Type-safe abstractions without runtime overhead.

C# Generics — Constraints, Variance, Generic Math

C# generics enable type-safe, reusable code without boxing overhead. Generic constraints (where T : IComparable<T>) let you call methods on T. Covariance (out T) and contravariance (in T) control assignability of generic interfaces. .NET 9 Generic Math (INumber<T>) lets you write arithmetic algorithms once for all numeric types. The "allows ref struct" anti-constraint opens generics to Span<T> and ref structs.

Interactive Pipeline Flow

FoundationCore LogicProduction

🔤 Type Param — <T, TKey, TValue>

Declare type parameters in angle brackets: class Repository<T>. Multiple parameters: IDictionary<TKey, TValue>.

Architecture Diagrams

Generic Math with INumber<T>: one method for all numeric types.

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Key Design Decisions

⚡Value types: zero boxing with generics

List<int> stores ints directly on the heap array — no boxing. Compare with ArrayList which boxes every int to object.

📐Prefer IEnumerable<out T> for read-only APIs

Covariance lets callers pass IReadOnlyList<Derived> where IEnumerable<Base> is expected. Use out T in interface return positions.

🔢Generic Math eliminates numeric overloads

Replace Sum(int[]), Sum(double[]), Sum(decimal[]) with a single Sum<T>(T[]) where T : INumber<T>.

Discussion