lambda 表达式拼接

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<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);
        }
    }
    public static class LambdaHelper
    {
        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);
        }
        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);
        }
    }
}

 

使用方法1:

 

 

TyMall.Entity.DbEntities db = new DbEntities();
Expression<Func<tb_advert, bool>> 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<Preson> l = new List<Preson>();
    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<Func<Preson, bool>> 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();

}