C#

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Data.SqlServerCe;

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            MainForm frmLogin = new MainForm();
            frmLogin.ShowDialog();
        }

        private void btnLogin_Click(object sender, System.EventArgs e)
        {
            if (txtuser.Text.Equals(""))
            {
                MessageBox.Show("Please Enter User Name", "Organiser", MessageBoxButtons.OK, MessageBoxIcon.Information);
                return;
            }
            if (txtpwd.Text.Equals(""))
            {
                MessageBox.Show("Please Enter Password", "Organiser", MessageBoxButtons.OK, MessageBoxIcon.Information);
                return;
            }
            int count = 0;
           
            con = new SqlCeConnection();
            con.ConnectionString = "Data Source=Database.sdf";
            com = new SqlCeCommand("select * from [User]", con);
            con.Open();
            if (con.State == ConnectionState.Open)
            {
                SqlCeDataReader dtr = com.ExecuteReader();
                while (dtr.Read())
                {
                    us = dtr["username"].ToString();
                    ps = dtr["password"].ToString();
                    if (us == txtuser.Text && ps == txtpwd.Text)
                    {
                        count = 1;                                             

                    }
                    else count = 2;                    
                }
            }            
            if (count == 1)
            {
                MessageBox.Show("Login successful " + us, "Login", MessageBoxButtons.OK, MessageBoxIcon.Information);
                ChangePassword pass = new ChangePassword();
                pass.ShowDialog();

            }
            else if (count == 2)
            {
                MessageBox.Show("WRONG USER NAME OR PASSWORD ", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);

            }
        }
        private void btnRegister_Click(object sender, System.EventArgs e)
        {
            this.Close();
            this.Dispose();
            RegisterUser reg = new RegisterUser();
            reg.ShowDialog();
        }
        private void btnExit_Click(object sender, System.EventArgs e)
        {
            this.Close();
            this.Dispose();
            Application.Exit();
        }

        private void MainForm_Load(object sender, EventArgs e)
        {
                        
        }

        private void MainForm_Load_1(object sender, EventArgs e)
        {

        }
    }
}

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Data.SqlServerCe;

        private void btnCancel_Click(object sender, System.EventArgs e)
        {
            this.Close();
            this.Dispose();            
        }

        private void btnSave_Click(object sender, System.EventArgs e)
        {
            MainForm mf = new MainForm();
            
            int count = 0;
            string connectionString = "Data Source=..\\..\\Database.sdf";
            SqlCeConnection conn = new SqlCeConnection(connectionString);
            conn.Open();
            if (conn.State == ConnectionState.Open)
            {
                SqlCeCommand com;
                com = new SqlCeCommand("select * from [User]", conn);
                SqlCeDataReader dtr = com.ExecuteReader();
                while (dtr.Read())
                {
                   string pass = dtr["password"].ToString();
                   username = dtr["username"].ToString();
                   password = txtNew.Text;
                   if (pass == txtOld.Text)
                   {
                       count = 1;
                       string query = "UPDATE [User] SET password = '" + password + "' WHERE username = '" + username + "'";
                       try
                       {
                           SqlCeCommand myCommand1 = new SqlCeCommand();
                           myCommand1.CommandText = query;
                           myCommand1.Connection = conn;
                           myCommand1.ExecuteNonQuery();
                           MessageBox.Show(username +"'s password changed to " + password, "Password change", MessageBoxButtons.OK, MessageBoxIcon.Information);

                       }
                       catch (Exception ex)
                       {
                           MessageBox.Show(ex.Message, "Problem.");
                       }
                   } else count = 2;
                   if (count == 2)
                   {
                       MessageBox.Show("WRONG USER NAME OR PASSWORD ", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                   }
                }
            }
            conn.Close(); 
        }
     }
}