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Generics

Definition​

Generics allow you to define type-parameterized classes, interfaces, methods, and delegates. By introducing type parameters (e.g., <T>), you write code that works with any data type while preserving compile-time type safety and avoiding the performance cost of boxing/unboxing.

Why Generics Exist​

Problem without GenericsSolution with Generics
ArrayList stores object — boxing value types causes overheadList<T> stores values directly — no boxing
Casting from object can fail at runtimeType errors are caught at compile time
Duplicate code for each data typeOne implementation for all types
// Without generics — runtime risk, boxing overhead
ArrayList list = new ArrayList();
list.Add(42);          // boxes int to object
int value = (int)list[0]; // unboxes, runtime cast

// With generics — compile-time safety, no boxing
List<int> list = new List<int>();
list.Add(42);          // no boxing
int value = list[0];   // no cast needed
Key Benefits
  • Type safety at compile time
  • Code reuse without duplication
  • Better performance — no boxing for value types
  • Self-documenting code via explicit type parameters

Core Concepts​

Generic Classes​

Generic classes use one or more type parameters in their definition. The .NET Base Class Library ships with many generic collections.

// Built-in generic classes
List<string> names = new List<string> { "Alice", "Bob" };
Dictionary<string, int> ages = new Dictionary<string, int>
{
    ["Alice"] = 30,
    ["Bob"] = 25
};

// Custom generic class
public class Repository<T>
{
    private readonly List<T> _items = new();

    public void Add(T item) => _items.Add(item);

    public T? FindById(Func<T, bool> predicate) =>
        _items.FirstOrDefault(predicate);

    public IReadOnlyList<T> GetAll() => _items.AsReadOnly();
}

Generic Methods​

Methods can introduce their own type parameters independent of the containing class. The compiler often infers the type arguments from usage.

public class Utilities
{
    // Generic method with type inference
    public T GetDefault<T>() => default(T);

    // Explicit type arguments when inference is not possible
    public T ConvertTo<T>(object value) => (T)value;

    // Generic method with constraint
    public T Max<T>(T a, T b) where T : IComparable<T>
        => a.CompareTo(b) >= 0 ? a : b;
}

// Usage
Utilities util = new Utilities();
int max = util.Max(10, 20);             // type inferred as int
string def = util.GetDefault<string>();  // explicit — returns null

Generic Interfaces​

// Built-in
public interface IComparable<T>
{
    int CompareTo(T? other);
}

// Custom generic interface
public interface IRepository<T> where T : class
{
    T GetById(int id);
    void Add(T entity);
    void Delete(T entity);
    IEnumerable<T> GetAll();
}

public class InMemoryRepository<T> : IRepository<T> where T : class
{
    private readonly List<T> _store = new();
    public T GetById(int id) => throw new NotImplementedException();
    public void Add(T entity) => _store.Add(entity);
    public void Delete(T entity) => _store.Remove(entity);
    public IEnumerable<T> GetAll() => _store;
}

Generic Delegates​

// Built-in generic delegates
Func<int, int, int> add = (a, b) => a + b;           // returns int
Action<string> log = msg => Console.WriteLine(msg);   // returns void
Predicate<int> isEven = n => n % 2 == 0;              // returns bool

// Custom generic delegate
public delegate TResult Transformer<TInput, TResult>(TInput input);

Constraints​

Constraints restrict the types that can be used as type arguments, enabling the compiler to know what operations are available.

ConstraintDescription
where T : structT must be a value type
where T : classT must be a reference type
where T : new()T must have a parameterless constructor
where T : BaseClassT must derive from BaseClass
where T : IInterfaceT must implement IInterface
where T : unmanagedT must be an unmanaged type (no reference fields)
where T : notnullT must be a non-nullable type
where T : EnumT must be an enum
// Multiple constraints
public class Factory<T> where T : class, new()
{
    public T Create() => new T();
}

// Constraint on multiple type parameters
public class Pair<TFirst, TSecond>
    where TFirst : struct
    where TSecond : class
{
    public TFirst First { get; }
    public TSecond Second { get; }
}

Default Value Expression​

T value = default(T);  // null for reference types, zero-initialized for value types

// In generic context
public T? FindOrDefault<T>(IEnumerable<T> source, Func<T, bool> predicate)
{
    foreach (T item in source)
        if (predicate(item)) return item;
    return default;  // default(T) — shorthand
}

Covariance and Contravariance​

Variance annotations allow generic type parameters to be used polymorphically.

