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What are generics and why use them? What problem do generics solve compared to writing a method or class that accepts 'object'? How do you declare a generic class with a type parameter, and how do you instantiate it with a specific type? What are generic constraints and why are they needed? Why can't you use operators like + directly on an unconstrained generic T? How do generic type constraints ('where T : ...') restrict what types can be used, and what are the common constraint kinds? How does generic type inference work when calling a generic method, and when do you need to specify the type argument explicitly? What is covariance ('out T') and contravariance ('in T') in generic interfaces, and how does IEnumerable<out T> demonstrate covariance? What are generics in C#, and what specific problems do they solve compared to writing type-specific code? How are generics implemented for value types vs reference types in the CLR?
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What is covariance and contravariance in generics?

Intermediate

Answer

out (covariant) lets you use a more-derived type where a less-derived is expected (IEnumerable<out T>); in (contravariant) lets a less-derived be used where more-derived is expected (IComparer<in T>).
IEnumerable<object> objs = new List<string>(); // covariance
Real-world example Returning IEnumerable<Animal> from a method that actually yields Dogs.
All 17 Generics questions