如何以不那么繁琐的方式编写带有约束的通用接口的扩展方法

Question

interface interface1<Coordinate> where Coordinate : IComparable<Coordinate>
{
    Coordinate getPosition(XYZ p);
}

static class CreateOrderOnASpaceExtensions
{
    public static IOrderedEnumerable<T> OrderElements<Coordinate, T>(this interface1<Coordinate> myInterface, IEnumerable<T> elements, Func<T, XYZ> elementToPoint)
        where Coordinate : IComparable<Coordinate>
    =>
    elements.OrderBy(
        x => myInterface.getPosition(elementToPoint(x)),
        Comparer<Coordinate>.Create((x, y) => x.CompareTo(y)
        )
    );
}

Should I really write where Coordinate: IComparable<Coordinate> in the extension method declaration?

I already said that Coordinate is comparable when I created interface1 .

Without that:

There is no boxing convertion or type paramater convertion from Coordinate to Comparable

Answer 1

I'll use === to show to which part the where applies in each of your generic definitions:

interface interface1<Coordinate>
                     ==========
                     where Coordinate : IComparable<Coordinate>
{
    ...
}

and

static class CreateOrderOnASpaceExtensions
{
    public static IOrderedEnumerable<T> OrderElements<Coordinate, T>(this interface1<Coordinate> myInterface, IEnumerable<T> elements, Func<T, XYZ> elementToPoint)
                                                      ==========
                                                      where Coordinate : IComparable<Coordinate>
}

Without the 2nd where , the compiler will see that you are trying to feed an unconstrained type Coordinate (which could then be anything) into interface1<> , which has a constraint , making the unconstrained Coordinate not allowed.

The fact that interface1 has a constraint does not mean that the "world" outside of it will magically also be restricted if you try to connect it; it already needs to fit before it can be allowed.

An analogy: you made a railroad which requires trains. You first need to make a train, and then you can put in on the railroad. There's no magic that makes any vehicle that you put on it a train.