Can interfaces have private methods (C# 8+)?

Short answer: If so, why? Yes, C# 8 introduced private methods in interfaces. Purpose: share implementation among default interface methods without exposing them publicly. interface ILogger { void Log(string message) => LogInternal(message); private void LogInternal(string msg) => Console.WriteLine(msg); }

Example code

If so, why? Yes, C# 8 introduced private methods in interfaces. Purpose: share implementation among default interface methods without exposing them publicly. interface ILogger
{
void Log(string message) => LogInternal(message);
private void LogInternal(string msg) => Console.WriteLine(msg);
}

Real-world example (ShopNest)

ShopNest’s Order keeps _items private and exposes AddItem() so totals stay correct—callers cannot put a negative quantity directly into the list.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.