lambda 表达式拼接

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类库:

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

namespace Mvc.Util
{
    public class ParameterRebinder : ExpressionVisitor
    {
        private readonly Dictionary map;
        public ParameterRebinder(Dictionary map)
        {
            this.map = map ?? new Dictionary();
        }
        public static Expression ReplaceParameters(Dictionary 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);
        }
    }
    public static class LambdaHelper
    {
        public static Expression Compose(this Expression first, Expression second, Func 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(merge(first.Body, secondBody), first.Parameters);
        }
        public static Expression> And(this Expression> first, Expression> second)
        {
            return first.Compose(second, Expression.And);
        }
        public static Expression> Or(this Expression> first, Expression> second)
        {
            return first.Compose(second, Expression.Or);
        }
    }
}

使用方法1:

   
            TyMall.Entity.DbEntities db = new DbEntities();
            Expression> expression = t => true;
            expression = expression.And(t => t.id == 1);
            expression = expression.And(t => t.sort > 1);
            var rs=db.tb_advert.Where(expression);

使用方法2:

        static void Main(string[] args)
        {
            //模拟数据源
            List l = new List();
            for (int i = 1; i <= 100; i++) {

                Preson p = new Preson();
                p.id = i;
                p.age = i;
                l.Add(p);
            }
            //定义表达式
            Expression> ex = t => true;
            ex = ex.And(t => t.id >30);
            ex = ex.And(t => t.age > 25);
            ex = ex.And(t => t.age < 35);

            //查询  注意:AsQueryable
            var rs =l.AsQueryable().Where(ex).ToList(); ;
            foreach(Preson p in rs){
                Console.WriteLine(p.id);
            }
            Console.Read();

        }