Interfaces have always been a way to define a contract that classes must implement but was always limited to instance members.

In C# 11.0, you can now define static abstract or virtual members on an interface.

Code

Now

public interface IConcateable<T> where T:IConcateable<T>
{
    static abstract T Concat(T a, T b);
}

Before

public interface IConcateable<T> where T:IConcateable<T>
{
    T Concat(T other);
}

Notes

• This feature is part of the work in enabling generic math support

More information

• Tutorial