关于c＃：了解为什么NullReferenceException未处理？

Understand why NullReferenceException was unhandled?

本问题已经有最佳答案，请猛点这里访问。

我是一个初学者，我正在用C语言编写一个简单的Windows窗体程序(对我来说不是那么容易，但对一个专业的程序员来说)，它的工作原理是这样的：一个窗体(contactForm类)从用户那里获得输入(名字、姓氏、地址、电子邮件和电话号码)，并通过属性将其存储到contact类中的变量中。contact类从其他三个类(地址、电子邮件和电话)调用属性和方法来存储和验证用户的输入。contact类还包含一个override-toString方法，该方法将包含用户输入的变量格式化为字符串。在这一点上，我尝试用contactForm调用这个toString方法，并在一个消息框中显示该字符串，它工作得很好。但后来我尝试用另一个类(customer)调用该方法，该类具有contact类的对象，这是一个灾难。编译器不断为contact类(m_contact)的对象(表示为空)重新发出nullreferenceexception。代码如下：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42

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。这是m_contact.validate方法：

1
2
3
4
5
6
7
8
9

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;
}

为了更清晰起见，这是email.checkemail方法，但上述检查方法绝对不是问题：

1
2
3
4
5
6
7
8
9

public bool CheckEmail()
{
bool ok = false;
if ((!string.IsNullOrEmpty(m_personal)) || (!string.IsNullOrEmpty(m_work)))
ok = true;
else
ok = false;
return ok;
}

这是来自contactForm类的代码，显示了contact类的toString方法，完全没有问题：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103

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，主窗体调用customer-toString方法以在消息框上显示消息。这是主窗体中的方法：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

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());
}
}

为什么客户类中的m_联系人为空，而不是contactForm类中的联系人？抱歉，这篇文章篇幅太长了，我真的很难理解。谢谢您！