private string conn;
private MySqlConnection connect;
string user = txt_username.Text;
string pass = passwordbox_psw.Password;

private void DbConnection()
{
    try
    {
        conn = "*connectionstring*";
        connect = new MySqlConnection(conn);
        connect.Open();
    }
    catch (MySqlException ex)
    {
        this.ShowMessageAsync("Connection to the database refused. n", ex.Message);
    }
}

private bool ValidateLogin(string user, string pass)
{
    DbConnection();
    MySqlCommand cmd = new MySqlCommand();
    cmd.CommandText = "Select * from user where username=@user
    and password=@password limit 1";
    cmd.Parameters.Add("@user", MySqlDbType.VarChar).Value = txt_username.Text;
    cmd.Parameters.Add("@password",MySqlDbType.VarChar).Value= passwordbox_psw.Password;
    cmd.Connection = connect;
    MySqlDataReader login = cmd.ExecuteReader();
    if (login.Read())
       {
          connect.Close();
          return true;
       }
    else
       {
          connect.Close();
          return false;
       }
}

public static bool CheckPassword(string password, string hashedpassword)
{
   if (BCryptHelper.CheckPassword(password, hashedpassword))
       return true;
   else
       return false;
}