C#

namespace Math
{
    public partial class NewMathForm : Form
    {
        public NewMathForm()
        {
            InitializeComponent();
            int questionCounter = 1;

            for (; questionCounter < 50; )
            {
                addition(ref questionCounter);
            }
        }


        private void sumTextBox_KeyPress(object sender, KeyPressEventArgs e)
        {
            sumTextBox.SelectionStart = 0;
        }

        public void shuffle(ref int[] array)
        {
            Random rng = new Random();

            // The number of items left to shuffle (loop invariant).
            int count = array.Length;         

            while (count > 1)
            {
                // 0 <= k < n.
                int k = rng.Next(count);           
                count--;
                // count is now the last pertinent index;
                int temp = array[count];
                // swap array[n] with array[k] (does nothing if k == n).
                array[count] = array[k];
                array[k] = temp;
            }
        }

        private void parseNumbers(ref int[] array)
        {
            double addend1 = 0;
            double addend2 = 0;

            string number;

            char num1;
            char num2;

            for (int count = 0; count < 50; count++)
            {
                number = array[count].ToString();
                num1 = number[0];

                int length = number.Length;

                if (length < 2)
                {
                    char zero = '0';
                    num2 = zero;
                }

                else
                    num2 = number[1];

                addend1 = System.Char.GetNumericValue(num1);
                addend2 = System.Char.GetNumericValue(num2);

                array[count] = Convert.ToInt32(addend1);
                count++;
                array[count] = Convert.ToInt32(addend2);
            }
        }

        private void addition(ref int tickUp)
        {
            int[] numbers = new int[100];
            int counter = 0;
            int questionCounter = 1;
            int numberOfQuestions = 50;

            // Read numbers 0 to 99 into an array.
            for (counter = 0; counter < 100; counter++)
            {
                numbers[counter] = counter;
            }

            // Randomly shuffle the array of numbers
            shuffle(ref numbers);

            // Parse the first 50 numbers into tens and ones digits, then use those numbers for the problem.
            parseNumbers(ref numbers);

            //for (counter = 0; counter < 50; )
            //{
                if (sumTextBox.Text == "")
                {
                    // Write addends to label.
                    addendOneLabel.Text = numbers[questionCounter].ToString();
                    addendTwoLabel.Text = numbers[questionCounter + 1].ToString();
                }

                tickUp++;


           // }
        }

        private void checkAnswerButton_Click(object sender, EventArgs e)
        {
            checkAnswer();
        }

        public void checkAnswer()
        {

            int sum = Convert.ToInt32(sumTextBox.Text);
            int number1 = Convert.ToInt32(addendOneLabel.Text);
            int number2 = Convert.ToInt32(addendTwoLabel.Text);
            int answer = number1 + number2;

             if (answer == sum)
                    correctAnswer();
             else
                    incorrectAnswer(answer);

            sumTextBox.Text = "";
        }  

        private void correctAnswer ()
        {
            MessageBox.Show ("That is correct!");
        }

        private void incorrectAnswer (int sum)
        {
            MessageBox.Show ("That is incorrect! The correct answer is " + sum + ".");
        }

    }
}