C#.NETInterview PrepBackend

100 C# Interview Questions & Answers

August 19, 202660 min read

This covers the C#-language-level ground an interview loop draws from — data types, the CLR's memory model, delegates/events/generics, collections, async internals, and concurrency primitives. A few questions here overlap with OOP principles or Dependency Injection specifically — where that's the case, this gives the C# angle and points to the dedicated 52 OOP interview questions and 35 Dependency Injection interview questions posts for the deeper design-level treatment.

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C# Fundamentals

Q1. What is C# and what are its key features?

C# is Microsoft's statically-typed, object-oriented language running on .NET — type-safe, garbage-collected, and compiled to intermediate language (IL) that the CLR JIT-compiles to native code at runtime. Key features: full OOP support, generics, LINQ, async/await, pattern matching, and (since C# 8) nullable reference types for compile-time null-safety checking.

Q2. Explain the basic structure of a C# program.

csharp

// Modern C# (top-level statements, .NET 6+) — no explicit Main() needed
Console.WriteLine("Hello, world!");

// Classic structure:
namespace MyApp;

public class Program
{
    public static void Main(string[] args)
    {
        Console.WriteLine("Hello, world!");
    }
}

Q3. What are the different types of data types available in C#?

Category	Examples
Value types	int, double, bool, char, struct, enum
Reference types	class, string, array, interface, delegate
Nullable value types	int?, DateTime? — via Nullable<T>

Q4. What is the difference between value types and reference types?

csharp

struct Point { public int X, Y; }
Point p1 = new Point { X = 1 };
Point p2 = p1;         // COPIES the whole struct
p2.X = 99;              // p1.X is still 1

class Box { public int X; }
Box b1 = new Box { X = 1 };
Box b2 = b1;            // COPIES the reference, not the object
b2.X = 99;               // b1.X is now 99 too — same object

	Value type	Reference type
Storage	Typically the stack (or inline within a containing object)	The heap, referenced by a pointer stored on the stack
Assignment	Copies the entire value	Copies the reference — both variables point to the same object
Default	struct, int, bool, enum	class, string, array, delegate

Q5. What are nullable types in C#?

csharp

int? age = null;               // Nullable<int>
if (age.HasValue) Console.WriteLine(age.Value);
int actual = age ?? 0;         // null-coalescing default

Value types can't natively hold null (a plain int always has some numeric value) — Nullable<T> (int? is shorthand for Nullable<int>) wraps a value type to add "no value" as a legitimate state, which is essential for things like an optional database column mapped to a C# property.

Q6. Can you describe what namespaces are and how they are used in C#?

csharp

namespace MyCompany.Orders;

using System.Collections.Generic;   // brings another namespace's types into scope

A namespace organizes types hierarchically and prevents naming collisions — two classes named Order can coexist as MyCompany.Orders.Order and ThirdParty.Sdk.Order without conflict, and a using directive avoids having to fully qualify every reference to a type from another namespace.

Q7. Explain the concept of boxing and unboxing in C#.

csharp

int i = 42;
object boxed = i;        // boxing — the value type is wrapped in a heap object
int unboxed = (int)boxed; // unboxing — copied back out into a value type

Boxing wraps a value type in a heap-allocated object so it can be treated as object (or an interface it implements) — necessary because value types don't naturally live on the heap, but it costs an allocation and a copy. Unboxing reverses it. Both are a real, sometimes-overlooked performance cost when they happen inside a hot loop (a common source of unexpected allocations pre-generics, e.g. adding ints to an ArrayList).

Q8. What is Type Casting and what are its types in C#?

Cast type	Example	Fails how
Implicit	int i = 5; long l = i;	Never — only allowed when no data can be lost
Explicit	double d = 5.7; int i = (int)d;	May lose data (truncates 5.7 to 5) silently
as	obj as string	Returns null on failure instead of throwing
is	if (obj is string s)	Pattern-matches and safely extracts the typed value

Q9. What are operators in C# and can you provide examples?

Category	Examples
Arithmetic	+, -, *, /, %
Comparison	==, !=, <, >, <=, >=
Logical	&&, \|\|, !
Null-related	??, ??=, ?., !
Bitwise	&, \|, ^, ~, <<, >>

Q10. What is the difference between == operator and .Equals() method?

csharp

string a = "hello";
string b = "hel" + "lo";
Console.WriteLine(a == b);        // true — string overrides == for value comparison
Console.WriteLine(a.Equals(b));   // true — same reason

object x = new object();
object y = new object();
Console.WriteLine(x == y);        // false — default == is reference comparison for object

For reference types, == defaults to reference equality unless the type overloads it (string and records both do, for value-based comparison) — .Equals() is virtual and can be overridden per type for genuine content-based equality, which is why user-defined classes should override both consistently if value equality is intended.

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