.NET Developer Interview Questions UK: 8 Real Questions and What Interviewers Want

Most .NET Developer interviews in the UK follow a pattern, and knowing it beats guessing. Below are eight questions from our bank of 25, each with a note on what the interviewer is really after.

Eight .NET Developer interview questions, and what the interviewer is thinking

1. What is the difference between .NET Framework, .NET Core and modern .NET, and which would you use for a new project?

Why they ask: They want to confirm you understand the platform landscape and make sensible choices.

2. Explain the difference between value types and reference types in C#, and what boxing is.

Why they ask: A staple technical question to test your grasp of the C# memory model.

3. How does async and await work in C#, and what are common pitfalls?

Why they ask: Asynchronous programming is central to modern .NET and a frequent interview topic.

4. What is dependency injection in .NET, and how does the built-in container handle service lifetimes?

Why they ask: DI is fundamental in ASP.NET Core, so they check your practical understanding.

5. How does Entity Framework Core work, and how do you avoid performance problems such as N+1 queries?

Why they ask: EF Core is the most common ORM in UK .NET roles, and performance knowledge separates levels.

6. A production API is responding slowly under load. How would you investigate and fix it?

Why they ask: They want to see systematic diagnosis rather than guessing.

7. Tell me about a time you improved the performance or reliability of a .NET application.

Why they ask: They want a real example with measurable impact.

8. What does a typical day as a .NET developer involve?

Why they ask: They want to confirm you understand the day-to-day reality of the role.

A sample answer to question 1

What is the difference between .NET Framework, .NET Core and modern .NET, and which would you use for a new project?

The .NET Framework is the original Windows-only platform, now in maintenance mode, still found in many legacy enterprise applications. .NET Core was the cross-platform, open-source rewrite, and from version 5 it was simply renamed .NET, so today we talk about .NET 8 as the long-term support release and .NET 10 as the newer LTS. For a new project I'd choose the current LTS version of .NET, because it is faster, runs on Windows, Linux and containers, is actively patched and has the latest C# features. I'd only use .NET Framework when maintaining an existing application that depends on Windows-only technologies such as WCF server or Web Forms, and even then I'd plan a gradual migration, using tools like the .NET Upgrade Assistant and the strangler pattern. I'd also pay attention to support dates, because running out-of-support runtimes is a security and compliance risk.

Common mistake to avoid: Recommending .NET Framework for new builds; Not knowing release cycle

Want the rest?

Full model answers, key points and common mistakes for all 25 questions are on the .NET Developer interview questions page. The first five are free, no sign-up needed.