Lambda 表达式查询条件拼接 - 鞍山老菜鸟 - 博客园

baacloud免费翻墙vpn注册使用

来源: Lambda 表达式查询条件拼接 – 鞍山老菜鸟 – 博客园

public static class PredicateBuilder
    {    
      /// <summary>              /// 可以理解为创建一个初始化为True的Predicate。              /// 注意:如果你是要创建一个OR组成的Predicate就不能把它初始化为True因为这样这个表达试永远为True了。              /// </summary>              /// <typeparam name="T"></typeparam>              /// <returns></returns>
     public static Expression<Func<T, bool>> True<T>() { return f => true; }  
      /// <summary>              /// 可以理解为创建一个初始化为False的Predicate。              ///注意:如果你是要创建一个AND组成的Predicate就不能把它初始化为False因为这样这个表达试永远为False了。              /// </summary>              /// <typeparam name="T"></typeparam>              /// <returns></returns>
public static Expression<Func<T, bool>> False<T>() { return f => false; }
        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // build parameter map (from parameters of second to parameters of first)
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.And);
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.Or);
        }

        class ParameterRebinder : ExpressionVisitor
        {
            private readonly Dictionary<ParameterExpression, ParameterExpression> map;

            public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
            {
                this.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 p)
            {
                ParameterExpression replacement;
                if (map.TryGetValue(p, out replacement))
                {
                    p = replacement;
                }
                return base.VisitParameter(p);
            }
        }
    }

调用方法

/*
       Lambda 表达式查询条件拼接

       调用实例

        var predicate = PredicateBuilder.True<Orders>();
        if (!string.IsNullOrWhiteSpace(select.so_text))
        {
            predicate = predicate.And(d => d.price > 10);
            predicate = predicate.And(d => d.price < 12);
        }


        var list = db.Orders.Where(predicate).ToList();
      全And:            var where = PredicateBuilder.True<int>();            where = where.And(x => x >= 50);            where = where.And(x => x <= 70);            var res = list.Where(where.Compile());

       全Or:            var list  = Enumerable.Range(1, 100);            var where = PredicateBuilder.False<int>();            where = where.Or(x => x == 50);            where = where.Or(x => x == 70);            var res = list.Where(where.Compile());

       var list = Enumerable.Range(1, 100);            var where = PredicateBuilder.True<int>();            where = where.And(x => x >= 50);            where = where.And(x => x <= 70);            var subwhere = PredicateBuilder.False<int>();            subwhere = subwhere.Or(x => x == 60);            subwhere = subwhere.Or(x => x == 61);            where = where.And(subwhere);            var res = list.Where(where.Compile());
*/
赞(0) 打赏
分享到: 更多 (0)

觉得文章有用就打赏一下文章作者

支付宝扫一扫打赏

微信扫一扫打赏