Build a Simple Windows Forms Login Application in C#

using System;
using System.Data;
using System.Data.SqlClient;
using System.Windows.Forms;

namespace LoginDemoApplication
{
    public partial class frmLogin : Form
    {
        // database connection string
        string connectionString = "Server=127.0.0.1;Database=DEMODB;uid=sa;pwd=YourPassword;";

        public frmLogin()
        {
            InitializeComponent();
        }

        private void btnLogin_Click(object sender, EventArgs e)
        {
            // validate your fields
            if (string.IsNullOrEmpty(txtUserName.Text))
            {
                MessageBox.Show("User Name can not be empty");
                txtUserName.Focus();
                return;
            }
            if (string.IsNullOrEmpty(txtPassword.Text))
            {
                MessageBox.Show("Password can not be empty");
                txtPassword.Focus();
                return;
            }

            // connection with database
            SqlConnection con = new SqlConnection(connectionString);
            SqlCommand cmd = new SqlCommand("Select * from UserMaster where UserName=@username", con);
            cmd.Parameters.AddWithValue("@username", txtUserName.Text);
            con.Open();
            SqlDataAdapter adapt = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            adapt.Fill(ds);
            con.Close();

            if (ds.Tables[0].Rows.Count == 0)
            {
                MessageBox.Show("Invalid User Name/Password");
                return;
            }

            // check password
            if (ds.Tables[0].Rows[0]["Password"].ToString() != txtPassword.Text)
            {
                MessageBox.Show("Invalid User Name/Password");
                return;
            }

            frmMaster.DisplayName = ds.Tables[0].Rows[0]["DisplayName"].ToString();
            frmMaster master = new frmMaster();
            this.Hide();
            master.Show();
        }

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

        private void frmLogin_FormClosing(object sender, FormClosingEventArgs e)
        {
            Application.Exit();
        }
    }
}

using System;
using System.Windows.Forms;

namespace LoginDemoApplication
{
    public partial class frmMaster : Form
    {
        public static string DisplayName;

        public frmMaster()
        {
            InitializeComponent();
        }

        private void btnLogout_Click(object sender, EventArgs e)
        {
            frmLogin login = new frmLogin();
            login.Show();
            this.Hide();
        }

        private void frmMaster_Load(object sender, EventArgs e)
        {
            lblUser.Text = DisplayName;
        }

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

        private void frmMaster_FormClosing(object sender, FormClosingEventArgs e)
        {
            Application.Exit();
        }
    }
}

CREATE TABLE UserMaster
(
    id          INT IDENTITY(1,1) PRIMARY KEY,
    UserName    VARCHAR(50) NOT NULL,
    Password    VARCHAR(50) NOT NULL,
    DisplayName VARCHAR(50) NOT NULL,
    Active      INT NOT NULL DEFAULT (1)
);
GO

INSERT INTO UserMaster (UserName, Password, DisplayName, Active) VALUES ('Admin', 'Adminpass', 'Admin User', 1);
INSERT INTO UserMaster (UserName, Password, DisplayName) VALUES ('User1', 'userpas', 'User One');