string userName = LoginUser.UserName;
            string password = LoginUser.Password;
            string comm = "SELECT Username,Password,Clientname,Role FROM Users WHERE Username = '" + userName + "';";
            bool rememberUserName = LoginUser.RememberMeSet;

            SqlConnection conn = new SqlConnection(connstring);
            conn.Open();

            SqlCommand command = new SqlCommand(comm, conn);
            SqlDataAdapter da = new SqlDataAdapter(command);
            DataTable dt = new DataTable();
            da.Fill(dt);
            DataRow dr = dt.NewRow();
            if (dt != null)
            {
                //logic
            }

if (dt.Rows.Count > 0)

if(dt != null && dt.Rows.Count > 0)

SqlCommand command = new SqlCommand(comm, conn);
        using (var reader = command.ExecuteQuery())
        {
            if (reader.Read())
            {
                //logic
                var userName = reader.GetString(0);
                var password = reader.GetString(1);
                // etc
            }
        }

try
{
   //dataTable operations
}
catch(IndexOutOfRangeException)
{
}

if (ds.Tables[0].Rows.Count > 0)