我是初学者,我在c#编写一个简单的Windows窗体程序(对我来说并不容易,但对于专业程序员),其工作方式如下:表单(ContactForm类)获取输入(来自用户的名字,姓氏,地址,电子邮件和电话号码)并通过“属性”将其存储在Contact类中的变量中。 Contact类调用其他三个类(地址,电子邮件和电话)中的属性和方法来存储和验证用户的输入。 Contact类还包含一个覆盖ToString方法,该方法将包含用户输入的变量格式化为字符串。 此时,我尝试使用ContactForm调用此ToString方法并在MessageBox中显示字符串,并且它工作正常。但后来我尝试用另一个类Customer调用该方法,该类有一个Contact类的对象,这是一场灾难。编译器继续将Contact类(m_contact)的对象的NullReferenceException与null相关联。这是代码:
public class Customer
{
private Contact m_contact;
private int m_ID;
public Customer()
{
}public Customer(Contact contactIn)
{
}public Customer(Contact contactIn, int id)
{
if (m_contact != null)
{
m_contact = new Contact();
m_contact = contactIn;
}
m_ID = id;
}public Contact ContactData
{
get { return m_contact; }
set {
if (value != null)
m_contact = value; }
}public int ID
{
get { return m_ID; }
set { m_ID = value; }
}
public override string ToString()
{
string strOut = string.Empty;
**if ((!string.IsNullOrEmpty(m_contact.ToString())) && (m_contact != null) && (m_contact.Validate()))**
{
strOut = m_contact.ToString();
}
else
strOut = null;
return strOut;
}
}
} 星号中的行是发现错误的行。这个是m_contact.Validate方法:
public bool Validate()
{
bool ok = false;
if (!string.IsNullOrEmpty(m_firstName) && (!string.IsNullOrEmpty(m_lastName)) && m_address.CheckData() && m_email.CheckEmail() && m_phone.CheckPhone())
ok = true;
else
ok = false;
return ok;
}
为了更清晰,这是email.CheckEmail方法,但上面的检查方法肯定不是问题:
public bool CheckEmail()
{
bool ok = false;
if ((!string.IsNullOrEmpty(m_personal)) || (!string.IsNullOrEmpty(m_work)))
ok = true;
else
ok = false;
return ok;
}
这是ContactForm类中显示Contact类的ToString方法的代码,完全没问题:
public partial class ContactForm : Form
{
private Contact m_contact;
private bool m_closeForm;
public ContactForm(string title)
{
InitializeComponent();
m_contact = new Contact();
this.Text = title;
InitializeGUI();
}public Contact ContactData
{
get { return m_contact; }
set
{
if (value != null)
m_contact = value;
}
}private void InitializeGUI()
{
txtFirstName.Text = string.Empty;
txtLastName.Text = string.Empty;
txtHomePhone.Text = string.Empty;
txtCellPhone.Text = string.Empty;
txtEMailBus.Text = string.Empty;
txtEMailPr.Text = string.Empty;
txtStreet.Text = string.Empty;
txtCity.Text = string.Empty;
txtZip.Text = string.Empty;
FillCountryComboBox();
cmbCountry.SelectedIndex = (int)Countries.Sverige;
m_closeForm = true;
}private void FillCountryComboBox()
{
cmbCountry.Items.AddRange(Enum.GetNames(typeof(Countries)));
}
private void MainForm_FormClosing(object sender, FormClosingEventArgs e)
{
if (m_closeForm)
e.Cancel = false;
else
e.Cancel = true;
}public bool ReadInput()
{
m_contact.FirstName = txtFirstName.Text;
m_contact.LastName = txtLastName.Text;
m_contact.AddressData = ReadAddress();
m_contact.EmailData = ReadEmail();
m_contact.PhoneData = ReadPhone();
bool ok = m_contact.Validate();
if (ok)
{
return true;
}
else
{
MessageBox.Show("Please provide at least your firstname, lastname, phone number, email address, city and country");
return false;
}
}private Address ReadAddress()
{
Address m_address = new Address();
m_address.Street = txtStreet.Text;
m_address.City = txtCity.Text;
m_address.ZipCode = txtZip.Text;
m_address.Country = (Countries)cmbCountry.SelectedIndex;
return m_address;
}private Email ReadEmail()
{
Email m_email = new Email();
m_email.Work = txtEMailBus.Text;
m_email.Personal = txtEMailPr.Text;
return m_email;
}private Phone ReadPhone()
{
Phone m_phone = new Phone();
m_phone.Home = txtHomePhone.Text;
m_phone.Other = txtCellPhone.Text;
return m_phone;
}
private void btnOK_Click(object sender, EventArgs e)
{
if (ReadInput())
{
UpdateGUI();
m_closeForm = true;
this.Close();
}
else
m_closeForm = false;
}public void UpdateGUI()
{
string strOut = m_contact.ToString();
MessageBox.Show(strOut);
}
}
}
} 此外,MainForm调用Customer ToString方法以在MessageBox上显示消息。这是MainForm中的方法:
private void btnAdd_Click(object sender, EventArgs e)
{
ContactForm contForm = new ContactForm("Add a new customer");
Customer cust = new Customer();
int index = lstResults.SelectedIndex;
if (index != -1)
contForm.ContactData = customerMngr.GetCustomer(index).ContactData;
if (contForm.ShowDialog() == DialogResult.OK)
{
lstResults.Items.Clear();
if (contForm.ReadInput() && (!String.IsNullOrEmpty(cust.ToString()) && (cust != null)))
{
MessageBox.Show(cust.ToString());
}
}
为什么Customer类中的m_contact为null,而不是ContactForm类中的m_contact?对不起这篇文章的篇幅,我很难理解。谢谢!
答案 0 :(得分:0)
首先,您必须检查m_contact
是否为空,然后尝试在该对象上调用方法ToString
。否则你得到NullReferenceException,因为你在null对象上调用方法。
基本上,重写
**if ((!string.IsNullOrEmpty(m_contact.ToString())) && (m_contact != null) && (m_contact.Validate()))**
{
strOut = m_contact.ToString();
}
else
strOut = null;
为:
if (m_contact != null && !string.IsNullOrEmpty(m_contact.ToString()) && m_contact.Validate())
{
strOut = m_contact.ToString();
}
else
strOut = null;