EntityFramework动态多条件查询与Lambda表达式树 +分页查询

初次使用linq to sql 对于查询语句还不熟悉，特此记录下

查询分页方法（转）

    /// <summary>
        /// 分页查询 + 条件查询 + 排序
        /// </summary>
        /// <typeparam name="Tkey">泛型</typeparam>
        /// <param name="pageSize">每页大小</param>
        /// <param name="pageIndex">当前页码</param>
        /// <param name="total">总数量</param>
        /// <param name="whereLambda">查询条件</param>
        /// <param name="orderbyLambda">排序条件</param>
        /// <param name="isAsc">是否升序</param>
        /// <returns>IQueryable 泛型集合</returns>
        public IQueryable<_dpsui_logs> LoadPage_log<Tkey>(int pageSize, int pageIndex, out int total, Expression<Func<_dpsui_logs, bool>> whereLambda, Func<_dpsui_logs, Tkey> orderbyLambda, bool isAsc)
        {
            total = MyBaseDbContext.Set<keyou_dpsui_logs>().Where(whereLambda.Compile()).Count();
            if (isAsc)
            {
                var temp = MyBaseDbContext.Set<_dpsui_logs>().Where(whereLambda.Compile())
                             .OrderBy<_dpsui_logs, Tkey>(orderbyLambda)
                             .Skip(pageSize * (pageIndex - 1))
                             .Take(pageSize);
                return temp.AsQueryable();
            }
            else
            {
                var temp = MyBaseDbContext.Set<keyou_dpsui_logs>().Where(whereLambda.Compile())
                           .OrderByDescending<_dpsui_logs, Tkey>(orderbyLambda)
                           .Skip(pageSize * (pageIndex - 1))
                           .Take(pageSize);
                return temp.AsQueryable();
            }
        }

使用多条件查询帮助类需要修改的地方
MyBaseDbContext.Set<keyou_dpsui_logs>().Where(whereLambda)
修改为：
MyBaseDbContext.Set<keyou_dpsui_logs>().Where(whereLambda.Compile())

多条件查询帮助类

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;

namespace DPS
{
   public static class DynamicLinqExpressions
    {
        //多条件查询帮助类
        public static Expression<Func<T, bool>> True<T>() { return f => true; }
        public static Expression<Func<T, bool>> False<T>() { return f => false; }

        //注意this
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,
                                                            Expression<Func<T, bool>> expr2)
        {
            var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
            return Expression.Lambda<Func<T, bool>>
                  (Expression.Or(expr1.Body, invokedExpr), expr1.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,
                                                             Expression<Func<T, bool>> expr2)
        {
            var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
            return Expression.Lambda<Func<T, bool>>
                  (Expression.And(expr1.Body, invokedExpr), expr1.Parameters);
        }
    }
}

对于上面的修改做了下修改，由于多条件查询用到了上面的连接的方法。在执行时报错了，于是在网上再到了如下解决办法
来源：https://www.cnblogs.com/fengri/p/3376431.html

修改后运行正常

正常调用

  var eps = DynamicLinqExpressions.True<_dpsui_logs>();// 多条件查询集合
            DateTime date =DateTime.Parse( dateTimeInput1.Value.ToString("yyyy-MM-dd "));
            if (date.ToString().Split('/')[0] != "0001")
            {
                //关于时间 由于需求是查询特定某一天的数据。数据库中 时间字段包含 00:00:00
                //于是转换了下
                eps=eps.And(u =>u.time.Value.ToString("yyyy-MM-dd ") == date.ToString("yyyy-MM-dd "));
            }
            string username = txtuserName.Text;
            if (username != "")
            {
                eps = eps.And(u => u.username == username);
            }
            string opt = comboBox1.SelectedItem.ToString();
            if (opt != "请选择")
            {
                eps = eps.And(u => u.opt == opt);
            }
            int pagesize = int.Parse(cmd_pagecount.SelectedItem.ToString());//每页显示多少条数据
            int count = 0;//返回的页数
            //调用分页
            var log = dPShelper.LoadPage_log(pagesize, CurrentPage, out count, eps, u => u.id, true);
            txtpage.Text = count.ToString();//有多少条数据
            txtpagesizeCount.Text = string.Format("{0}", (count + 1) / pagesize + 1); ;//得到总页数
            txtpagesize.MaxValue = int.Parse(txtpagesizeCount.Text);

            DGV1.AutoGenerateColumns = false;
            DGV1.DataSource = log.ToList();//绑定数据的时候要 toLisst() 不然会报异常

更新2018-10-09

由于要查询两张表A和B，
要根据A中的ID查询B表中相应的数量
所以更新了下分页

/// <summary>
        /// card_A 是自己定义的实体类
        /// </summary>
        /// <returns></returns>
        //已发卡查询
        public IQueryable<Log.card_A> LoadPage_dpcinfo<Tkey>(int pageSize, int pageIndex, out int total, Expression<Func<dpcinfo, bool>> whereLambda, Func<dpcinfo, Tkey> orderbyLambda, bool isAsc)
        {
            total = MyBaseDbContext.Set<dpcinfo>().Where(whereLambda).Count();
            if (isAsc)
            {
                var temp = MyBaseDbContext.Set<dpcinfo>()
                                .Where(whereLambda)
                                .OrderBy<dpcinfo, Tkey>(orderbyLambda)
                                .Skip(pageSize * (pageIndex - 1))
                                .Take(pageSize)
                                .GroupJoin(MyBaseDbContext.cardda_mxz, d => d.dpcid, c => c.dpcid, (d, c) => new Log.card_A { dpcid = d.dpcid, addr = d.addr, count_add = c.Count() }) ;
                return temp.AsQueryable();
            }
            else
            {
                var temp = MyBaseDbContext.Set<dpcinfo>()
                               .OrderByDescending<dpcinfo, Tkey>(orderbyLambda)
                               .Skip(pageSize * (pageIndex - 1))
                               .Take(pageSize)
                                .GroupJoin(MyBaseDbContext.cardda_mxz, d => d.dpcid, c => c.dpcid, (d, c) => new Log.card_A { dpcid = d.dpcid, addr = d.addr, count_add = c.Count() });
                return temp.AsQueryable();
            }

        }

注意：Log.card_A是自己定义的实体类。根据界面上需要显示的数据源定义的一个实体类

绑定数据

做了个查询优化
1、当用户查询 刷新时，从当前页开始刷新，不会跳转到第一页
2、当显示行数增加，触发查询时，如当前页*显示页数超过数据总量时，自动显示最后一页
反之正常显示当前页数

 public void GOselect_card()
        {
            int pagesize = int.Parse(txtpageNow_card.Text);//当前页数
            int pagesizeCount = int.Parse(txtpagesizeCount_card.Text);//总页数
            int pagecount = int.Parse(cmd_pagecount_card.SelectedItem.ToString());//显示多少条
            int count_user = int.Parse(txtpage_card.Text);//总条数
            if (pagesize > 0 || pagesize <= pagesizeCount)
            {
                if ((pagesize * pagecount) > count_user && count_user != 0)
                {
                    txtpageNow_card.Value = int.Parse(txtpagesizeCount_card.Text);//最大页数
                    pagesizeCount = (count_user - 1) / pagecount + 1;//得到总页数
                    selectcard(pagesizeCount);
                }
                else
                {
                    selectcard(pagesize);
                }
            }
        }