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，并试图简化我可能在某个应用程序中使用的东西。我已经做了10多年的C#开发者，但直到现在才有理由跳进表达式

我试图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扩展方法，以便在创建自动映射时使用

示例地图：

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

让

Increment（）

为我做

+1

我已经环顾四周，意识到一个

ExpressionVisitor

可能是我所需要的，但我不确定从那里走到哪里

这看起来很相似，但我无法将这项工作转化为我的用例

您可以使用，而不是使用+1。 或者我们可以让它真正通用

大概是这样的：

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, TDestination>(this IMappingExpression<TSource, TDestination> expression, 
        Expression<Func<TDestination, string>> destinationMember, 
        Expression<Func<TSource, string>> sourceMember)
    {
        return expression.CustomAction(destinationMember, sourceMember, s => s.ToUpper());
    }


    public static IMappingExpression<TSource, TDestination> CustomAction<TSource, TDestination, TDestinationMember, TSourceMember>(
        this IMappingExpression<TSource, TDestination> expression, 
        Expression<Func<TDestination, TDestinationMember>> destinationMember, 
        Expression<Func<TSource, 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;
    }
}

---

谢谢如果我不增加整数结果，而是想对字符串结果执行一些转换，该怎么办？例如，假设我想

ToUpper（）

原始表达式的结果？@SvenGrosen同样的想法适用，您只需更改我将int设置为string的位置或您正在操作的任何值类型。@SvenGrosen我已使其更通用，以便您可以自定义操作并进行自己的扩展methods@SvenGrosen如果完全回答了您的问题，请将其标记为已回答：）

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, TDestination>(this IMappingExpression<TSource, TDestination> expression, 
        Expression<Func<TDestination, string>> destinationMember, 
        Expression<Func<TSource, string>> sourceMember)
    {
        return expression.CustomAction(destinationMember, sourceMember, s => s.ToUpper());
    }


    public static IMappingExpression<TSource, TDestination> CustomAction<TSource, TDestination, TDestinationMember, TSourceMember>(
        this IMappingExpression<TSource, TDestination> expression, 
        Expression<Func<TDestination, TDestinationMember>> destinationMember, 
        Expression<Func<TSource, 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;
    }
}

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

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