Question

我正在尝试编写简单的成员资格类。第一类名称是Customer，它有其他类，如Silver_Customer，Gold_Customer，它们继承自Customer类。

我在简单的Windows应用程序中使用这些类：

    public Customer customer;
    public Form_MURAT_TURAN()
    {
        InitializeComponent();
    }
    private void Form1_Load(object sender, EventArgs e)
    {
        Product p1 = new Product("Blouse", 152.80);
        Product p2 = new Product("T-Shirt", 50.25);
        .....
        lbProducts.Items.Add(p1);
        lbProducts.Items.Add(p2);
        .....
    }

    private void btnCustomer_Click(object sender, EventArgs e)
    {
        Customer customer = new Standard_Customer(txtName.Text, txtSurname.Text, 0);
        customer.Name = "Mark 1";
        customer.TotalAmount = 5;
        gbCustomer.Enabled = false;
        gbProduct.Enabled = true;

        set_info(customer.customerType(), customer.Name + " " + customer.Surname, customer.TotalAmount);
    }

    private void btnAddToBasket_Click(object sender, EventArgs e)
    {
        customer.Name = "Mark 2";
    }

除了btnAddToBasket_Click方法之外，一切正常。 customer.Name =“Mark 2”; line给我NullReferenceException错误，但customer.Name =“Mark 1”行有效。

Answer 1

你为什么不试试：

private void btnCustomer_Click(object sender, EventArgs e)
{
        this.customer = new Standard_Customer(txtName.Text, txtSurname.Text, 0);
        customer.Name = "Mark 1";
        customer.TotalAmount = 5;
        gbCustomer.Enabled = false;
        gbProduct.Enabled = true;

        set_info(customer.customerType(), customer.Name + " " + customer.Surname, customer.TotalAmount);
    }

在创建新客户时尝试使用this.customer。

Answer 2

在btnCustomer_Click中，您没有设置全局customer对象。您正在设置它的本地版本。按如下方式更改您的代码：

private void btnCustomer_Click(object sender, EventArgs e)
{
    customer = new Standard_Customer(txtName.Text, txtSurname.Text, 0);

继承对象中的空引用问题

2 个答案: