本文原出处：http://blog.csdn.net/knight94/archive/2006/04/06/652394.aspx 稍作修正

最近，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;

                // Set dialog result with OK

                this.DialogResult = DialogResult.OK;

            }

            else

            {

                // Wrong username or password

                nLoginCount++;

                if( nLoginCount == MAX_LOGIN_COUNT )

                    // Over 3 times

                    this.DialogResult = DialogResult.Cancel;

                else

                {

                    MessageBox.Show( "Invalid user name and password!" );

                    txtUserName.Focus();

                    this.DialogResult=DialogResult.none;

                }

            }

        }

        private void btnCancel_Click(object sender, System.EventArgs e)

        {

            // Set dialog result with Cancel

            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)

{

    // Check whether form is closed with dialog result

    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 = "";

        }

    }