Code:

        // Program.cs

        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            // ...
            
            bool flag = false;            
            short role = 0;

            using (FrmLogin login = new FrmLogin())
            {
                if ((login.ShowDialog() == DialogResult.OK) && login.IsValidLogin)
                {
                    flag = true;
                    role = login.UserRole;
                }
            }

            if (flag)
            {                
                try
                {
                    switch (role)
                    {
                        case 1:
                            using (MainForm form = new MainForm())
                            {
                               Application.Run(form);
                            }
                            break;

                        // add cases here                        

                        default:
                            break;
                    }
                }
                catch (Exception exc)
                {
                    // process Exception...
                }
            }
        }

Code:

        // FrmLogin.cs

        public FrmLogin()
        {
            InitializeComponent();

            this.passTries = 3;

            // set what is needed
        }

        public bool IsValidLogin
        {
            get
            {
                return isValidLogin;
            }
        }

        public short UserRole
        {
            get
            {
                return userRole;
            }
        }

        private void FrmLogin_Load(object sender, EventArgs e)
        {
            // set what is needed...
  
            this.ActiveControl = this.txtUser;
            this.txtUser.Focus();
            this.txtUser.Text = string.Empty;
            this.txtPassword.Text = string.Empty;
        }

        private void btnCancel_Click(object sender, EventArgs e)
        {            
            Application.Exit();
        }

        private void btnLogin_Click(object sender, EventArgs e)
        {     
                this.btnLogin.Enabled = false;

                this.Cursor = Cursors.AppStarting;

                Application.DoEvents();

                try
                {
                        this.Authorize();
                }
                catch (Exception exception)
                {                   
                        // log error...
                        this.isValidLogin = false;
                        this.Cursor = Cursors.Default;
                        this.btnLogin.Enabled = true;                    
                }
         }

         private void Authorize()
         {
             // do authorization

             // set isValidLogin property

             // set userRole value

             if (!this.isValidLogin)
             {
                 this.txtPassword.Clear();
                 this.txtPassword.Focus();

                 this.passTries--;

                 MessageBox.Show("Login Error");
              }

              this.Cursor = Cursors.Default;
              this.btnLogin.Enabled = true;

              if ((this.isValidLogin == true) || (this.passTries < 1))
              {
                  base.DialogResult = DialogResult.OK;
                  base.Close();
              }
         }

         private bool isValidLogin = false;
         private short passTries = 3;
         private short userRole = 0;