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top 10 senior .NET developer interview questions

9 min readpublished 10 August 2022
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When you’re looking for senior .NET developer jobs, preparing for senior .NET developer interview questions is essential. While you should start by exploring the top 11 .NET interview questions and answers, you also need to dive into more advanced interview questions, since they are almost guaranteed to come up during your interview.

The answers to the questions below were provided by EPAM Anywhere’s Certified .NET Technical Interviewer and Chief Software Engineer, Diego Parra. Here’s a look at the top 10 senior .NET developer interview questions and answers.

1. What is dependency injection?

This is one of the classic senior .NET developer interview questions designed to test your knowledge. Ideally, you want to provide a detailed explanation, showing that you have ample expertise.

Dependency injection involves providing the objects that objects require — known as the object’s dependencies — instead of having the object construct the dependencies itself. This process allows developers to mock or stub out the various dependencies, an approach that’s incredibly helpful during testing.

It also allows the developer to follow SOLID patterns more easily, since it is based on dependency inversion and inversion of container principles, which are two powerful techniques to avoid high coupling in code.

2. How can you relate SOLID principles and dependency injection?

This is another senior .NET developer interview question that aims to explore your knowledge, so you’ll want to be reasonably thorough.

Dependency injection helps us to enforce SOLID principles in the following ways:

  • Single responsibility: You can enforce single responsibility by detecting code that should not be in the current class, creating a class/interface pair, and using IoC to inject the new dependency. Dependency injection also assumes the responsibility of creating new object instances and managing their lifecycle.
  • Dependency inversion: The dependency injection framework will automatically inject the dependencies in your code, but you still need to reduce your coupling by encapsulating it in a lower-level abstraction.

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3. Which types of locking mechanisms do you know in C#?

With senior .NET developer interview questions like this, you need to demonstrate knowledge while referencing your personal experience in the answer. Along with describing the locking mechanisms, you can add details about your past use of them.

.NET provides different classes and keywords to allow developers to create thread-safe code and lock specific code zones.

  • Lock keyword/Monitor: Creates a basic code zone where only one thread can enter at the same time. This is precisely the same as using Monitor.Enter/Exit class.
  • Mutex: This is similar to Monitor but can be named and shared between processes and async code (which lock keyword cannot).
  • SemaphoreSlim: This is a lightweight version of Semaphore which works within the same application. It allows you to fine-tune the number of threads that can enter into the critical zone.
  • ManualResetEvent/AutoResetEvent: These classes can be shared among threads to allow precise control regarding when some code should wait and when it can execute. The major difference here is that the code is not restricted to a critical zone.

These are the main mechanisms, but there are many other specialized tools in C# to control thread concurrency.

4. How do you make thread-safe code?

With interview questions of this nature, you need to draw on your knowledge and experience.

There are many ways to make your code thread-safe. The most important approach, however, is to avoid the need for thread-safe code. That will save you from some future headaches.

If we attempt to avoid shared state and mutable structures, and use pure functions most of the time, the code will be automatically thread-safe.

There are certain situations in which we cannot use immutable structures due to factors like memory restrictions. To overcome these situations, we can use Concurrent Collections such as ConcurrentDictionary, or use adequate locking mechanisms as explained in the previous question.

5. What are the differences between class and struct?

This is one of the senior .NET developer questions that makes its way into many interviews. One of the primary reasons it appears so frequently is that even some senior developers cannot answer it correctly, allowing the interviewer to identify the best candidates with greater ease.

There are many differences. The most important ones that you want to include are:

  • Classes are passed by reference while structs are passed by value.
  • Classes are hosted on the heap, and structs are allocated on the stack.
  • Structs are more limited than classes.
  • Class objects are created using the new keyword, while struct instances are not.
  • You can assign a null value to a class variable, but you cannot do this for a struct variable.
  • You can have two class variables pointing to the same object. If you have two struct variables, however, they are entirely different structures even though their data is the same.
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6. What are the differences between class and record in C#?

When you answer senior .NET developer interview questions that ask about differences, you need to focus on your knowledge. Your answer can also reference your experience if you want to discuss use cases. Doing so can help you respond to questions like these in a more compelling way.

The main difference between class and record type in C# is that a record’s primary purpose is storing data, while a class defines responsibility. Additionally, records are immutable, but classes are not.

Other differences between class and record type include:

  • We define records using the record keyword instead of the class keyword.
  • Records should not have any state changes after instantiation, while classes change properties (of course, we could have record state changes, but doing so would lose the full benefit of immutability).
  • We create new records from existing ones when we want to change state with a keyword. With classes, we modify the existing instances.
  • Records automatically define Equals, ToString, and GetHashCode methods.
  • When comparing two different records, value comparison is used, while classes use reference comparison.
  • Records are a better choice when defining Database Rows, Value Objects, or Data Transfer Objects; pure value objects that don’t contain logic at all or contain very small chunks of logic (more helper methods than business ones).

