从父类型实例化一个孩子

Question

I have a base type which stores information about a question in a question pool for a system which generates practice question sets to help people study for multiple choice tests. The only information that is stored in it are the answers, the prompt and question number. When I create a practice test, I need to augment the type with some properties to store the answer submitted (the test can be taken in parts), and so I created the following classes:

public class MultipleChoiceQuestion
{
    public Int32 Number { get; internal set; }
    public String Prompt { get; internal set; }
    public MultipleChoiceAnswer[] Choices { get; internal set; }

    // constructors, etc...
}

public class PracticeTestQuestion : MultipleChoiceQuestion
{
    public MultipleChoiceAnswer AnswerSelected { get; set; }

    // is this right?
    public PracticeTestQuestion(MultipleChoiceQuestion question)
    {
        ...
    }
}

Originally I had the MultipleChoiceQuestion as just a member of PracticeTestQuestion, but it added a lot of extra dots in my property accessors, and so I changed it to inherit the class as listed above. Currently I am assigning all of the properties line for line in the constructor, and but it feels sort of cumbersome, and I was wondering if there is a better way.

The C# compiler doesn't like upsizing types for good reasons, so my question is what is the best way to go about instantiating my PracticeTestQuestions from their MultipleChoiceQuestion base types?

Answer 1

I would add a constructor to MultipleChoiceQuestion that takes another MultipleChoiceQuestion . If necessary it can assign each of the properties in the constructor, but that's more appropriate, since it has the knowledge of its own properties.

Then in PracticeTestQuestion , you can just do:

public PracticeTestQuestion(MultipleChoiceQuestion question) : base(question) { }

and be done with it.

Answer 2

My first gut reaction is to use a factory to create questions. You ask the factory for a MCQ or a PTQ and it creates the right one.

This is also more extensible to essay questions, true false, etc.