User Roles For windows mobile app!!

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C#

.NET

I am developing a Windows mobile app,and i need to create some user roles,depending from the user.How can i set user roles,whats the best way to make this type of authentication?My app also connects to an outside sql server database.any idea,pls share!!

I have done this type of authentication,after creating a sql table with username and password colums,i chek if they exists...

namespace MobileAPPLICATION
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void button_login_Click(object sender, EventArgs e)
        {
            string _connectionstring = @"Server='.\SQLEXPRESS';Initial Catalog=LOGIN;USER ID=SA;PWD=;";
            SqlConnection _connection = new SqlConnection(_connectionstring);

            if (txtpwd.Text == "")
            {
                MessageBox.Show("Password is empty!");
                txtpwd.Focus();
            }
            else
            {
                    _connection.Open();

                           
                    string _sql = "SELECT * FROM [LOGIN] where username=@username and password=@password;";
                    SqlCommand _command = new SqlCommand(_sql, _connection);
                    _command.Parameters.AddWithValue("@username", txtusername.Text);
                    _command.Parameters.AddWithValue("@password", txtpwd.Text);

                    if (_command != null)
                    {

                        //commands to go to the main menu form!
                    }
                    else
                    {
                        MessageBox.Show("Invalid Username/Password!");
                    }
            }
        }

        private void button_reset_Click(object sender, EventArgs e)
        {
            txtusername.Text = "";
            txtpwd.Text = "";
        }
        private void txtusername_TextChanged(object sender, EventArgs e)
        {

        }

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

        
        
    }
}