C# interview questions

1. What is C#?

C# is an object-oriented programming language from Microsoft. It lets you model real things as objects in code.

Full lesson: Csharp intro

2. How do you declare a variable in C#?

You write a data type, then a name, then a value. The type tells the compiler what the variable can hold.

int age = 30;
string name = "Ana";

Full lesson: Csharp variables

3. What is a data type in C#?

A data type defines the kind of value a variable can store. Common types are int, double, bool, and string.

int count = 5;
double price = 9.99;
bool isReady = true;

Full lesson: Csharp data types

4. How do you write an if statement in C#?

An if statement runs a code block when a condition is true. The condition must give a bool value.

int a = 10;
if (a < 20)
{
    Console.WriteLine("a is small");
}

Full lesson: Csharp if

5. How do you add an else branch to an if statement?

The else block runs when the if condition is false. Only one of the two blocks runs.

int a = 100;
if (a < 20)
{
    Console.WriteLine("small");
}
else
{
    Console.WriteLine("large");
}

Full lesson: Csharp if else

6. How do you declare an array in C#?

You write the element type, then empty brackets, then the name. You create the array with the new keyword.

int[] numbers = new int[5];
numbers[0] = 10;

Full lesson: Csharp array

7. How does a for loop work in C#?

The init part runs one time. Then the condition is checked and the body runs. The increment runs after each pass.

for (int i = 0; i < 3; i++)
{
    Console.WriteLine(i);
}

Full lesson: Csharp for loop

8. How does a while loop work in C#?

A while loop repeats a block while a condition stays true. The condition is checked before each pass.

int i = 0;
while (i < 3)
{
    Console.WriteLine(i);
    i++;
}

Full lesson: Csharp while loop

9. How do you create a string in C#?

You use the string keyword and assign text in double quotes. The string keyword is an alias of System.String.

string first = "Rowan";
string last = "Atkinson";
string full = first + last;

Full lesson: Csharp string

10. How do you define a class in C#?

A class definition starts with the class keyword and a name. The class body holds members inside curly braces.

class Box
{
    public double length;
    public double width;
}

Full lesson: Csharp class

11. What is an indexer in C#?

An indexer lets an object use square brackets like an array. It uses get and set accessors with an index.

public string this[int index]
{
    get { return names[index]; }
    set { names[index] = value; }
}

Full lesson: Csharp indexer

12. What is a Hashtable in C#?

A Hashtable stores key and value pairs. You look up a value with its key.

Hashtable table = new Hashtable();
table["one"] = 1;
Console.WriteLine(table["one"]);

Full lesson: Csharp hashtable

13. How do you read a text file in C#?

Use the System.IO namespace for file work. File.ReadAllText reads the whole file into a string.

using System.IO;
string text = File.ReadAllText("data.txt");
Console.WriteLine(text);

Full lesson: Csharp text files

14. What is a common mistake with a while loop?

A missing increment keeps the condition true forever. The loop then never stops. Change the loop variable inside the body.

int i = 0;
while (i < 3)
{
    Console.WriteLine(i);
    i++;
}

Full lesson: Csharp while loop

15. What is a common mistake with array indexes?

An array of size 5 has indexes 0 to 4. Index 5 is out of range and throws an exception.

int[] numbers = new int[5];
numbers[4] = 10;
// numbers[5] = 20; // error

Full lesson: Csharp array class

16. What is the C# Array class?

The Array class is the base class for all arrays in C#. It provides methods to create, copy, and sort arrays.

int[] nums = { 3, 1, 2 };
Array.Sort(nums);
Console.WriteLine(nums[0]); // 1

Full lesson: Csharp array class

17. How do you use a foreach loop in C#?

A foreach loop repeats a block for each item in a collection. You do not need an index variable.

string[] names = { "Ann", "Bob" };
foreach (string name in names)
{
    Console.WriteLine(name);
}

Full lesson: Csharp for loop

18. How do you write a nested if statement in C#?

You place one if statement inside another if block. The inner if runs only when the outer condition is true.

int a = 100;
if (a == 100)
{
    if (a < 200)
    {
        Console.WriteLine("Yes");
    }
}

Full lesson: Csharp nested if

19. How do you create a BitArray in C#?

You create a BitArray with a size in bits. Each bit is false by default.

BitArray bits = new BitArray(8);
Console.WriteLine(bits.Length); // 8

Full lesson: Csharp bitarray

20. What is an interface in C#?

An interface defines a contract of methods and properties. A class that implements the interface must provide the code for all members.

interface IShape
{
    double Area();
}
class Square : IShape
{
    public double Side;
    public double Area() { return Side * Side; }
}

Full lesson: Csharp interface

21. How does inheritance work in C#?

A derived class inherits members from a base class. The derived class can add new members or override base members.

class Animal { public void Eat() { Console.WriteLine("Eating"); } }
class Dog : Animal { }
Dog d = new Dog();
d.Eat();

Full lesson: Csharp inheritance

22. How do you use a SortedList in C#?

A SortedList stores key-value pairs and keeps them sorted by key. You can access items by key or by index.

SortedList sl = new SortedList();
sl.Add(2, "B");
sl.Add(1, "A");
Console.WriteLine(sl[1]); // A

Full lesson: Csharp sortedlist

23. How do you write text to a file in C#?

You use the StreamWriter class to write text to a file. Call Write or WriteLine to add text, then Close the writer.

using (StreamWriter sw = new StreamWriter("test.txt"))
{
    sw.WriteLine("Hello");
}

Full lesson: Csharp text files

24. What is a common mistake with a for loop in C#?

A common mistake is an off-by-one error in the condition. For example, using <= instead of < can cause an extra iteration.

for (int i = 0; i <= 3; i++) // runs 4 times, not 3
{
    Console.WriteLine(i);
}

Full lesson: Csharp for loop

25. What is a common mistake with an if statement in C#?

A common mistake is using the assignment operator = instead of the equality operator ==. This assigns a value and does not compare.

int a = 5;
if (a = 10) // error: cannot convert int to bool
{
}

Full lesson: Csharp if

26. What is a common mistake with a foreach loop in C#?

A common mistake is trying to change the collection inside a foreach loop. This causes an InvalidOperationException.

