Result of mapper is NULL? How do I map my api request to a dto?

Question

I'm making a call to an api, and then deserializing that response to a list, when i am then trying the response to a new list:

    public async Task<IEnumerable<RoadDto>> GetRoadStatusDetail()
    {
        List<Road> road = await CallApi();

        return road
            .Select(x => _mapper.Map(x)); 

    }


    private async Task<List<Road>> CallApi()
    {
        HttpClient client = new HttpClient();
        client.BaseAddress = new Uri(baseURL);

        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        HttpResponseMessage res = await client.GetAsync(baseURL);

        if (res.IsSuccessStatusCode)
        {
            var roadResponse = res.Content.ReadAsStringAsync().Result;

           var road = JsonConvert.DeserializeObject<List<Road>>(roadResponse);

           return road;
        }

        return null;
    }

I am not using auto mapper, rather I am using this generic method:

public interface IMapToNew<in TIn, out TOut>
{
    TOut Map(TIn model);
}

The error I am getting is

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
_mapper was null.

I'm not sure why it would be null, i've created a mapping class here that should handle that?

   public class RoadToRoadDtoMapper : IMapToNew<Road, RoadDto>
{
    public RoadDto Map(Road model)
    {
        return new RoadDto
        {
            DisplayName = model?.DisplayName,
            StatusSeverity = model?.StatusSeverity,
            StatusSeverityDescription = model?.StatusSeverityDescription
        };
    }

}

Does this answer your question? [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — yaakov, Dec 16 '19 at 02:47
Having a variable defined in your scope isn't enough... it needs to be initialized as well. Null reference means that object was never initialized that's throwing the error or that property doesnt exist — Jawad, Dec 16 '19 at 02:49
@Jawad can you give an example? im not too sure what you mean? — dros, Dec 16 '19 at 02:54
The error states that `_mapper` is null. Make sure it's being injected properly. — Matt U, Dec 16 '19 at 03:26
is this dependency injection? I think i understand now, my mapping class 'RoadToRoadDtoMapper' is not being read by my _mapper injector? — dros, Dec 16 '19 at 03:35

kaffekopp · Accepted Answer · 2019-12-16T07:04:42.633

The _mapper variable is null, as the error states. DI your mapper interface to your controller:

private readonly IMapToNew<Road, RoadDto> _mapper;

// Make sure your controller constructor takes your mapper interface.
public MyController(IMapToNew<Road, RoadDto> mapper)
{
    // Assign the constructor parameter to your instance 
    _mapper = mapper; variable.
}

In order for dependency injection to work, you need to register your interface and the implementation in Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services.AddScoped<
        IMapToNew<Road, RoadDto>,
        RoadToRoadDtoMapper>();
}

Result of mapper is NULL? How do I map my api request to a dto?

1 Answers1