代码之家 › 专栏 › 技术社区 › Sven Grosen

c更改linq表达式的返回值

automapper linq c#

4

Sven Grosen · 技术社区 · 7 年前

一些背景:这只是我在玩 Expression 第一次使用对象,并尝试简化在我们的某个应用程序中可能使用的内容。我已经做了十多年的C开发人员,但直到现在我才有理由跳入表达式。

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扩展方法来在创建automapper映射时使用。

实例图:

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

我要做的是创建一个扩展方法,这样我就不必这么做了 + 1 任何地方都可以这样写:

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

并拥有 Increment() 做 + 1 为了我。

我环顾四周,意识到 ExpressionVisitor 可能是我需要的,但我不确定从那里去哪里?

这个 question 看起来很相似,但我无法将该工作翻译成我的用例。

1 回复 | 直到 7 年前

1

Victor Procure sachin BR 7 年前

你可以使用 Expression.Increment 而不是做+1。或者我们可以让它真正通用。

像这样:

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;
    }
}

输出:

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

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