了解为什么 NullReferenceException 未得到处理？

Question

我是初学者，我正在用 c# 编写一个简单的 Windows 表单程序（对我来说不是那么容易，但对专业程序员来说），它的工作方式如下：一个表单 (ContactForm class) 从用户那里获取输入（名字、姓氏、地址、电子邮件和 phone n），并通过属性将其存储到联系人 class 中的变量中。 Contact class 从其他三个 classes（Address、Email 和 Phone）调用属性和方法来存储和验证用户的输入。 Contact class 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，它运行良好。但是后来我尝试用另一个 class、Customer 调用该方法，它有一个 Contact class 的对象，这是一场灾难。编译器不断为表示为 null 的 Contact class (m_contact) 的对象关联 NullReferenceException。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;
    }

为了更清楚起见，这是email.CheckEmail方法，但上面那些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 class 中显示联系人 class 的 ToString 方法的代码，完全没有问题：

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 class 中的 m_contact 为空，而 ContactForm class 中的却不是？抱歉 post 的篇幅，我真的很难理解。谢谢！

Answer 1

首先，您必须检查 m_contact 是否不为空，然后尝试对该对象调用方法 ToString。否则你会得到 NullReferenceException，因为你在空对象上调用方法。

基本上，重写

 **if ((!string.IsNullOrEmpty(m_contact.ToString())) && (m_contact != null) && (m_contact.Validate()))**
        {
            strOut = m_contact.ToString();
        }
        else
            strOut = null;

至：

 if (m_contact != null && !string.IsNullOrEmpty(m_contact.ToString()) && m_contact.Validate())
 {
     strOut = m_contact.ToString();
 }
 else
     strOut = null;

了解为什么 NullReferenceException 未得到处理？

Understand why NullReferenceException was unhandled?

c#

nullreferenceexception