Unhandled exception. System.TypeLoadException: A ByRef-like type cannot be used as the type for an instance field in a non-ByRef-like type.

RefStructProblem.cs

var line = new Line(new PointStruct(1, 2), new PointStruct(3, 4));
line.Start.Swap();
Console.WriteLine(line);

record struct Line {

    private ref PointStruct _start;
    private ref PointStruct _stop;

    public ref PointStruct Start => ref _start;
    public ref PointStruct Stop => ref _stop;

    public Line(PointStruct start, PointStruct stop) {
        _start = start;
        _stop = stop;
    }
}

record struct PointStruct {

    public int X;
    public int Y;

    public PointStruct(int x, int y) {
        this.X = x;
        this.Y = y;
    }

    public void Swap() => this = new PointStruct(this.Y, this.X);

    public override string ToString() => $"({X},{Y})";
}