ExecuteReader() error อะไรก็ไม่รู้ครับ รบกวนด้วยครับ

String sql = " SELECT MName,MLast FROM member WHERE MUsername = ";
sql += " '" + txtUsername.Text + "' ";
sql += " AND MPassword =  ";
sql += " '" + txtPassword.Text + "' ";

        string sql = string.Format(" SELECT MName,MLast FROM member WHERE MUsername = '{0}' AND MPassword = '{1}'"
            , txtUsername.Text
            , txtPassword.Text);