Interfaces have always been a way to define a contract that classes must implement. Since C# 8.0 they could also contain static members with implementations, but there was no way to require implementing types to provide a static member.

In C# 11.0, you can now define static abstract or virtual members on an interface and call them through a type parameter.

Code

Now

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

static T ConcatAll<T>(IEnumerable<T> items) where T : IConcatable<T>
{
    T result = T.Empty;
    foreach (var item in items)
        result = T.Concat(result, item);
    return result;
}

Before

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

// No way to ask T for an empty value, so the caller must supply one
static T ConcatAll<T>(T empty, IEnumerable<T> items) where T : IConcatable<T>
{
    T result = empty;
    foreach (var item in items)
        result = result.Concat(item);
    return result;
}

Notes

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

More information

• Language reference
• Tutorial