如何用C#写一个简单的Login窗口

看到网上经常会问如何进行窗口跳转，大多数的问题都是牵扯到Login窗口。其实，在Visual Studio 6以来，比较正确的做法，是判断Login窗口的返回值，然后决定是否打开主窗体，那么在C#中也是一样的。

具体做法如下：
首先，创建Login窗口，然后添加相应的输入框和按钮，设置窗口的AcceptButton为窗体的确认按钮，而CancelButton为窗体的取消按钮。例如：
this.AcceptButton = this.btnOK;
this.CancelButton = this.btnCancel;

定义确定按钮以及取消按钮事件，如下：
private void btnOK_Click(object sender, System.EventArgs e)
{
// Here is to use fixed username and password
// You can check username and password from DB
if( txtUserName.Text == "Admin" && txtPassword.Text == "nopassword" )
{
// Save login user info
uiLogin.UserName = txtUserName.Text;
uiLogin.Password = txtPassword.Text;

this.DialogResult = DialogResult.OK;
}
else
{
nLoginCount++;
if( nLoginCount == MAX_LOGIN_COUNT )
this.DialogResult = DialogResult.Cancel;
else
{
MessageBox.Show( "Invalid user name and password!" );
txtUserName.Focus();
}
}
}

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

然后，在Login窗体的Closing事件中，要进行处理，如下：
private void frmLogin_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
if( this.DialogResult != DialogResult.Cancel &&
this.DialogResult != DialogResult.OK )
e.Cancel = true;
}

除此外，Login窗体一些辅助代码如下：
private int nLoginCount = 0;
private const int MAX_LOGIN_COUNT = 3;

private UserInfo uiLogin;
public frmLogin( ref UserInfo ui )
{
//
// Required for Windows Form Designer support
//
InitializeComponent();

// Set login info to class member
uiLogin = ui;
}

调用的时候，要修改程序的Main函数，如下：
/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
UserInfo ui = new UserInfo();
frmLogin myLogin = new frmLogin( ref ui );
if( myLogin.ShowDialog() == DialogResult.OK )
{
//Open your main form here
MessageBox.Show( "Logged in successfully!" );
}
else
{
MessageBox.Show( "Failed to logged in!" );
}
}

而附加的UserInfo类如下：
/// <summary>
/// User info class
/// </summary>
public class UserInfo
{
private string strUserName;
private string strPassword;
public string UserName
{
get{ return strUserName;}
set{ strUserName = value; }
}
public string Password
{
get{ return strPassword;}
set{ strPassword = value;}
}
public UserInfo()
{
strUserName = "";
strPassword = "";
}
}