c#在线考试系统登陆功能的一段代码，登陆有有关问题

c#在线考试系统登陆功能的一段代码，登陆有问题
麻烦大神们看看这段代码是不是缺少什么还是有问题，为什么我每次登陆的时候显示的都是“操作数据库出错”

namespace MySchool
{
    /// <summary>
    /// 登录窗体
    /// </summary>
    public partial class LoginForm : Form
    {
        public LoginForm()
        {
            InitializeComponent();
        }

        // “登录”按钮的单击事件
        private void btnLogin_Click(object sender, EventArgs e)
        {
            bool isValidUser = false;   // 标识是否为合法用户            
            string message = "";

            if (ValidateInput())  // 验证输入成功，显示窗体
            {
                // 验证用户是否为合法用户
                isValidUser = ValidateUser(
                    txtLoginId.Text,
                    txtLoginPwd.Text,
                    cboLoginType.Text,
                    ref message
                    );

                // 如果是合法用户，就转到相应的窗体
                if (isValidUser)
                {
                    // 记录登录用户名和登录类型
                    UserHelper.loginId = txtLoginId.Text;
                    UserHelper.loginType = cboLoginType.Text;

                    switch (cboLoginType.Text)
                    { 
                        case "学员":
                            // 试题选择窗体对象
                            StudentForm studentForm = new StudentForm();
                            studentForm.loginForm = this;
                            // 显示窗体
                            studentForm.Show();
                            break;
                        default:
                            MessageBox.Show("抱歉，功能尚未开通！","提示",MessageBoxButtons.OK,MessageBoxIcon.Information);
                            break;
                    }

                    // 登录窗体隐藏
                    this.Visible = false;                    
                }
                else  // 验证合法用户失败，给出提示
                {
                    MessageBox.Show(message,"登录提示",MessageBoxButtons.OK,MessageBoxIcon.Warning);
                }
            } 
        }

        // 验证用户的输入
        // 成功返回 True，失败返回 False
        private bool ValidateInput()
        {
            if (txtLoginId.Text.Trim() == "")
            {
                MessageBox.Show("请输入用户名", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                txtLoginId.Focus();
                return false;
            }
            else if (txtLoginPwd.Text.Trim() == "")
            {
                MessageBox.Show("请输入密码", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                txtLoginPwd.Focus();
                return false;
            }
            else if (cboLoginType.Text.Trim() == "")
            {
                MessageBox.Show("请选择登录类型", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                cboLoginType.Focus();
                return false;
            }
            else
            {
                return true;
            }
        }

        // 验证用户是否合法
        // 传入用户名、密码、登录类型
        // 合法返回 True，不合法返回 False
        // message 参数用来记录验证失败的原因
        private bool ValidateUser(string loginId, string loginPwd, string loginType, ref string message)
        {
            string tableName = "";  // 要查询的数据表的名称
            bool result = true;     // 返回值

            // 根据登录类型，确定数据表名称
            switch (loginType)
            { 
                case "教员":
                    tableName = "Teacher";
                    break;
                case "学员":
                    tableName = "Student";
                    break;
                case "管理员":
                    tableName = "Admin";
                    break;
                default:
                    message = "登录类型错误！";
                    result = false;
                    break;                  
            }            

            // 查询字符串
            string sql = string.Format(
                "SELECT COUNT(*) FROM {0} WHERE LoginId='{1}' AND LoginPwd='{2}'",
                tableName,loginId, loginPwd);

            try
            {
                // 创建 Command 对象
                SqlCommand command = new SqlCommand(sql, DBHelper.connection);

                // 打开数据库连接
                DBHelper.connection.Open();

                // 验证是否为合法用户
                int count = (int)command.ExecuteScalar();
                if (count < 1)
                {
                    message = "用户名或密码不存在！";
                    result = false;
                }
                else
                {
                    result = true;
                }
            }
            catch (Exception ex)
            {
                message = "操作数据库出错！";
                Console.WriteLine(ex.Message);
                result = false;
            }
            finally
            {
                // 关闭数据库连接
                DBHelper.connection.Close();
            }            

            return result;
        }

        // 退出登录
        private void btnCancel_Click(object sender, EventArgs e)
        {
            this.Close();
            Application.Exit();
        }
    }
}
------解决思路----------------------
如果你按我说的改了,传入的参数数量也要改成2个啊,密码就别传进去了