如何通过 C# 中的事件处理程序将 class 的实例传回

Question

So I am just getting my head around how Event Handlers work.

I have a WinForms app which has a panel control on the main form. I fill the panel with a login page. The login page will search the DB for the user and check the password is correct when the user clicks the login button. Providing the login details are correct, it then triggers the event handler to tell the main form the login was successful. It should then store the Users details in the main form ready for when the panel is filled with the next control. When the user logs in, it creates an instance of a class called User. How can I pass that User back to the main form so it knows which user is currently logged in?

I hope this makes sense.

This is in the ctrl:

public partial class ctrlLoginPage : UserControl
{
    public event EventHandler RegisterButtonClicked;
    public event EventHandler LoginButtonClicked;
    public User activeUser = new User();
    public ctrlLoginPage()
    {
        InitializeComponent();
    }

    private void btnRegister_Click(object sender, EventArgs e)
    {
        RegisterButtonClicked(this, e);
    }

    public void btnLogon_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = "server=DESKTOP-05NVBR2\\SQLEXPRESS;database=GymAppPrototype;UID=sa;password=Mabg123";
        con.Open();

        SqlDataReader dataReader = null;
        SqlCommand myCommand = new SqlCommand("select * from Customer where Email ='" + txtEmail.Text + "';", con);
        dataReader = myCommand.ExecuteReader();
        while (dataReader.Read())
        {
            activeUser.forename = dataReader["Forename"].ToString();
            activeUser.surname = dataReader["Surname"].ToString();
            activeUser.email = dataReader["Email"].ToString();
            activeUser.password = dataReader["Password"].ToString();
            activeUser.mobile = dataReader["MobileNumber"].ToString();
        }
        con.Close();
        if (txtPassword.Text == activeUser.password)
        {
            MessageBox.Show("Login Successful");
            LoginButtonClicked(this, e);
        }
        else
        {
            MessageBox.Show("Login Failed");
        }
        
    }
   
}

Answer 1

It all depends on the way you design your classes and their integration but given the example above you can use a custom EventArgs or better yet a custom EventHandler.

Don't pass 'e' to the 'LoginButtonClicked' event. Instead use custom event args class containing logged in user data. For that you need to define your event handler a tab bit different though:

event EventHandler<MyEventArgs> LoginButtonClicked;

Then define your event args class:

public class MyEventArgs : EventArgs{
public MyEventArgs(User activeUser) {
    ActiveUser = activeUser;
}
  public User ActiveUser { get; }
}

Now you can pass a new instance of the new class instead of 'e':

LoginButtonClicked(this, new MyEventArgs(activeUser));

The subscriber of the event can then use e.ActiveUser to access the logged in user info, for example:

myLoginPanel.LoginButtonClicked += (s, e) => { /* do something with e.ActiveUser */ };

Answer 2

While it's entirely possible to use the standard event-based messaging in your application, I would question whether it's even necessary here. Depending on your workflow, it might be simplest to show a different form and block until the user logs in/registers, or cancels it:

//MainForm
public void GetUser()
{
    using var userWindow = new UserForm();
    if (userWindow.ShowDialog(this) == DialogResult.OK)
    {
        return userWindow.SelectedUser;
    }

    return null;
}

//UserForm
public class UserForm : Form
{
    public User SelectedUser { get; private set; }

    private void btnRegister_Click(object sender, EventArgs e)
    {
        SelectedUser = // registration logic

        DialogResult = DialogResult.OK;

        Close();
    }

    private void btnLogon_Click(object sender, EventArgs e)
    {
        SelectedUser = // logon logic

        DialogResult = DialogResult.OK;

        Close();
    }

    private void btnCancel_Click(object sender, EventArgs e)
    {
        DialogResult = DialogResult.Cancel;

        Close();
    }
}