必须声明标量变量 "@cid"。

出错代码：

        public bool Delete_List(int cID)
        {
            StringBuilder strSql = new StringBuilder();
            strSql.Append("delete from Emp");
            strSql.Append(" where C_Id = @C_Id");
            SqlParameter[] parameters = {
                                        new SqlParameter("@C_Id",SqlDbType.Int,4)};
            parameters[0].Value = cID;
            int rows = Helper.SQLHelper.ExcuteSQL(strSql.ToString());
            if (rows > 0)
            {
                return true;
            }
            else
            {
                return false;
            }
        }

出错原因：

没有在执行sql语句的ExcuteSQL函数中申明标量变量

修改方法： int rows = Helper.SQLHelper.ExcuteSQL(strSql.ToString(),parameters);

将ExcuteSQL函数改为:

        /// <summary>
        /// 执行SQL语句，返回受影响的记录数
        /// </summary>
        /// <param name="strSql">SQL语句</param>
        /// <param name="cmdParms"></param>
        /// <returns>受影响的行数</returns>
        public static int ExcuteSQL(string SQLString, params SqlParameter[] cmdParms)
        {
            using (SqlConnection connection = new SqlConnection(GetConnectionString()))//连接数据库
            {
                using (SqlCommand cmd = new SqlCommand())
                {
                    try
                    {
                        PrepareCommand(cmd, connection, null, SQLString, cmdParms);//关联SQL语句
                        int rows = cmd.ExecuteNonQuery();
                        cmd.Parameters.Clear();
                        return rows;
                    }
                    catch (System.Data.SqlClient.SqlException e)
                    {
                        throw e;
                    }
                }
            }
        }