7. Endpoint response time troubleshooting

Here’s the question:

An endpoint returns a result of 5 seconds, but the requirement says 2 seconds is a max. How are you going to troubleshoot it?

This is one of the common senior .NET developer interview questions that require the developer to have some hands-on experience. Mainly, this is because questions like these have no exact answer. Luckily, you can follow this checklist when facing troubleshooting questions of this nature to create an answer that’s thorough and impressive:

  • Break it down: Systems are composed of different subsystems, so we need to break them down and detect which one is causing the slowness. One example: for a classic web application, we have a frontend app, network latency, backend app, external services, and databases, so we need to identify which component is slow.
  • Measure it: Once we have detected the component that is delaying our processes, we need to understand how long it is taking to do its job. To do that, we could use code profilers, database profilers, and logging. Once we are able to measure the slowness, we need to ask: is the component slow with all the invocations or just specific ones?
  • Narrow down the options: Once we realize which component is slow, how slow it is, and whether the slowness is tied to a specific case or is experienced generally, we have effectively narrowed our options:

If slowness is general: This may affect the current environment. Check things like Processor usage, Memory usage, Number of requests per second, and any other infrastructure-related problem. General slowness can also be caused by flaws in the architecture and components not scaling correctly.

If slowness is specific: We need to identify which parameters are causing the service to be slow. Normally, we do this by checking logs. Once we have isolated the offending parameters, we can create specific tests to replicate the issue in a controlled environment, and perform code changes or database index tuning based on the test results.

8. How does the lazy loading approach work?

Here’s another knowledge-based senior .NET developer interview question. With your answer, you’re mainly describing how the lazy loading approach functions. Questions like this also create an opportunity for you, since your response can include references to your past use of lazy loading if you want to personalize it a bit.

Lazy loading is an approach that helps us defer the loading of an object until the point when we need it, and then store it in memory.

Some advantages of using lazy loading include:

  • Minimizing the start-up time of the application
  • Enabling applications to consume less memory until the moment when they need to load an object
  • Reducing the number of external services or database calls because lazy loading acts as a small in-memory cache
  • Preventing other threads from entering the object initialization code at the same time, making it thread-safe (note: a loaded object can be thread-unsafe)

Of course, this can add complexity to our code, so it is a tool that should only be used to solve specific problems.

9. What is the difference between async calls and parallel calls?

Interviewers ask this or a similar interview question for several reasons. One of the main reasons is that these are concepts that are frequently mixed up or misunderstood, and some people answer that async code runs in parallel, which is not always the case.

The main difference lies in how we control the task awaiters. If we’re launching async code instructions and not awaiting them, then we’re effectively loading them in parallel:

However, there is a problem with this code because we don’t know when the method executions are finished, rendering the result variables useless. Fortunately for us, C# has many tools that help control the asynchronous flow of method calls.

Let’s say we have methods A, B, and C, and we want to send A and B in parallel, invoking C after that since it depends on the A and B results. In this situation, we can use Task.WaitAll to effectively wait until A and B are finished, and then launch method C.

Note that there is no guarantee from the .NET framework that it will execute tasks A and B in parallel. Everything is managed by the Task model, and it could be busy with other tasks.

Finally, there are other options in C# to manage this scenario, such as Task.WhenAll, Parallel class, or async for each.

10. How do you create a LINQ extension method?

Since this is one of the more technical senior .NET developer interview questions, you’ll need a highly detailed response. In some cases, you may even need to create code during the interview to answer questions of this nature, though that isn’t always the case.

Extension methods are really useful because they allow you to add methods to an existing class without modifying it.

To go further in this explanation, we can also use extension methods over generic types or base interfaces, which will add the new method to any object matching the generic or the instance signature.

LINQ methods normally work with IEnumerable<T> or IQueryable<T> generic interfaces, which are implemented by all common collections in the .NET framework, such as arrays, lists, dictionaries, hashsets, etc.

On top of that, most LINQ methods are chainable, since the return type is the same. In the case of IEnumerable<T>, it takes one or many predicates as a parameter, so the logic is customizable.

When we combine all these concepts, creating a new LINQ method is straightforward. For example, if we want a method that creates a Dictionary but updates records if it finds duplicates instead of throwing exceptions, the code could be:

Final tips

Reviewing these senior .NET developer interview questions and taking time to practice your answers is a solid starting point. Along with showing you what questions you can expect to face, the list above can give you insights into what your responses need to cover.

Simply reviewing senior .NET developer questions isn’t enough. You’ll also want to prepare in other ways to ensure that your interview goes smoothly. So, make sure you:

  • Review the fundamentals of any programming language, skill, platform, tool, or solution you may need to discuss during the meeting.
  • Select interview-appropriate attire in advance, and take a moment to ensure that your internet connection is reliable to maintain a video call.

By following the steps above, you’re going the extra mile to set yourself up for success. Ultimately, doing so increases your odds of creating exceptional answers to the interview questions, exceeding expectations throughout the hiring process, and securing a job offer that can boost your career.

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