List<int> nums = new List<int> { 1, 2, 3 };
foreach (int n in nums)
{
    nums.Add(4); // throws exception
}

Full lesson: Csharp for loop

27. What is a common mistake with a SortedList in C#?

A common mistake is adding a duplicate key. This throws an ArgumentException.

SortedList sl = new SortedList();
sl.Add(1, "A");
sl.Add(1, "B"); // throws exception

Full lesson: Csharp sortedlist

28. What is a common mistake with an interface in C#?

A common mistake is not implementing all interface members in the class. The compiler will report an error.

interface IShape { double Area(); }
class Square : IShape { } // error: does not implement Area

Full lesson: Csharp interface

29. What is a common mistake with inheritance in C#?

A common mistake is trying to inherit from multiple base classes. C# does not support multiple class inheritance.

class A { }
class B { }
class C : A, B { } // error

Full lesson: Csharp inheritance

30. What is a common mistake with file writing in C#?

A common mistake is not closing the StreamWriter. This can lock the file and lose data.

StreamWriter sw = new StreamWriter("test.txt");
sw.WriteLine("Hello");
// missing sw.Close()

Full lesson: Csharp text files

31. What is the basic structure of a C# program?

A C# program has a namespace, a class, and a Main method. The Main method is the entry point. The program starts at Main.

using System;
class Program {
  static void Main() {
    Console.WriteLine("Hello");
  }
}

Full lesson: Csharp basic syntax

32. What is an ArrayList in C#?

An ArrayList is a dynamic array. It can grow and shrink at run time. It stores items as objects.

using System.Collections;
ArrayList list = new ArrayList();
list.Add(10);
list.Add("text");

Full lesson: Csharp arraylist

33. What does the Array class provide in C#?

The Array class gives methods for all arrays. It has methods such as Sort, Reverse, and Length. These methods help you work with array data.

int[] nums = {3, 1, 2};
Array.Sort(nums);
Console.WriteLine(nums[0]);

Full lesson: Csharp array class

34. How do you use a do while loop in C#?

A do while loop runs the body first. Then it checks the condition. The body runs at least one time.

int i = 0;
do {
  Console.WriteLine(i);
  i++;
} while (i < 3);

Full lesson: Csharp while loop

35. How do you use a BitArray in C#?

A BitArray stores bits as true or false values. You create it from a byte array or a bool array. It is useful for compact flags.

using System.Collections;
bool[] flags = {true, false, true};
BitArray bits = new BitArray(flags);
Console.WriteLine(bits[0]);

Full lesson: Csharp bitarray

36. How do you use a Hashtable in C#?

A Hashtable stores key and value pairs. It uses a hash code to find each key fast. Keys must be unique.

using System.Collections;
Hashtable table = new Hashtable();
table["one"] = 1;
Console.WriteLine(table["one"]);

Full lesson: Csharp hashtable

37. How do you use an indexer in C#?

An indexer lets an object use square brackets. You define it with the this keyword. It gives array-like access to class data.

class Box {
  private int[] data = new int[5];
  public int this[int i] {
    get { return data[i]; }
    set { data[i] = value; }
  }
}

Full lesson: Csharp indexer

38. How do you define an interface in C#?

An interface lists method signatures with no body. A class that uses the interface must implement each method. The interface keyword starts the definition.

interface IShape {
  double Area();
}
class Square : IShape {
  public double Area() { return 4.0; }
}

Full lesson: Csharp interface

39. How do you use inheritance in C#?

A derived class inherits members from a base class. You write a colon and the base class name. The derived class can add new members.

class Animal {
  public void Eat() { Console.WriteLine("eat"); }
}
class Dog : Animal {
  public void Bark() { Console.WriteLine("bark"); }
}

Full lesson: Csharp inheritance

40. What is a common mistake with an ArrayList in C#?

A common mistake is to store mixed types and forget the cast. ArrayList stores objects, so you must cast items on read. A wrong cast causes a run-time error.

ArrayList list = new ArrayList();
list.Add(5);
int n = (int)list[0];

Full lesson: Csharp arraylist

41. What is a common mistake with a do while loop in C#?

A common mistake is to forget the update inside the loop. The condition then stays true and the loop never stops. Always change the loop variable in the body.

int i = 0;
do {
  Console.WriteLine(i);
  i++;
} while (i < 3);

Full lesson: Csharp while loop

42. What is a common mistake with a Hashtable key in C#?

A common mistake is to use a missing key. A Hashtable returns null for a missing key. A null value can cause a later error.

Hashtable t = new Hashtable();
if (t.ContainsKey("one")) {
  Console.WriteLine(t["one"]);
}

Full lesson: Csharp hashtable

43. What is a common mistake with an indexer in C#?

A common mistake is to skip the bounds check. A bad index can throw an exception. Check the index before you use it.

public int this[int i] {
  get {
    if (i < 0 || i >= data.Length) return 0;
    return data[i];
  }
}

Full lesson: Csharp indexer

44. What is a common mistake with a StreamReader in C#?

A common mistake is to skip the Close call. An open reader can lock the file. Use a using block to close it for you.

using (StreamReader sr = new StreamReader("a.txt")) {
  string line = sr.ReadLine();
  Console.WriteLine(line);
}

Full lesson: Csharp text files