C#更改Linq表达式的返回值

一些背景：这只是我第一次玩Expression对象并尝试简化我在其中一个应用程序中使用的东西.我已经是一名C#开发人员超过10年了,但直到现在才有理由跳到表达式.

我正在尝试生成一个扩展方法来修改表达式的返回值.更具体地说,我正在尝试添加一个在创建AutoMapper地图时使用的扩展方法.

示例地图：

map.CreateMap<Widget,WidgetModel>()
   .ForMember(x => x.Count,opts => opts.MapFrom(src => src.Count + 1));

我想要做的是创建一个扩展方法,所以我不必在任何地方都这样做,只需将其写成：

map.CreateMap<Widget,WidgetModel>()
   .Increment(x => x.Count,src => src.Count);

并且让Increment()为我做1.

我已经环顾四周,意识到ExpressionVisitor可能就是我所需要的,但我不确定从那里去哪里？

这question看起来很相似,但我无法将这项工作转化为我的用例.

解决方法

您可以使用 Expression.Increment而不是1.
或者我们可以使它非常通用.

像这样的东西：

void Main()
{
    var mapperConfiguraiton = 
        new MapperConfiguration(cfg => 
            cfg.CreateMap<Widget,WidgetModel>()
               .Increment(x => x.CountD,src => src.Count)
               .toupper(x => x.Name,src=>src.Name));

    var widget = new Widget {Count = 3,Name="Jimmy"};
    var mapper = mapperConfiguraiton.CreateMapper();

    var model = mapper.Map<WidgetModel>(widget);
}

public class Widget {
    public int Count {get; set;}
    public string Name {get;set;}
}

public class WidgetModel {
    public int Count {get; set;}
    public string Name {get;set;}
}

public static class MapperExtensions {
    public static IMappingExpression<TSource,TDestination> Increment<TSource,TDestination>(this IMappingExpression<TSource,TDestination> expression,Expression<Func<TDestination,int>> destinationMember,Expression<Func<TSource,int>> sourceMember) 
    {
        return expression.CustomAction(destinationMember,sourceMember,s => s + 1);
     }

    public static IMappingExpression<TSource,TDestination> toupper<TSource,string>> destinationMember,string>> sourceMember)
    {
        return expression.CustomAction(destinationMember,s => s.toupper());
    }


    public static IMappingExpression<TSource,TDestination> CustomAction<TSource,TDestination,TDestinationMember,TSourceMember>(
        this IMappingExpression<TSource,TDestinationMember>> destinationMember,TSourceMember>> sourceMember,Expression<Func<TSourceMember,TDestinationMember>> transform)
    {
       var sourceMemberExpression = (MemberExpression)sourceMember.Body;        
       var sourceParameter = Expression.Parameter(typeof(TSource));

       var expr = Expression.Invoke(transform,Expression
                                    .MakeMemberAccess(sourceParameter,sourceMemberExpression.Member));

        var lambda = (Expression<Func<TSource,TSourceMember>>)
            Expression.Lambda(expr,sourceParameter);

        var newExpression = expression.ForMember(
             destinationMember,opts => opts.MapFrom(lambda));

        return newExpression;
    }
}

输出：

widget.Count = 3
widget.Name = "Jimmy"

model.Count = 4
model.Name = "JIMMY"