How to inform the controller why the object creation has failed so an appropriate response can be returned?

Question

In my MVC web application the controller calls a service layer which takes any relevant request parameters and builds the model for the view e.g.

Controller

var model = _modelService.Build(id);
return View("ViewName", model);

If there are multiple reasons why the model service will fail to build a valid model, how do I communicate that to the controller so it can return an appropriate response?

Below is a contrived example.

Model Service

var entity = _dataService.Get(id);
if (entity == null) {
  return null;
}

var entityMeta = _dataService.GetMeta(id);
if (entityMeta == null) {
  return null;
}

return new Model(entity);

Controller

var model = _modelService.Build(id);

// Null because of invalid id
if (model == null) { 
  return View("Invalid Id");
}

// Null because of missing meta data
if (model == null) {
  return View("Invalid meta data");
}
return View("ViewName", model);

In the example above there are two reasons why the model is null, but there is no way to inform the controller what has happened because all it gets is a null object back.

I can think of some possible solutions: throw an exception (which doesn't feel right - this isn't necessarily an unexpected outcome), move some of this logic to the controller (the general advise is to keep controllers slim), create a generic ModelResult object which contains the model or some information about why it has failed etc.

I feel like this is a common situation so there must be a standard pattern to apply.

Answer 1

The simplest way is to beef up the service layer. So instead of the controller puzzling out the reason, you can have the result and errors/messages returned from the service layer.

var entity = _dataService.Get(id);
if (entity == null) {
  return null;
}

var entityMeta = _dataService.GetMeta(id);
if (entityMeta == null) {
  return null;
}

return new EntityOperationResult 
{
    Entity = entity,
    Messages = new List<Message>(),
};

Where an error might look like:

return new EntityOperationResult 
{
    Entity = null,
    Messages = new List<Message>() { new Message("Invalid foobar") }
};

This way, the service layer deals entirely with the entities and returns validation messages, but the controller can puzzle out what happened and instantiate the model from the entity.