C# - User Login only checking the newest record for the username

Question

I am making a login system using C#, which successfully checks if the user's credentials match up, but for some reason, whenever I write a username that is not the newest record, it gives me the "Username doesn't exist" error, but if it is written with the correct password, it still logs in.

using (SqlConnection conn = new SqlConnection())
        {
            conn.ConnectionString = "Data source=(local);Initial Catalog=GameStore;Integrated Security=True";
            conn.Open();
            SqlCommand command = new SqlCommand("SELECT * FROM Staff", conn);

            //Username
            String TextBoxUsername = textBox1.Text;
            //Lower Casing
            TextBoxUsername = TextBoxUsername.ToLower();
            //Password
            String TextBoxPassword = textBox2.Text;      

            using (SqlDataReader reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    //Username
                    string DatabaseUsername = (string)reader["StaffUserName"];
                    //Lower Casing
                    DatabaseUsername = DatabaseUsername.ToLower();
                    //Password
                    string DatabasePassword = (string)reader["StaffPassword"];


                    //If Username Matches One In DB
                    if (DatabaseUsername == TextBoxUsername)
                    {
                        WarningLabel.Visible = false;
                        WarningLabel.Text = "";

                        //If Password Matches One In DB
                        if(DatabasePassword == TextBoxPassword)
                        {
                            WarningLabel.Visible = false;
                            WarningLabel.Text = "";
                            MessageBox.Show("Logging In");
                        }
                        //Wrong Password
                        else
                        {
                            WarningLabel.Visible = true;
                            WarningLabel.Text = "Incorrect Password";
                        }

                    }

                    //Username doesn't exist in DB
                    else

                    {
                        WarningLabel.Visible = true;
                        WarningLabel.Text = "Username doesn't exist";

                    }
                }
            }
        }
    }

Answer 1

Yeah that's cause you are doing a select * ... and storing the data in same string variable which overwrites all the data and obviously remains with the last record and thus the behavior

            while (reader.Read())
            {
                //Username
                string DatabaseUsername = (string)reader["StaffUserName"];

Better would be filter the record based on your input like below and then your posted code should work fine

select * from stuff
where StaffUserName = @uname
and StaffPassword = @pwd;