How do generics and constraints work in C#?
Assesses fundamental understanding of C# conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Generics let you write type-safe, reusable code without boxing or casts. The type parameter is resolved at compile time, and the JIT specialises for value types while sharing code for reference types.
public T Max<T>(T a, T b) where T : IComparable<T>
=> a.CompareTo(b) > 0 ? a : b;
public class Repo<T> where T : class, new() { }
Constraints include where T : class, struct, new(), a base class, one or more interfaces, notnull and unmanaged. Multiple constraints are separated by commas and new() must be last. Without a constraint you can only use members of object, or use EqualityComparer<T>.Default.
Covariance (out) and contravariance (in) on generic interfaces and delegates let IEnumerable<string> be used as IEnumerable<object>. Generics improve performance by avoiding boxing and improve safety by removing casts.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.