c# - 如何制作登录系统C#

Question

关于登录系统，我的代码有问题。当我在数据库中添加 1 个用户名和密码时，它可以正常工作。但是当我在数据库中添加另外 1 个用户名和密码时，我的 else 语句将弹出两次。当 3 用户名和密码时，我的 else 语句将弹出 trice。依此类推......这是我的代码..如果你有比我更好的代码，请展示它。

SqlConnection con = new SqlConnection("Data Source=.\\SQLEXPRESS;AttachDbFilename=C:\\Users\\JEDMARC\\Desktop\\VS v1.0.0\\Voting System v1.0.0\\Voting System v1.0.0.mdf;Integrated Security=True;User Instance=True");
    SoundPlayer t = new SoundPlayer(@"C:\Users\JEDMARC\Documents\welcome.wav");

    private void btnEnter_Click(object sender, EventArgs e)
    {
 if (cmbToE.Text == "HomeRoom Election" && comboBox1.Text == "English")
        {
            con.Open();

            SqlCommand da = new SqlCommand("SELECT * FROM RegistrationTable", con);

            SqlDataReader reader = null;
            reader = da.ExecuteReader();
            while (reader.Read())
            {
                if (tbUsername.Text == (reader["Username"].ToString()) && tbPassword.Text == (reader["Password"].ToString()))
                {
                    MessageBox.Show("*Choose your best candidate. Use a Combobox.\n\n*After choosing, click Submit button to pass your vote!\n\n                           VOTE WISELY!", "How to vote?", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
                    UserHRForm x = new UserHRForm();
                    x.Show();
                    t.Play();
                    this.Close();
                }
                else
                {
                    SystemSounds.Hand.Play();
                    MessageBox.Show("Access Denied! Account doesn't exist!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Hand);                        
                }
            }
        }
}

Answer 1

您的 while 循环会检查数据库中的每个用户，因此它一次命中正确的用户，两次命中错误的用户。要按照您现在编写的代码的方式执行此操作，您必须在 while 循环中使用一个标志来跟踪您是否找到了正确的用户/密码组合。像这样的东西:

        bool foundUser = false;

        while (reader.Read())
        {
            if (tbUsername.Text == (reader["Username"].ToString()) && tbPassword.Text == (reader["Password"].ToString()))
            {
                foundUser = true;
                break;
            }
        }


        if (foundUser) {
                MessageBox.Show("*Choose your best candidate. Use a Combobox.\n\n*After choosing, click Submit button to pass your vote!\n\n                           VOTE WISELY!", "How to vote?", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
                UserHRForm x = new UserHRForm();
                x.Show();
                t.Play();
                this.Close();
            }
            else
            {
                SystemSounds.Hand.Play();
                MessageBox.Show("Access Denied! Account doesn't exist!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Hand);                        
            }

但这是一种非常低效的代码使用。相反，let SQL handle the filter for you .这就是它的设计目的。

        con.Open();

        SqlCommand da = new SqlCommand("SELECT * FROM RegistrationTable WHERE Username=@Username AND Password=@Password", con);
        da.Parameters.Add("@Username ", SqlDbType.Varchar);
        da.Parameters["@Username "].Value = tbUsername.Text;
        da.Parameters.Add("@Password", SqlDbType.Varchar);
        da.Parameters["@Password"].Value = tbPassword.Text;

        SqlDataReader reader = null;
        reader = da.ExecuteReader();

            if (da.HasRows)
            {
                MessageBox.Show("*Choose your best candidate. Use a Combobox.\n\n*After choosing, click Submit button to pass your vote!\n\n                           VOTE WISELY!", "How to vote?", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
                UserHRForm x = new UserHRForm();
                x.Show();
                t.Play();
                this.Close();
            }
            else
            {
                SystemSounds.Hand.Play();
                MessageBox.Show("Access Denied! Account doesn't exist!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Hand);                        
            }
        }