C# Function Problem I Dont Understand Why

The reason that some questions are asked multiple times is due to the way the code handles the case where a randomly selected question has been asked before.

Consider on the first time through (when no questions have been asked before), let's say Question 5 is randomly selected. The if statement will be false and will mark Question 5 as 'has been asked' and will return the number 5. So far so good.

However, some time later, Random.Next might return 5 again. This time the if statement is true and you recursively seek another number (if you're lucky). Let's say that this call to GetRandomNumber returns, let's say, the value 8. However this value is discarded and the original function call proceeds to return the value 5 (which is an already asked question number).

You might want to consider what happens when you run out of questions?

Perhaps a better approach is to build a list of question numbers, and then as they are used, remove them from the list, until the list is empty.

using System;
using System.Collections.Generic;

namespace TwentyQuestions
{
    public class Quiz
    {
        private const int NUM_QUESTIONS = 20;
        private List<int> UnaskedQuestions = new List<int>();
        private Random _rnd = new Random();

        public Quiz()
        {
            for (int i = 0; i < NUM_QUESTIONS; i++)
                UnaskedQuestions.Add(i+1);
        }

        public void Run()
        {
            try
            {
                while (true)                
                    AskQuestion(GetRandomNumber());                                        
            }
            catch (ApplicationException Ex)
            {
                Console.WriteLine(Ex.Message);
            }
        }

        public int GetRandomNumber()
        {
            if (UnaskedQuestions.Count == 0) 
                throw new ApplicationException("No more questions");
            int returnpoint = _rnd.Next(UnaskedQuestions.Count);
            int questionNumber = UnaskedQuestions[returnpoint];
            UnaskedQuestions.RemoveAt(returnpoint);
            return questionNumber;
        }       

        public void AskQuestion(int QuestionID)
        {
            Console.Write("Answer question " + QuestionID.ToString() + 
                " (or blank to exit)? ");
            string answer = Console.ReadLine();
            if (String.IsNullOrEmpty(answer)) 
                throw new ApplicationException("Blank answer");
        }
    }
}

Using a little Linq we can simplify this, and make it "flexible:"

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

private const int NumberOfAvailableQuestions = 100;

private const int NumberOfQuestions = 20;

private Dictionary<int,bool> TestResults = new Dictionary<int,bool>(); 

private List<string> AvailableQuestions = new List<string>(NumberOfAvailableQuestions);

private List<string> currentQuestionList = new List<string>(NumberOfQuestions); 

private Random getRand = new Random(DateTime.Now.Millisecond);

// for testing
private void mockInitializeQuestions()
{
    for (int i = 0; i < NumberOfAvailableQuestions; i++)
    {
        AvailableQuestions.Add(string.Format("question #{0}", i + 1));
    } 
}

private void mockStartTest()
{
    // initialize
    if(AvailableQuestions == null) mockInitializeQuestions();

    TestResults.Clear();
    currentQuestionList.Clear();

    // copy the available questions
    string[] copiedQuestions = new string[NumberOfAvailableQuestions];
    AvailableQuestions.CopyTo(copiedQuestions);

    // random sort the copied list of all questions
    // and take the first #NumberOfQuestions of them
    currentQuestionList = copiedQuestions.OrderBy(q => getRand.Next(0, NumberOfAvailableQuestions)).ToList().GetRange(0, NumberOfQuestions);

    // start the test
    foreach (string question in currentQuestionList)
    {
        // show the question, get an answer
        // update the TestResults Dictionary

        // for testing only
        Console.WriteLine(question);
    }
}

Good questions to ask:

1. why do you copy all the available questions ? because I have made the assumption/guess that the "original" list of all questions should be kept in order-created for good reason.

2. isn't it wasteful to copy all those strings ? no: 'CopyTo performs a "shallow copy."

3. if you claim that 'CopyTo performs a reference-only copy (shallow) then doesn't sorting the copy sort the original list ? no.

I would guess that in the future what are now the 'strings will be replaced by instances of some Class that will contain other data needed to present a question with things like "multiple choice" answers: when that's the case then the cost any copying will be minimal since it's just references copied.