使用AutoMapper时Expression的转换

此文章为转载：http://www.bubuko.com/infodetail-699735.html

参考链接：

http://q.cnblogs.com/q/34480/   dudu有回复，其中一个链接http://*.com/questions/7424501/automapper-for-funcs-between-selector-types可以打开，并且本人主要参考此链接进行修改

下文链接为automapper相关源码

http://www.symbolsource.org/Public/Metadata/NuGet/Project/AutoMapper/3.3.0-ci1017/Release/Unsupported,Version%3Dv0.0/AutoMapper/AutoMapper/Internal/MappingExpression.cs?ImageName=AutoMapper

以下为转载内容：

问题描述

项目中使用AutoMapper进行VO&DTO&Entity的互相映射，但是默认Map方法不支持Expression的转换。如

Expression<Func<Entity,bool>> fun = _ => _.A == "A";

希望转换成

Expression<Func<Dto,bool>> fun = _ => _.A == "A";

似乎解决方案就是解析ExpressionTree并映射替换节点。正好找到了人家的提问和解决方案

http://*.com/questions/7424501/automapper-for-funcs-between-selector-types

改造一下支持泛型，代码如下：

 #region using namespace

 using System;
 using System.Collections.Concurrent;
 using System.Linq.Expressions;
 using System.Reflection;
 using AutoMapper;
 using AutoMapper.QueryableExtensions;

 #endregion

 namespace Core
 {
     public static class FunctionCompositionExtensions
     {
         private static readonly ConcurrentDictionary<Tuple<Type, Type>, Tuple<MethodInfo, Expression>> Dictionary =
             new ConcurrentDictionary<Tuple<Type, Type>, Tuple<MethodInfo, Expression>>();

         private static readonly MethodInfo Method = typeof (FunctionCompositionExtensions).GetMethod("Compose",
             BindingFlags.NonPublic | BindingFlags.Static);

         public static Expression<Func<TDestination, bool>> MapExpression<TSource, TDestination>(
             this Expression<Func<TSource, bool>> selector)
         {
             var bulidMethod = Dictionary.GetOrAdd(new Tuple<Type, Type>(typeof (TSource), typeof (TDestination)), _ =>
             {
                 var expression = Mapper.Engine.CreateMapExpression<TDestination, TSource>();
                 return new Tuple<MethodInfo, Expression>(
                     Method.MakeGenericMethod(typeof (TDestination), typeof (bool), typeof (TSource)), expression);
             });
             return
                 bulidMethod.Item1.Invoke(null, new object[] {selector, bulidMethod.Item2}) as
                     Expression<Func<TDestination, bool>>;
         }

         public static Expression<Func<TX, TY>> Compose<TX, TY, TZ>(this Expression<Func<TZ, TY>> outer,
             Expression<Func<TX, TZ>> inner)
         {
             return Expression.Lambda<Func<TX, TY>>(
                 ParameterReplacer.Replace(outer.Body, outer.Parameters[], inner.Body),
                 inner.Parameters[]);
         }

         public static Expression<Predicate<TX>> ComposePredicate<TX, TZ>(this Expression<Predicate<TZ>> outer,
             Expression<Func<TX, TZ>> inner)
         {
             return Expression.Lambda<Predicate<TX>>(
                 ParameterReplacer.Replace(outer.Body, outer.Parameters[], inner.Body),
                 inner.Parameters[]);
         }
     }

     class ParameterReplacer : ExpressionVisitor
     {
         private readonly ParameterExpression _parameter;
         private readonly Expression _replacement;

         private ParameterReplacer(ParameterExpression parameter, Expression replacement)
         {
             _parameter = parameter;
             _replacement = replacement;
         }

         public static Expression Replace(Expression expression, ParameterExpression parameter, Expression replacement)
         {
             return new ParameterReplacer(parameter, replacement).Visit(expression);
         }

         protected override Expression VisitParameter(ParameterExpression parameter)
         {
             if (parameter == _parameter)
             {
                 return _replacement;
             }
             return base.VisitParameter(parameter);
         }
     }
 }

测试

 public class Entity
 {
 public string A { get; set; }
 }

 public class Dto
 {
 public string A { get; set; }
 }

 Mapper.CreateMap<Entity, Dto>();
 Mapper.CreateMap<Dto, Entity>();

 var list = new List<Dto>()
 {
 new Dto() {A = "A"},
 new Dto() {A = "B"},
 new Dto() {A = "C"},
 new Dto() {A = "D"},
 new Dto() {A = "E"},
 };

 //Predicate<Entity> fun = _ => _.A =="A";
 Expression<Func<Entity,bool>> funEntity = _ => _.A == "A";

 var query = list.Where(funEntity.MapExpression<Entity, Dto>().Compile());
 Assert.True(query.Count() == );

 Expression<Func<Entity, bool>> funEntity2 = _ => _.A == "F";
 var query2 = list.Where(funEntity2.MapExpression<Entity, Dto>().Compile());
 Assert.True(query2.Count() == );