Why is ITransformFactory.Build called when ITransformFactory.Validate returned false?

[Ghostbird]

What I would have expected is that it worked something like this pseudo-csharp:

// Build route transform usage information based on validation
var routeTransforms = routes
  .SelectMany((route) => route.Transforms
    .Select(transform => transformFactories
      .Where(factory => factory.Validate(new(route), transform.Values))
      .Select(factory => (route, factory, transform.Values))
    )
  );
// Build route transforms
routeTransforms.ForEach((route, factory, values) => factory.Build(new(route), values));

However instead the same progressive iteration is used for both Validate and Build. Which means that Build is called a lot with data that's not valid. Which means that each Build implementation has to reimplement the entirety of the Validate method. Which means that Validate might as well not be implemented and just return true.

Why is this so weird? Or do I fundamentally misunderstand something about how to use the transform factories?

[MihaZupan]

Which means that Build is called a lot with data that's not valid.

It's important to distinguish between "invalid data" and "data that doesn't apply to this factory".

Validate does two things:

• Decide whether the values apply to this factory at all (the returned bool)
• If it does, populate the errors list (on the context argument) for everything wrong

Build should check whether the values apply to this factory, and if yes, do something interesting.
YARP won't call Build if the factory applies, but the input is invalid.

So you don't have to reimplement Validate instead of Build - you can choose to assume that inputs are already valid. You do still have to check if the factory applies at all.

[Ghostbird]

I wrote a debug wrapper for the ITransformFactory and analysed how it worked through that. I now understand how it works, but I still think it results in too much repeated code. I still think a design whereby the conclusion whether an ITransformFactory applies to this route from the Validate call would determine whether Build is called at all would be more sensible, but I don't understand all the nuances of YARP and it might be that I've overlooked a valid use-case that makes the current implementation a better choice.