计算器应用程序崩溃

我正试图用C#构建一个简单的计算器应用程序，但我不知道为什么在执行以下步骤时它会崩溃。

1. 输入0.2
2. 单击减法
3. 输入0

应用程序立即崩溃。我认为这与Zero（）函数有关，因为这是单击Zero按钮时执行的操作。条件语句是为了处理不应该发生的情况，比如连续密码等等。这是源代码。顺便说一句，其他数字的功能是相同的。

    public partial class MainWindow : Window
    {
        protected double firstNumber, secondNumber;
        protected string textBoxContents;
        protected int selectedFunction;
        public MainWindow()
        {
            InitializeComponent();
            firstNumber = 0;
            secondNumber = 0;
            selectedFunction = 0;
            textBoxContents = "0";
        }
        private void Zero(object sender, RoutedEventArgs e)
        {
            if (Convert.ToDouble(textBoxContents) > 0 || textBoxContents[textBoxContents.Length - 1] == '.')
            {
                if(selectedFunction != 0)
                    textBoxContents = textBoxContents + "0";
            }
            else if (textBoxContents == null)
            {
                textBoxContents = textBoxContents + "0";
            }
            ResultBox.Content = textBoxContents;
        }
        private void One(object sender, RoutedEventArgs e)
        {
            textBoxContents = textBoxContents + "1";
            ResultBox.Content = textBoxContents;
        }
        private void Decimal(object sender, RoutedEventArgs e)
        {
            textBoxContents = textBoxContents + ".";
            ResultBox.Content = textBoxContents;
        }
        private void Addition(object sender, RoutedEventArgs e)
        {
            firstNumber = Convert.ToDouble(textBoxContents);
            textBoxContents = null;
            selectedFunction = 1;
        }
        private void Subtraction(object sender, RoutedEventArgs e)
        {
            firstNumber = Convert.ToDouble(textBoxContents);
            textBoxContents = null;
            selectedFunction = 2;
        }
        private void Multiplication(object sender, RoutedEventArgs e)
        {
            firstNumber = Convert.ToDouble(textBoxContents);
            textBoxContents = null;
            selectedFunction = 3;
        }
        private void Division(object sender, RoutedEventArgs e)
        {
            firstNumber = Convert.ToDouble(textBoxContents);
            textBoxContents = null;
            selectedFunction = 4;
        }
        private void Result(object sender, RoutedEventArgs e)
        {
            secondNumber = Convert.ToDouble(textBoxContents);
            double thirdNumber = 0;
            switch (selectedFunction)
            {
                case 1:
                    thirdNumber = firstNumber + secondNumber;
                    break;
                case 2:
                    thirdNumber = firstNumber - secondNumber;
                    break;
                case 3:
                    thirdNumber = firstNumber * secondNumber;
                    break;
                case 4:
                    thirdNumber = firstNumber / secondNumber;
                    break;
                default:
                    break;
            }
            textBoxContents = Convert.ToString(thirdNumber);
            ResultBox.Content = textBoxContents;
        }
        private void ClearEverything(object sender, RoutedEventArgs e)
        {
            textBoxContents = null;
            firstNumber = 0;
            secondNumber = 0;
            selectedFunction = 1;
            ResultBox.Content = Convert.ToString(0);
        }
        private void ToggleNegative(object sender, RoutedEventArgs e)
        {
            if (Convert.ToDouble(textBoxContents) != 0)
            {
                textBoxContents = Convert.ToString(Convert.ToDouble(textBoxContents) * -1);
                ResultBox.Content = textBoxContents;
            }
            else
                ResultBox.Content = Convert.ToString(0);
        }
    }

private void Zero(object sender, RoutedEventArgs e)
{
    if (Convert.ToDouble(textBoxContents) > 0 ||
        textBoxContents[textBoxContents.Length - 1] == '.')
    {
        if(selectedFunction != 0)
            textBoxContents = textBoxContents + "0";
    }
    else if (textBoxContents == null)
    {
        textBoxContents = textBoxContents + "0";
    }
    ResultBox.Content = textBoxContents;
}

private void Zero(object sender, RoutedEventArgs e)
{
    var dblVal = Convert.ToDouble(textBoxContents.Text);
    textBoxContents.Text = dblVal.ToString();
    ResultBox.Content = textBoxContents.Text;
}