Is this a violation of the Liskov Substitution Principle?
- by Paul T Davies
Say we have a list of Task entities, and a ProjectTask sub type. Tasks can be closed at any time, except ProjectTasks which cannot be closed once they have a status of Started. The UI should ensure the option to close a started ProjectTask is never available, but some safeguards are present in the domain:
public class Task
{
public Status Status { get; set; }
public virtual void Close()
{
Status = Status.Closed;
}
}
public ProjectTask : Task
{
public override void Close()
{
if (Status == Status.Started)
throw new Exception("Cannot close a started Project Task");
base.Close();
}
}
Now when calling Close() on a Task, there is a chance the call will fail if it is a ProjectTask with the started status, when it wouldn't if it was a base Task. But this is the business requirements. It should fail. Can this be regarded as a violation?
