Generics let you write classes and methods parameterized by a type, providing type safety and reusability without code duplication or casting. Constraints restrict what types a generic parameter can be, enabling safe use of their members. Generics are pervasive in C# (the entire collections library uses them).
=> arr[i];
=> arr[i];
x = Get([] { , , }, );
Generics move type errors from runtime (casting object) to compile time, and eliminate casts — safe, reusable code.
public class Box<T> // a generic class
{
private T _value;
public void Set(T value) => _value = value;
public T Get() => _value;
}
var box = new Box<string>(); // T = string, type-safe
// generic method
public T Max<T>(T a, T b) where T : IComparable<T> // constraint (see below)
=> a.CompareTo(b) > 0 ? a : b;
where T : class // T must be a reference type
where T : struct // T must be a value type
where T : new() // T must have a parameterless constructor
where T : IComparable<T> // T must implement an interface (so you can call its members)
where T : BaseClass // T must derive from BaseClass
// example: constraint lets you call CompareTo safely
public T Max<T>(T a, T b) where T : IComparable<T>
=> a.CompareTo(b) > 0 ? a : b; // CompareTo is available because of the constraint
Constraints (where T : ...) restrict what T can be, which lets you safely use that type's members (e.g. constraining to IComparable<T> lets you call CompareTo). Without a constraint, T could be anything, so you can't assume any members.
List<T>, Dictionary<K,V>, IEnumerable<T>, Task<T>, Nullable<T> ...
→ the entire collections library and much of .NET is generic.
Generics are fundamental to C# — they're used pervasively (the entire collections library, Task<T>, IEnumerable<T>, and countless APIs are generic), so understanding them is essential for any C# development.
Generics solve a real problem: writing reusable code that works for multiple types while preserving type safety — the alternative (using object and casting) sacrifices compile-time safety and requires error-prone casts, while duplicating code per type is unmaintainable.
Generics give you reusable, type-safe code with no casting and compile-time error checking. Constraints (where T : ...) are an important complement: they restrict what a type parameter can be, which both enables safe use of the type's members (constraining to an interface lets you call its methods) and ensures correct usage.
Understanding generic classes and methods, how type inference works, and constraints (class, struct, new(), interface/base-class constraints) is necessary for both using the generic-heavy .NET libraries effectively and writing your own reusable, type-safe abstractions.
Since generics underpin C#'s collections and much of the framework, and since they're the key to reusable type-safe code, mastering generics and constraints is core, frequently-applied knowledge for effective C# development and a common topic reflecting understanding of type-safe, reusable design.