Advanced C# Generics: How Covariance and Contravariance Work in Practice

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A follow-up to an introductory post on C# generics, this piece addresses reader feedback requesting more senior-level content and a contravariant collection base class example. The article draws a clear distinction between merely using generics and intentionally designing with them. It explains that generics are invariant by default, meaning a List<string> cannot be assigned to a List<object>, even though string derives from object. Covariance, marked with the 'out' keyword, is safe when a type only produces values of T, while contravariance, marked with 'in', is safe when a type only consumes T. The post includes complete working code examples to illustrate each concept, including custom covariant and contravariant interfaces.

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