83. Generic supertypes#

In addition to generic classes, we can also define and implement generic interfaces as well as inherit from generic classes. The same principles apply. In this chapter we’ll focus on generic interfaces.

Key point

  • If the subtype is generic then the constructed generic supertype can remain “open” (e.g. Cycle<T> : ISequence<T>).

  • If the subtype is non-generic then the constructed generic supertype must be “closed” (e.g. Alphabet : ISequence<char>.

When implementing a generic interface or inheriting from a generic type then we must construct the generic supertype if we want to create a non-generic type.

Let’s imagine that we want to model the idea of different sequences. Some classic sequences include the sequence of natural numbers, the alphabet, and the Fibonacci sequence. But we can easily imagine more esoteric sequences like the sequence of customers entering a cafe in a simulation game. What unifies these different types is the idea of being able to ask for the next element in the sequence.

Implementing generic interfaces#

We’ll start by creating a generic interface called ISequence<T>. Any class implementing the interface must define a method T Next() which simply provides the next element of type T in the sequence. Let’s also add a default method T[] Take(int n) that calls Next n number of times and returns the result as an array of elements of type T.

interface ISequence<T>
    // Should return the next element in the sequence.
    T Next();

    // Returns an array of the next n elements from the sequence.
    T[] Take(int n)
        T[] output = new T[n];
        for(int i=0; i<n; i++)
            output[i] = Next();
        return output;

In the code below we indicate that the non-generic classes NaturalSequence and Alphabet implement the constructed generic interfaces ISequence<int> and ISequence<char> respectively.

class NaturalSequence : ISequence<int>
    int current = 0;

    // Returns the next natural number.
    public int Next()
        => current++;
class Alphabet : ISequence<char>
    int i = -1;
    char[] letters = new char[] { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' };

    // Returns the next letter in the alphabet, cycling back to 'A' after 'Z'.
    public char Next()
        => letters[i = (i + 1) % letters.Length];

Since our subtypes of the generic interface are non-generic we have to “close” the constructed generic supertype. In other words, we cannot say that NaturalSequence or Alphabet implements the interface ISequence<T>. If we could, then there would be no way for the user of our subtype to specify what T should be.


While Alphabet is a subtype of ISequence<char> it is not a subtype of ISequence<T>. This means you can’t use an instance of type Alphabet when an ISequence<T> is expected but you can use it when an ISequence<char> is expected.

Since the classes NaturalSequence and Alphabet are both non-generic there’s no need to construct them when we want to use them. They are both already actual types.

ISequence<int> naturals = new NaturalSequence();
Console.WriteLine(String.Join(", ", naturals.Take(10)));
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
ISequence<char> alphabet = new Alphabet();
Console.WriteLine(String.Join(", ", alphabet.Take(40)));
A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z, A, B, C, D, E, F, G, H, I, J, K, L, M, N

If however we want a generic subtype which is parameterized over the same type as the supertype, then we don’t have to “close” the constructed generic supertype. In the code below we’re defining a generic class called Cycle<T> that implements the interface ISequence<T>.

class Cycle<T> : ISequence<T>
    int i = -1;
    T[] elems;

    public Cycle(T[] items)
        => elems = items;

    public T Next()
        => elems[i = (i + 1) % elems.Length];

Since the class Cycle<T> is a generic class we must construct it in order to create an actual type. In the examples below we’re using it to create a cycle of characters and a cycle of strings.

ISequence<char> rgb = new Cycle<char>(new char[] { 'R', 'G', 'B' });
Console.WriteLine(String.Join(", ", rgb.Take(10)));
R, G, B, R, G, B, R, G, B, R
ISequence<string> light = new Cycle<string>(new string[] { "Red", "Orange", "Green", "Orange" });
Console.WriteLine(String.Join(", ", light.Take(10)));
Red, Orange, Green, Orange, Red, Orange, Green, Orange, Red, Orange


Any constructed generic type from Cycle<T> (such as Cycle<int>) will be a subtype of any constructed generic type from ISequence<T> if the type argument used in place of T is the same (such as ISequence<int>).

See also

There’s more nuance to substitutability in generic interfaces but we’ll explore that in the chapter on variant generic interfaces.

Inheriting from generic classes#

Now that we’ve grasped the principles behind implementing generic interfaces, let’s pivot our attention to inheritance in generics. Inheriting from a generic class works just like inheriting from a non-generic class except that if the subclass is non-generic then the constructed generic superclass must be “closed”.

It’s the same logic as when implementing generic interfaces. If the subclass is generic and we pass the type parameter as a type argument to the construction of the generic superclass then we can keep the generic superclass “open”.

Meaning we don’t have to use an actual type (like string) as a type argument for the superclass if the type is also a type parameter in the subclass. However, if we don’t use our type parameters as type arguments in the super class then we must of course close those parameters of the constructed generic super class.

The generic class MonoPair<T> inherits from the generic class Pair<T1, T2>. It does not change or add any behavior but merely restricts users of the class to a single type parameter which means that all pairs must have the same type in both positions.

class Pair<T1, T2> {
    public T1 Item1 { get; init; }
    public T2 Item2 { get; init; }

    public Pair(T1 item1, T2 item2) {
        Item1 = item1;
        Item2 = item2;
class MonoPair<T> : Pair<T, T> {
    public MonoPair(T item1, T item2)
        : base(item1, item2) { }


In the definition of MonoPair<T> above, the T on the left-hand side is a type parameter, while the two T’s on the right are type arguments.

On the left-hand side we declare that the generic class MonoPair<T> is a generic type parameterized over one type that we call T. On the right-hand side however we use T as the type argument for both generic type parameters in the generic type Pair<T1, T2> so that whatever T is it is used as the type for both T1 and T2.

Inheriting from generic interfaces#

It’s worth noting that it’s also possible to use generic types in interface inheritance. The same rules apply.

Any type parameter of the constructed supertype that isn’t matched with a type parameter of the generic subtype must be “closed”.

We must supply type arguments for all type parameters of the inherited interfaces and if our inheriting interface is generic then those type arguments can be type parameters of the inheriting interface.

In the example below we keep the constructed generic supertype open.

interface ISequence<T>
    T Next();

interface ICycle<T> : ISequence<T>
    void Reset();

In the example below however we close the constructed generic supertype.

interface ICharSequence : ISequence<char>
    void SetUpperCase(); // Changes to uppercase.
    void SetLowerCase(); // Changes to lowercase.


In this chapter, we’ve delved generic interfaces and inheritance, highlighting the nuances between non-generic and generic subtypes. As we look ahead, we’ll delve deeper into the world of variance and explore variant generic interfaces, which offers even greater flexibility when subtyping constructed generic types. But before that, our journey will take us to generic methods and type constraints. See you in the next chapter.