// Covariance (out) — type can only appear as output
IEnumerable<Derived> derived = new List<Derived>();
IEnumerable<Base> bases = derived;  // valid because IEnumerable<out T>

// Contravariance (in) — type can only appear as input
IComparer<Base> baseComparer = new BaseComparer();
IComparer<Derived> derivedComparer = baseComparer;  // valid because IComparer<in T>

// Custom covariant interface
public interface IProducer<out T>
{
    T Produce();
}

// Custom contravariant interface
public interface IConsumer<in T>
{
    void Consume(T item);
}
Variance Limitations
  • Variance only applies to interfaces and delegate types, not classes or structs.
  • out parameters are output-only; in parameters are input-only.
  • Value types do not support variance — IEnumerable<int> cannot be assigned to IEnumerable<object>.

Generic Type Inference​

The compiler can infer type arguments from method arguments, making calls cleaner.

// Compiler infers T from arguments
var result = Max(3, 5);           // T inferred as int
var names = new[] { "a", "b" };
var first = names.FirstOrDefault(); // T inferred as string

When to Use​

  • Collections and data structures — any time you need a container that works with multiple types.
  • Repositories and services — IRepository<T> pattern for data access layers.
  • Utility and helper methods — swap, max, min, convert, and other type-agnostic operations.
  • Event handlers and callbacks — EventHandler<TEventArgs>, Func<T>, Action<T>.
  • Avoid when there are only one or two concrete types and no reuse benefit — simplicity wins.

Common Pitfalls​

Over-Constraining​

// Bad — too restrictive, limits reusability
public class Cache<T> where T : class, IEntity, IComparable, new()

// Better — only constrain what you actually need
public class Cache<T> where T : class

Generic Type Casting​

// This does NOT compile — cannot cast to type parameter
T Convert(object value) => (T)value;

// Correct approach
T Convert<T>(object value) => (T)(dynamic)value;
// Or use Convert.ChangeType
T Convert<T>(object value) => (T)System.Convert.ChangeType(value, typeof(T));

Reflection with Generics​

// Getting the open generic type
Type openType = typeof(List<>);
Type closedType = openType.MakeGenericType(typeof(int));

// Checking generic type at runtime
if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(List<>))
{
    Type elementType = type.GetGenericArguments()[0];
}

new() Constraint Limitations​

// new() only works with public parameterless constructors
public T Create<T>() where T : new() => new T();

// Cannot use new() with types that have required parameters or internal constructors
new() Constraint

The new() constraint requires a public parameterless constructor. It cannot be satisfied by types with only internal/private constructors or types with required constructor parameters.

Key Takeaways​

  • Generics provide type safety, code reuse, and performance by eliminating boxing and runtime casts.
  • Use constraints to restrict type parameters and access specific members.
  • Covariance (out) and contravariance (in) enable polymorphic usage of generic interfaces.
  • The compiler can often infer type arguments — prefer letting it do so for cleaner code.
  • Avoid over-constraining; only apply constraints your implementation actually needs.

Interview Questions​

Q: What are generics and why use them? Generics let you define type-parameterized types and methods. They provide compile-time type safety, eliminate boxing/unboxing for value types, and enable reusable code without duplication.

Q: What are generic constraints? Constraints (where T : ...) restrict which types can be used as arguments, allowing the compiler to verify that specific members (methods, constructors, properties) are available on T.

Q: What is covariance and contravariance? Covariance (out) lets a generic interface accept more derived types as outputs. Contravariance (in) lets it accept more base types as inputs. Both enable polymorphic assignment of generic types.

Q: What is the difference between List<T> and ArrayList? ArrayList stores object, causing boxing for value types and requiring runtime casts. List<T> is generic — it stores values directly with no boxing and provides compile-time type safety.

Q: Can you inherit from a generic type? Yes. You can inherit from a closed generic type (class MyList : List<int>), or keep the parameter open (class MyList<T> : List<T>). You can also constrain the type parameter in the derived class.

References​