How to Pass a C# Interview: Top Questions and Answers

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How to Pass a C# Interview: Top Questions and Answers

A C#/.NET developer interview is a test of both theoretical knowledge of the platform and practical coding skills. Employers want to see not just someone who can write code, but an engineer who understands the internal workings of .NET, memory management, asynchrony, and SOLID principles.

In this article, we have compiled top-10 questions that are most often asked in C# interviews. You will learn not only the correct answers but also the pitfalls that interviewers pay attention to. The material will be useful for both junior developers and mid-level specialists.

1. Questions on OOP Basics in C#

Without understanding object-oriented programming, it is impossible to pass a C# interview. Interviewers test not only definitions but also the ability to apply principles in practice.

What OOP principles are implemented in C#?

C# fully supports the four pillars of OOP:

Encapsulation — hiding internal implementation through access modifiers (private, protected, internal).
Inheritance — a class can inherit behavior from one base class (single inheritance).
Polymorphism — the ability to work with different types through a single interface (virtual methods, abstract classes).
Abstraction — highlighting essential characteristics of an object and ignoring non-essential ones (interfaces, abstract classes).

Example question: "What is the difference between an abstract class and an interface?"

This is a classic. The key difference is that an interface defines only a contract (method signatures without implementation), while an abstract class can contain both implementation and state (fields). In C#, starting from version 8.0, interfaces can have default implementation, but this does not negate the conceptual difference.

// Abstract classpublic abstract class Animal{    public string Name { get; set; }    public abstract void MakeSound(); // without implementation    public void Sleep() => Console.WriteLine("Zzz..."); // with implementation}

// Interfacepublic interface IMovable{    void Move(); // contract only}

2. Memory Management and Garbage Collector

Understanding how the GC works is a sign of an experienced developer. Questions on this topic often serve as a filter between junior and mid-level.

How does the Garbage Collector work in .NET?

The GC works on the managed heap and frees memory from objects that are no longer referenced. It uses a generational model (Generation 0, 1, 2):

Gen 0 — short-lived objects (cleaned most often).
Gen 1 — a buffer between Gen 0 and Gen 2.
Gen 2 — long-lived objects (cleaned rarely).

Example question: "What is finalization and IDisposable?"

The finalizer (~ClassName()) is called by the GC before freeing memory, but its invocation time is non-deterministic. For deterministic cleanup of unmanaged resources, the IDisposable pattern is used:

public class ResourceHolder : IDisposable{    private bool _disposed = false;

    public void Dispose()    {        Dispose(true);        GC.SuppressFinalize(this);    }

    protected virtual void Dispose(bool disposing)    {        if (_disposed) return;        if (disposing) { /* free managed resources */ }        /* free unmanaged resources */        _disposed = true;    }

    ~ResourceHolder() => Dispose(false);}