Why strings are immutable, what that costs in loops, when to use StringBuilder, interpolation, correct string comparison, and null checks.
The four pillars of OOP in C#, overloading versus overriding, virtual, override and new, abstract class versus interface, access modifiers, static members, and composition versus inheritance.
Records and value equality, struct versus class, init-only and required properties, pattern matching and switch expressions, and extension methods.
Array versus List, how a Dictionary works internally, why custom keys need Equals and GetHashCode, HashSet, the collection interfaces, and the modify-while-iterating exception.
What generics are, why List of T beats collections of object, generic constraints, and a short introduction to covariance.
What a delegate is, Func, Action and Predicate, lambdas and closures including the for-loop trap, events versus delegates, and how events cause memory leaks.
Stack and heap, how the garbage collector works with its generations and the large object heap, finalizers, and how managed code still leaks memory.
Dependency injection and service lifetimes (transient, scoped, singleton), the five SOLID principles with small examples, and unit testing with xUnit and the Arrange-Act-Assert pattern.
The 25 mistakes that cost freshers the most marks in C# and .NET interviews, grouped by topic, with what to say or write instead.
The 30 coding problems freshers get asked most, grouped by technique, with the idea, the complexity and a tested C# solution for each.
The 50 most-asked C# and .NET interview questions for freshers, grouped by topic and marked High, Medium or Low priority, each with a short answer you can say out loud.
The order SQL really evaluates a query in, how NULL and three-valued logic change your results, ISNULL versus COALESCE, safe sorting and paging, and date-range filters.