使用 Automapper 映射两个集合时如何使用 BeforeMap Action？

Question

When mapping two single objects with automapper, the BeforeMap Action is working fine:

return _mapper.Map<Project, ProjectDto>(projects, opt =>
{
    opt.BeforeMap((src, dest) => dest.ProvidedLanguage = requestedLanguage);
});

But when mapping two collections, BeforeMap does not work as src and dest are of type object now:

return _mapper.Map<IEnumerable<Project>, IEnumerable<ProjectDto>>(projects, opt =>
{
    opt.BeforeMap((src, dest) => dest.ProvidedLanguage = requestedLanguage);
});

I need the provided language for the mapping itself, so I cannot set it after mapping. How can I pass the provided language to the destination objects when mapping two collections?

Answer 1

I premliminary solved it like this:

        var result = new List<ProjectDto>();
        foreach (var item in projects)
        {
            var dest = new ProjectDto() { ProvidedLanguage = requestedLanguage };
            result.Add(_mapper.Map(item, dest));
        }

I know, that it doesn't look elegant. if anybody knows a better way to solve it, don't hesitate to share your idea.

Answer 2

Using BeforeMap() to tweak destination object is dangerous. Running your code:

return _mapper.Map<Project, ProjectDto>(projects, opt =>
{
    opt.BeforeMap((src, dest) => dest.ProvidedLanguage = requestedLanguage);
});

will result in a NullReferenceException as dest object was not initialized yet by AutoMapper. See, BeforeMap() works best at performing some tweaks over source object, because any work over destination object can be overridden by forthcoming mapping process. If you want to do tweaks to your destination object, it's better to use AfterMap() . That way you'll have the mapping process already over and the certainty that your destination object was initialized. I'm not saying you shouldn't use BeforeMap() , I'm just saying be cautious with it, and use it when you really know what you're doing.

As of mapping the collections, they may seem to be object type, but that's just what they seem to be. Put a breakpoint in debugger to see their type is correct.

And you won't bee able to set ProvidedLanguage property on IEnumerable . If you want to set that property on every ProjectDto that's inside that IEnumerable , use a foreach loop for that:

return _mapper.Map<IEnumerable<Project>, IEnumerable<ProjectDto>>(projects, opt =>
{
    opt.AfterMap((source, destination) =>
    {
        foreach (var dto in destination)
        {
            dto.ProvidedLanguage = requestedLanguage;
        }
    });
});