C# 映射器的结果为空？如何将api请求映射到dto？

我正在调用一个api，然后将该响应反序列化到一个列表，然后尝试响应到一个新列表：

    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;
    }

我不确定它为什么会为null，我在这里创建了一个映射类来处理它

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

}

public class RoadToRoadDtoMapper:IMapToNew
{
公共道路地图（道路模型）
{
返回新道路
{
DisplayName=型号？.DisplayName，
StatusSeverity=型号？。StatusSeverity，
StatusSeverityDescription=型号？.StatusSeverityDescription
};
}
}

错误状态下，

\u mapper

变量为空。将您的映射器接口DI到控制器：

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.
}

---

这回答了你的问题吗？在范围中定义变量是不够的。。。它也需要初始化。Null引用表示从未初始化引发错误的对象，或者该属性未初始化exist@Jawad你能举个例子吗？我不太清楚你的意思？错误表明

\u mapper

为空。确保它被正确注入。这是依赖注入吗？我想我现在明白了，我的制图器注入器没有读取我的制图类“RoadToLoaddToMapper”？

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

}

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.
}

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