answer:
- An abstract class can contain implementation (methods with bodies), fields, and constructors. A derived class can inherit from an abstract class and may override its abstract members.
- An interface only contains method signatures (methods without bodies) and properties. A class or structure can implement multiple interfaces, whereas it can only inherit from one abstract class.
8. What is polymorphism in C#?
- Answer: Polymorphism is an OOP concept that allows methods to do different things based on the object that it is acting upon. In C#, it can be achieved through:
- Method overriding (runtime polymorphism).
- Method overloading (compile-time polymorphism).
9. What is LINQ and how is it used in C#?
- Answer: LINQ (Language Integrated Query) is a query syntax in C# that allows developers to perform queries on collections, databases, XML, and more, using a unified syntax. It simplifies data manipulation and retrieval, enabling the use of SQL-like queries directly within C# code.
Miscellaneous Questions
10. What is garbage collection in C#?
- Answer: Garbage collection is an automatic memory management feature in C#. The garbage collector (GC) reclaims memory occupied by objects that are no longer in use, thus preventing memory leaks and improving application performance.
11. How do you handle exceptions in C#?
- Answer: Exceptions in C# are handled using try, catch, and finally blocks. The code that may throw an exception is placed in the try block, catch blocks handle the exceptions, and the finally block contains code that runs regardless of whether an exception is thrown or caught.
12. Explain the concept of asynchronous programming in C#.
- Answer: Asynchronous programming in C# allows methods to run in the background while allowing the main thread to continue executing. The `async` and `await` keywords are used to define and call asynchronous methods, improving application responsiveness, especially in I/O-bound operations.
Conclusion
Remember to tailor your responses to reflect your knowledge and experiences, and be prepared to provide examples or elaborate on your answers during the interview. Good luck!
Here are some advanced interview questions and their answers related to C#:
1. What are the differences between `IEnumerable` and `IQueryable`?
Answer:
- `IEnumerable<T>` is optimized for in-memory operations. It pulls data from a collection, processes it on the client-side, and works with in-memory collections like Lists or Arrays.
- `IQueryable<T>` is optimized for querying data from external data sources such as databases. It does not execute the query until you enumerate it (e.g., using `ToList()`, `ToArray()`, etc.). `IQueryable` translates the expression tree into a query language (like SQL for databases), allowing for deferred execution and better performance when working with huge datasets.
2. What is the purpose of the `async` and `await` keywords in C#?
Answer:
- The `async` keyword is used to declare that a method is asynchronous. It enables the method to run asynchronously, allowing other tasks to run without blocking the calling thread.
- The `await` keyword is used to pause the execution of the async method until the awaited task completes. It does not block the thread while waiting, thus improving performance, especially in UI applications where responsiveness is essential.
3. Explain the concept of Dependency Injection and its benefits in C#.
Answer:
- Dependency Injection (DI) is a design pattern used to implement Inversion of Control (IoC), allowing for better separation of concerns and making code more testable and maintainable. It involves providing an object its dependencies rather than hard-coding them within the object.
Benefits of Dependency Injection:
- Improved Code Maintainability: Dependencies can be swapped out without modifying the dependent code.
- Easier Unit Testing: Dependencies can be mocked or stubbed when testing.
- Loose Coupling: The system becomes more flexible and less prone to issues caused by tightly bound components.
4. What are extension methods in C# and how are they implemented?
Answer:
- Extension methods allow you to add new methods to existing types without modifying the original type or creating a derived type. They provide a way to "extend" the functionality of the class.
Implementation:
1. Create a static class to contain your extension method.
2. Define a static method with the `this` keyword before the first parameter, which indicates the type you are extending.
```csharp
public static class StringExtensions
{
public static bool IsNullOrEmpty(this string str)
{
return string.IsNullOrEmpty(str);
}
}
```
5. What is garbage collection in C#? How does it work?
Answer:
- Garbage collection (GC) is the automatic memory management feature in C#. It automatically releases memory that is no longer in use, preventing memory leaks and optimizing memory usage.
How it works:
1. The GC runs when the system is low on memory or at specific intervals.
2. It tracks the references to objects in memory.
3. It identifies objects that are no longer reachable (i.e., no references are pointing to them).
4. It then releases that memory, allowing it to be reused.
6. What is the difference between a `struct` and a `class` in C#?
Answer:
- Allocation: Classes are reference types, meaning they are allocated on the heap and accessed via references. Structs are value types and are allocated on the stack (unless they are part of a heap-allocated object).
- Handling & Performance: Since structs are copied by value, they can lead to performance overhead if large structs are used frequently. Classes, being reference types, only require a reference to be passed around.
- Inheritance: Classes support inheritance; structs do not. You cannot inherit from a struct, but you can implement interfaces.
7. What is the purpose of `lock` statement in C#?
Answer:
- The `lock` statement is used to ensure that a block of code runs by only one thread at a time. It is a way to deal with race conditions in multi-threaded applications. By locking an object, other threads are prevented from entering a critical section of code until the lock is released.
```csharp
private readonly object _lock = new object();
public void ThreadSafeMethod()
{
lock (_lock)
{
// Critical section code goes here...
}
}
```
8. What is the difference between `override` and `new` keywords in method definitions?
Answer:
- The `override` keyword is used in a derived class to provide a specific implementation of a method that is already defined in its base class. The base class method must be marked with the `virtual` or `abstract` keyword.
- The `new` keyword is used to hide a method in the base class. The base method stays unchanged and can still be accessed via the base class reference.
```csharp
class BaseClass
{
public virtual void Display() { Console.WriteLine("Base Display"); }
}
class DerivedClass : BaseClass
{
public override void Display() { Console.WriteLine("Derived Display"); }
}
class AnotherDerivedClass : BaseClass
{
public new void Display() { Console.WriteLine("Another Derived Display"); }
}
```
9. How does C# implement immutability and what are its benefits?
Answer:
- Immutability in C# can be achieved using `readonly` fields or by creating immutable types using properties or structures that do not provide setters. If the object's state cannot be changed after it's created, it is considered immutable.
Benefits of Immutability:
- Thread Safety: Immutable objects are inherently thread-safe. Multiple threads can safely read without locks.
- Easier to Reason About: Since they do not change state, the behavior of the application becomes more predictable.
- Improved Performance: The use of immutable objects can lead to reduced overhead in scenarios involving concurrency and caching.
10. What are tuples in C# and how can they be used?
Answer:
- Tuples are data structures that can hold a fixed size collection of heterogeneously typed items. They can be defined using the `ValueTuple` type.
Usage:
You can create tuples to return multiple values from a method easily.
```csharp
public (int, string) GetPersonInfo()
{
return (1, "John Doe");
}
var person = GetPersonInfo();
Console.WriteLine($"ID: {person.Item1}, Name: {person.Item2}");
```
Starting from C# 7.0, tuples are more user-friendly with named elements, allowing for better readability:
```csharp
public (int Id, string Name) GetPersonInfo() => (1, "John Doe");
var (id, name) = GetPersonInfo();
Console.WriteLine($"ID: {id}, Name: {name}");
```
These questions and answers cover various aspects of C# and should help you prepare for an advanced interview effectively.
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