附加信息：对象引用未设置为对象的实例

Question

所以我得到了这个错误，我不太明白为什么我得到它。我曾尝试在这里寻找答案，但每个问题都非常独特，因此不适用于我的问题。

这是我目前所拥有的。

class customer{
    public string sFirstName { get; set; }
    public string sLastName { get; set; }
    public int iAge { get; set; }
}
class customerObject{
    public static List<customer> lstStaticUsers { get; set; }
    public customerObject(){
        lstStaticcustomer = new List<customer>();
    }
    public static void AddNewStaticUser(string FirstName, string LastName, int Age)
    {
        lstStaticcustomer.Add(new customer{sFirstName = FirstName, sLastName = LastName, iAge = Age});
    }
}

这是我的窗口

public partial class wndCustomerInfo : Window
{
    public wndCustomerInfo ()
    {
        InitializeComponent();
    }

    private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
    {
        this.Hide();
        e.Cancel = true;
    }

    private void btnSubmitForm_Click(object sender, RoutedEventArgs e)
    {
        customerObject.AddNewStaticcustomer(txtFirstName.Text, txtLastName.Text, Convert.ToInt32(txtAge.Text));
        lblMessage.Content = "Created new user: " + txtFirstName.Text;
    }
}

然后这是我打开上面那个的主窗口

public partial class MainWindow : Window
{

    wndCustomerInfo wndCustomerInfoForm;

    public MainWindow()
    {
        InitializeComponent();
        Application.Current.ShutdownMode = ShutdownMode.OnMainWindowClose;
        wndCustomerInfoForm= new wndCustomerInfo ();
    }

    private void btnSave_Click(object sender, RoutedEventArgs e)
    {
        this.Hide();
        wndCustomerInfoForm.ShowDialog();
        this.Show();
    }
}

如果有人可以提供帮助，那就太好了。我是 C# 的新手，所以请善待

Answer 1

制作customObject的构造函数static：

static customerObject(){
    lstStaticcustomer = new List<customer>();
}