TL;DR: Immutable objects can’t be changed after creation. In C#, this can make your code safer, easier to test, and bug-resistant, especially in multithreaded or async scenarios. Have you ever had a bug where some object mysteriously changed its value? Or spent hours debugging a weird race condition? Immutability might be the solution you need. In simple terms, immutable objects can’t be changed after they’re created. Instead of modifying an existing object, you create a new one with the updated values. It’s like the difference between editing a document and making a new copy with your changes. ...

Constructor chaining in C# reduces code duplication by allowing one constructor to call another. Learn how to use this technique effectively with examples, best practices, and when to avoid it.

TL;DR: A deadlock happens when threads block each other, waiting for resources that never become available. Most deadlocks in C# are caused by inconsistent lock ordering or mixing sync and async code. Recognize deadlocks by symptoms like app hangs, high thread count, or timeout exceptions. Prevent deadlocks by always acquiring locks in a consistent order, minimizing lock duration, and using lock timeouts. Prefer higher-level synchronization tools like SemaphoreSlim or concurrent collections to reduce risk. In async code, avoid blocking calls like .Wait() or .Result - use await all the way. Use debugging tools and thread dumps to detect and analyze deadlocks in production. Design your multithreaded code with prevention in mind; fixing deadlocks after deployment is much harder. A deadlock is one of the hardest bugs to solve in a multithreaded C# application. It doesn’t cause a crash or an obvious exception; it just brings your application to a silent, grinding halt. If you’ve ever seen a service become completely unresponsive under load for no apparent reason, a deadlock might be the culprit. ...

TL;DR Learn how to build custom route constraints in ASP.NET Core using IRouteConstraint. Use them to validate route parameters (like ensuring only alphabets or specific patterns), inject services like IUserService, and simplify controller logic. Bonus: works with minimal APIs, conventional routes, and supports DI in constraints via the new ASP.NET Core routing system. URL routing is a fundamental part of ASP.NET Core applications, determining how incoming requests are mapped to controller actions. While the framework provides numerous built-in route constraints like int, bool, and guid, there are situations where you need more sophisticated validation rules for your URL parameters. This is where custom route constraints become invaluable. ...

TL;DR: DIP (Dependency Inversion Principle): High-level modules should not depend on low-level modules; both depend on abstractions. DI (Dependency Injection): A technique to supply dependencies from outside a class, improving testability and flexibility. IoC (Inversion of Control): A broader concept where control of object creation and dependency resolution is delegated to a container or framework. Together, they enable decoupled, maintainable, and testable applications. DIP is a design principle, DI is a pattern, IoC is the overarching concept. I bombed this question in an interview once. ...