c#

System.ArgumentNullException: '值不能为空。（参数'）'

我收到此错误 ^。

我想在产品代码上按 CTRL Z 后，它会停止循环并继续下一行代码。但是我输入了 ctrl z 并且它转到 Quantity 行并在那之后中断。如果有人知道如何解决此错误，我认为这是因为 Quantity 无法返回 Null 值，因为它不像产品那样设置，但我找不到解决方法。

public class Transaction
{
    public string Name { get; set; }
    public int productCode { get; set; }
    public int productQuantity { get; set; }

    //constructor

    public Transaction(string cName)//, int pCode, int pQuantity
    {
        Name = cName;
    }

    public void AssessProducts()
    {

        int codeInput;
        int qtyInput = 0;
        decimal transTotal = 0;
        double qty201 = 0;
        double qty556 = 0;
        double count201 = 0;
        double count556 = 0;
        double count558 = 0;
        double count909 = 0;
        double count910 = 0;

        Console.WriteLine("Product Code: ");
        string ctrlzInput = Console.ReadLine();
        Console.WriteLine("Quantity: ");
        qtyInput = int.Parse(Console.ReadLine());

        while (ctrlzInput != null)
        {
            codeInput = int.Parse(ctrlzInput);

            if (codeInput == 201)
            {
                count201 = count201 + 19.99;
            }
            else if (codeInput == 556)
            {
                count556 = count556 + 59.99;
            }
            else if (codeInput == 558)
            {
                count558 = count558 + 20.50;
            }
            else if (codeInput == 909)
            {
                count909 = count909 + 105.99;
            }
            else if (codeInput == 910)
            {
                count910 = count910 + 16.99;
            }
            else
            {
                Console.WriteLine("Error! Wrong product code.");
            }

            transTotal = (decimal)((double)transTotal + count201 + count556 + count558 + count909 + count910);
            qty201 = qty201 + qtyInput;
            qty556 = qty556 + qtyInput;


          Console.WriteLine("Product Code: ");
          ctrlzInput = Console.ReadLine();
          Console.WriteLine("Quantity: ");
          qtyInput = int.Parse(Console.ReadLine());

        }
                
       Console.WriteLine($"Customer Name: {Name} nn");
       Console.WriteLine("Items Ordered:nn");
       Console.WriteLine("Product #    Product Name              Price            Qty         Line Totaln");
        if (count201 >= 1)
        {
            Console.Write($"Product 210 - ACME Anvil               @ $19.99       *{qty201}      = {qty201 * 19.99} n");
        }
        if (count556 >= 1)
            Console.Write($"Product 556 - ACME Dynamite            @ $59.99       *{qty556}      = {qty556 * 59.99} n");
        }
              
    }

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