大家帮我看看,出错在哪里啊

string sql = "insert into sgfsjbys values ('" + tb_shijian.Text.ToString().Trim() + "','" + tb_didian.Text.ToString().Trim();
        sql += "','" + ddl_tianqi.Text.ToString().Trim() + "','" + ddl_jsy.Text.ToString().Trim() + "','" + tb_sfz.Text.ToString().Trim();
        sql += "','" + ddl_clzbh.Text.ToString().Trim() + "','" + tb_clpzh.Text.ToString().Trim() + "','" + tb_clcd.Text.ToString().Trim();
        sql += "','" + tb_clxl.Text.ToString().Trim() + "','" + tb_sggc.Text.ToString().Trim() + "','" + tb_sgswrs.Text.ToString().Trim;
        sql += "','" + ddl_yjzr.Text.ToString().Trim() + "','" + tb_sgaqy.Text.ToString().Trim() + "','" + tb_djr.Text.ToString().Trim();
        sql += "','" + tb_djsj.Text.ToString().Trim() + "')";

红色的为出错的地方
出错原因:    运算符“+”无法应用于“string”和“方法组”类型的操作数