call function of class on instance of class - function

Question

I have some code that generates answers based on the user input. But in somecases i need to update the values later by calling SetAnswers But when i compile my code i get the following error:
NullReferenceException: Object reference not set to an instance of an object
I get this error on the line marked by the arrow.
See below for my code:
public class Generate_Questions : MonoBehaviour{
public Question q5, q4;
void Start(){
q4 = create_question("Select object to edit", EXTERNAL);
Visual_Question vq1 = new Visual_Question(1, q4, new vector(1,1,1), Ui, Canvas);
vq1.draw_question();
}
void Update(){
}
public class Visual_Question : Generate_Questions{
public Visual_Question(int order_id, Question q, Vector2 loc, Dictionary<string, RectTransform> ui, RectTransform canvas){
}
public void draw_question(){
q4.SetAnswers(new Answer[]{ <--------- this generates the error.
new Answer(null, "Select an option")
});
}
}
public class Question{
public string text;
public int answers_loc;
public List<Answer> answers;
public Question(string q_text, int answers_loc){
answers = new List<Answer>();
this.text = q_text;
this.answers_loc = answers_loc;
}
public void SetAnswers(Answer[] c_answers){
foreach(Answer answer in c_answers){
this.answers.Add(answer);
}
}
public bool CheckIfAnswersAvailable(){
if(answers.Count > 0){
return true;
}else{
return false;
}
}
public int QuestionLocation(){
return answers_loc;
}
}
public Question create_question(string text, int a_type){
Question Q = new Question(text, a_type);
return Q;
}
public interface IAnswer{
string GetText();
string GetDataType();
object GetValue();
Question GetNextQuestion();
}
public class Answer : IAnswer{
public string text;
public Question next = null;
public int? action = null;
public Element obj = null;
public string property = null;
public float? value = null;
public Answer(Question next, string text){
this.text = text;
this.next = next;
}
public Answer(Question next, string text, Element obj, int? action){
this.action = action;
this.text = text;
this.next = next;
this.obj = obj;
}
public Answer(Question next, string text, Element obj, int? action, string property, float? value){
this.action = action;
this.next = next;
this.text = text;
this.obj = obj;
this.property = property;
this.value = value;
}
public string GetText(){
return text;
}
public string GetDataType(){
throw new System.NotImplementedException();
}
public object GetValue(){
return value;
}
public Question GetNextQuestion(){
return next;
}
}
}
how would i go about fixing this problem? I am a complete newbie to c#. So my question may be already answered but i just dont know what i am looking for.

Answer 1

I assume that IAnswer[] is an interface and since you are trying to initialize an abstract object you get that runtime exception
NullReferenceException: Object reference not set to an instance of an object
if you want to create instance of IAnswer object you have to restructure it like class or structure.

Answer 2

Your class Visual_Question derives from Generate_Questions, so the member q4 that you use en draw_question is not initialized. This is not the member of Generated_Questions but a member of Visual_Question that is not initialized.

Answer 3

In Generate_Questions you are creating a new instance of Visual_Question and then immediately calling draw_question on that new instance. You now have 2 instances of a question (both derive from Generate_Questions), but only one of them has had the Start method, which initializes q4 called. If, however, you attempt to call Start from your second instance, you're going to find yourself in an infinite series of recursive calls and quickly crash with a different error (a stack overflow in this case).
One issue with the current code is that Generate_Questions sounds more like an action than a class. I'd suggest removing the inheritance from Visual_Question and make that an interface that you would implement on Question. Question should probably have the create_question method removed. That probably belongs in a MonoBehavior script (technically it's a factory method -- look up the factory pattern -- I'm not going to go into it here since this is a beginner topic).
Something like (obviously not complete):
public class Generate_Questions : MonoBehaviour
{
private IVisualQuestion q4;
void Start()
{
q4 = new Question("Select object to edit", EXTERNAL);
q4.DrawQuestion(new vector(1,1,1), Ui, Canvas)
}
void Update() {}
}
public interface IVisualQuestion
{
void DrawQuestion(Vector2 loc, Dictionary<string, RectTransform> ui, RectTransform canvas);
}
public class Question : IVisualQuestion
{
// ... insert the Question constructor and code here ...
// Implement VisualQuestion interface
public void DrawQuestion(Vector2 loc, Dictionary<string, RectTransform> ui, RectTransform canvas)
{
this.SetAnswers(new Answer[]{new Answer(null, "Select an option")});
}
}
In general, you probably don't need inheritance. As you learn more C#, you'll discover that when inheritance is going to help it will be clear. More often than not, using an interface is a far better and flexible approach. As a commenter noted, you probably don't want to inherit from MonoBehavior. You really only need that for classes that the Unity Engine is going to directly handle.
Another note: the convention in C# is to name methods, variables, etc. in PascalCase, not using underscores to separate words.