How and when to apply business rules?

Question

Hi,

Suppose I have a service StateService which has a method ChangeState.

ChangeState(State toState, DomainObject object)

I have business rules that checks whether the destination state is valid or not in the domain objects current "state", how can I technically check those rules without first setting the toState on the domain object? It feels wrong to first set the new state, run validation and if one or more rule breaks unset the state.

One solution I came up with is to create some context object that drives the validation eg. ChangeStateContext that contains the DomainObject along with the State that is to be set.

Another related question is how to report back from the ChageState call how it went?
I can collect all validation rules that broke and throw an exception with those rules that the caller can catch and handle accordingly or I can add a return type on the ChangeState-method like ValidationSummary that contains information about broken rules and such. What is best practices in these cases?

Answer 1

Can't you throw an exception from the ChangeState method when the state transformation is invalid? You can throw specific exceptions such as StateTransformationException or ValidationException that you can catch higher on the call stack. You can optionally include extra properties to these exception types so that you can communicate very precisely what went wrong to the user.

When you want to call multiple ChangeStates after a single user action, you will need a way to revert or rollback. What I usually do is using the unit of work pattern (supplied to me by LINQ to SQL and Entity Framework) and change all state within this unit of work. When an exception is thrown I throw away the complete unit of work with all its changes.