All 17 Generics questions
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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? What is covariance and contravariance in generics? 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? What problem do generic math / static abstract interface members solve?
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Why can't you use operators like + directly on an unconstrained generic T?

Intermediate

Answer

Because the compiler doesn't know T supports the operator. You must constrain T (e.g. to INumber<T>) or pass a delegate/strategy that performs the operation.
// error without constraint: T r = a + b;
T Add<T>(T a, T b) where T : INumber<T> => a + b;
Real-world example Generic aggregation helpers require a numeric constraint to add values.
All 17 Generics questions