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  • Expression表达式 实现and、or搜索

    用法:

            [HttpPost]
            public ActionResult GetBannerList(int pageIndex, int pageSize, string search)
            {
                Result result = new Result();
                var db = new Credit_CloudEntities();
    
                Expression<Func<Banner, bool>> expre = x => x.Status == 1;
                if (!string.IsNullOrEmpty(search))
                {
                    expre = ExpressionFuncExtender.And(expre, x => x.Title.Contains(search));
                }
    
                result.Total = db.Banner.Count(expre);
    
                var list = db.Banner.Where(expre).OrderBy(x => x.Sort).ThenByDescending(x=>x.AddTime)
                    .Skip((pageIndex - 1) * pageSize).Take(pageSize)
                    .ToList();
                result.Data = list;
                result.ResultCode = (int)ResultCode.Success;
                result.Message = "获取成功";
    
                return Json(result);
            }

    封装方法:

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace Common
    {
        public static class ExpressionFuncExtender
        {
            /// <summary>
            ///     以特定的条件运行组合两个Expression表达式
            /// </summary>
            /// <typeparam name="T">表达式的主实体类型</typeparam>
            /// <param name="first">第一个Expression表达式</param>
            /// <param name="second">要组合的Expression表达式</param>
            /// <param name="merge">组合条件运算方式</param>
            /// <returns>组合后的表达式</returns>
            public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second,
                Func<Expression, Expression, Expression> merge)
            {
                var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
                var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
                return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
            }
    
            /// <summary>
            ///     以 Expression.AndAlso 组合两个Expression表达式
            /// </summary>
            /// <typeparam name="T">表达式的主实体类型</typeparam>
            /// <param name="first">第一个Expression表达式</param>
            /// <param name="second">要组合的Expression表达式</param>
            /// <returns>组合后的表达式</returns>
            public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first,
                Expression<Func<T, bool>> second)
            {
                return first.Compose(second, Expression.AndAlso);
            }
    
            /// <summary>
            ///     以 Expression.OrElse 组合两个Expression表达式
            /// </summary>
            /// <typeparam name="T">表达式的主实体类型</typeparam>
            /// <param name="first">第一个Expression表达式</param>
            /// <param name="second">要组合的Expression表达式</param>
            /// <returns>组合后的表达式</returns>
            public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first,
                Expression<Func<T, bool>> second)
            {
                return first.Compose(second, Expression.OrElse);
            }
    
            private class ParameterRebinder : ExpressionVisitor
            {
                private readonly Dictionary<ParameterExpression, ParameterExpression> _map;
    
                private ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
                {
                    _map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
                }
    
                public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map,
                    Expression exp)
                {
                    return new ParameterRebinder(map).Visit(exp);
                }
    
                protected override Expression VisitParameter(ParameterExpression node)
                {
                    ParameterExpression replacement;
                    if (_map.TryGetValue(node, out replacement))
                        node = replacement;
                    return base.VisitParameter(node);
                }
            }
        }
    }
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  • 原文地址:https://www.cnblogs.com/zhuyapeng/p/11277022.html
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