FYI, how to use AutoMapper in custom ResultFilterAttribute

[dylanwa]

Please check comments in
https://app.pluralsight.com/library/courses/building-async-api-aspdotnet-core/discussion
this function broken in v5.0.1 and works in v4.0.1

[dylanwa]

Using v4.0.1, add this in ConfigureServices, works
Service.AddAutoMapper()

Using v5.0.1, add this in ConfigureServices, get error "Mapper not initialized"
Service.AddAutoMapper()
must add explicit initialization to make it work:
services.AddAutoMapper();
Mapper.Initialize(cfg => cfg.AddProfile());

[lbargaoanu]

https://github.com/AutoMapper/AutoMapper.Extensions.Microsoft.DependencyInjection#mappermap-usage

[dylanwa]

Thanks for the link.
It seems the static Map function is not recommended in v5.0.1 now.

Could you please give me an idea how to use _mapper.Map in a custom ResultFilterAttribute?

https://github.com/KevinDockx/BuildingAsyncAPIAspNetCore/blob/master/Finished%20sample/Books/Books.Api/Filters/BookResultFilterAttribute.cs

[lbargaoanu]

I would change the question to how do you do DI in general in a custom ResultFilterAttribute. I don't have an answer to that, but surely it's possible somehow.

[jbogard]

With any filter, they don't want you to do constructor injection. Filters typically have a singleton lifecycle. Instead, core ASP.NET extensions pass in a context parameter that then gives you access to an IServiceProvider. So that example should be:

using AutoMapper;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace Books.Api.Filters
{
    public class BookResultFilterAttribute : ResultFilterAttribute
    {
        public override async Task OnResultExecutionAsync(ResultExecutingContext context, 
            ResultExecutionDelegate next)
        {
            var resultFromAction = context.Result as ObjectResult;
            if (resultFromAction?.Value == null
               || resultFromAction.StatusCode < 200
               || resultFromAction.StatusCode >= 300)
            {
                await next();
                return;
            }
            
            var mapper = context.HttpContext.RequestServices.GetService<IMapper>();
            resultFromAction.Value = mapper.Map<Models.Book>(resultFromAction.Value);

            await next();
        }
    }
}

[dylanwa]

Awesome, thanks Jimmy, that's what I want!

Just a small change on your code and it works!

var mapper = (IMapper) context.HttpContext.RequestServices.GetService(typeof(IMapper)); resultFromAction.Value = mapper.Map<Models.Book>(resultFromAction.Value);

[jbogard]

FYI that GetService<T>() method is an extension method in the Microsoft.Extensions.DependencyInjection namespace.

[dylanwa]

You're right! 💯

[lock]

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