C# Interview Questions & Tips

By The Mighty Anomaly | Published: July 10, 2023

C# Interview Stats

We've hosted over 100k interviews on our platform. C# was the language of choice in those interviews 4% of the time. C# also had a slightly low succes rate – engineers who chose C# as their interview language passed their interviews 45% of the time.

Below is a distribution of programming languages and their popularity in technical interviews as well as success rates in interviews, by language.

C# was the language of choice in interviewing.io interviews 4% of the time

C# also had a slightly low succes rate – engineers who chose C# as their interview language passed their interviews 45% of the time.

C# Idioms & Idiosyncrasies

Each programming language comes with its unique set of idioms and idiosyncrasies, and C# is no different. Mastering these peculiarities is key to achieving fluency in the language and successfully navigating an interview.

  • Strongly-Typed Language: Unlike dynamically typed languages, C# requires explicit data type declaration which helps catch errors at compile-time.
  • LINQ (Language Integrated Query): This powerful feature allows developers to interact with data in a type-safe, declarative manner. However, it can also lead to overuse and performance issues if not managed correctly.
  • Nullable Reference Types: This feature, introduced in C# 8.0, aims to eliminate the infamous "null reference exceptions" by distinguishing nullable from non-nullable reference types.
  • Delegates and Events: These are fundamental to the C# language and .NET framework but can be challenging to understand and use correctly.

Common C# Interview Mistakes

Several common mistakes emerge when candidates use C# during technical interviews. Here are some examples:

Poor Data Structure Choices

C# provides a rich library of data structures, each with their specific usage scenarios and strengths. A common mistake is using inefficient data structures for the problem at hand. For example, using a List when a HashSet would provide a significant performance boost due to constant-time lookups.

// Instead of doing this...
List<int> myList = new List<int>();

// When lookups are frequent, do this...
HashSet<int> mySet = new HashSet<int>();

Overlooking Exception Handling

It's easy to overlook exception handling during an interview, but this mistake can be a signal that the candidate isn't considering edge cases and potential failures.

// Neglecting try-catch-finally
StreamReader sr = new StreamReader("nonexistentfile.txt");
string line = sr.ReadLine();

Poor Handling of Nulls

Nullable types and the null coalescing operator are important tools in C#. Mistakes can happen when they are not handled correctly. This is especially critical in the recent versions of C# with nullable reference types. Using these features makes your code more concise than if/else conditionals would and makes your solution cleaner overall.

// Instead of this...
if (myObject != null) 
{
    myObject.DoSomething();
}
else 
{
    myObject = new MyObject();
    myObject.DoSomething();
}

// You can do this...
(myObject ??= new MyObject()).DoSomething();

Overcomplicating Solutions

Sometimes the simplest solution is the best. Over-engineering and making the solution more complex than it needs to be is a common mistake. Avoid adding unnecessary patterns or features that complicate the code and make it harder to read. Understand the difference between code that is meant to show you understand a concept (like in interview questions) and code that is meant to be shipped and part of a production codebase.

// Instead of over-complicating with design patterns...
IList<int> numbers = new ReadOnlyCollection<int>(new List<int> {1, 2, 3, 4, 5});

// If all you're doing is iterating, do this...
int[] numbers = {1, 2, 3, 4, 5};

How to Demonstrate C# Expertise in Interviews

Use C# Data Structures

C# offers several structures that can make your code more efficient, such as Tuples and ValueTuples. Demonstrate your expertise by utilizing these when it makes sense.

public (int, int) GetMinMax(IEnumerable<int> numbers)
{
    int min = numbers.Min();
    int max = numbers.Max();
    return (min, max);
}

Use C# Conventions and Styles

Although this may seem minor, properly following C#'s naming and style conventions demonstrate your experience with the language. This includes things like PascalCasing public methods and properties, camelCasing private fields and parameters, using meaningful variable names, and keeping methods short and focused.

public class Customer
{
    private string name;

    public string Name
    {
        get { return name; }
        set { name = value; }
    }
}

Demonstrating your C# expertise during an interview is about more than just getting the code to work. It's about showing that you understand the language deeply, and that you know how to use its features effectively and appropriately.

Effectively Use Interfaces and Abstract Classes

The ability to properly use interfaces and abstract classes is essential in C#. Show that you understand when to use which, and how they can be used to create flexible and reusable code.

public interface IWorker
{
    void Work();
}

public class Employee : IWorker
{
    public void Work()
    {
        // Implementation here
    }
}

Leverage LINQ

As mentioned earlier, LINQ is a powerful tool in C#, but many engineers fail to leverage it during their interviews. This can be due to a lack of familiarity in C# or because of the hidden time and space complexities associated with these methods. It's ok to write a solution yourself when you're not sure if the LINQ equivalent has a poor complexity, but it is better to leverage LINQ and show mastery over C# to help you with simple tasks.

//Instead of this...
List<int> nums = new List<int> { 1, 2, 3, 4, 5 };
List<int> evenNums = new List<int>();

foreach (int num in nums)
{
    if (num % 2 == 0)
    {
        evenNums.Add(num);
    }
}

// Use LINQ like this...
List<int> evenNums = nums.Where(num => num % 2 == 0).ToList();

LINQ should be leveraged for simple tasks that make your code more concise without sacrificing readability. Here is another example:

// Instead of writing a loop to find the smallest number...
int smallest = int.MaxValue;
foreach (int num in numbers)
{
    if (num < smallest) smallest = num;
}

// You could simply use LINQ
int smallest = numbers.Min();

Know Delegates and Events

Many candidates struggle with the correct usage of delegates and events, which can lead to messy code and memory leaks. While these don't come up too often in interviews, it is worthwhile to know the ins-and-outs if you want to show mastery here.

// Incorrect event subscription/unsubscription
public delegate void Del();
public event Del MyEvent;

// Somewhere in the code
MyEvent += SomeMethod;
MyEvent -= SomeMethod;  // Forgot to unsubscribe the event handler

C# Interview Replays

Below you can find replays of mock interviews conducted on our platform in C#. The questions asked in these interviews tend to be language-agnostic (rather than asking about language-specific details and idiosyncrasies), but in these cases, the interviewee chose C# as the language they would work in.

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