步步为营-104-Lambda语句

1:Lambda的拼接

　　首先借助一个Lambda的帮助类　　

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

namespace LintwayCommon
{
    public static class PredicateBuilder
    {

        public static Expression<Func<T, bool>> True<T>() { return f => true; }
        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);
        }
    }
    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);
        }
    }
}

　　在使用的时候调用

  public ActionResult GetListDataBySearch(int pageSize, int pageIndex, string saleNO, string handleStatus)
        {
            if (pageIndex < 1) { pageIndex = 1; }
            if (pageSize < 1)
            {
                pageSize = 5;
            }
            var whereLambda = PredicateBuilder.True<SaleInfo>(); 
        
            if (!String.IsNullOrWhiteSpace(saleNO))
            {
               whereLambda= whereLambda.And(c => c.SaleNO.Contains(saleNO));
            }
            if (!String.IsNullOrWhiteSpace(handleStatus))
            {
              whereLambda=  whereLambda.And(c => c.handleStatus.Equals(handleStatus));
            }


            int total = 0;
            var saleInfoList = _SaleServer.LoadPageEntities(pageIndex, pageSize, out total, whereLambda, false, c => c.SaleNO);

            return Json(new { rows = saleInfoList, total = total, countPage = pageIndex }, JsonRequestBehavior.AllowGet);
        }