Dependecy Injection troubles: System.NullReferenceException: 'Object reference not set to an instance of an object.'

Question

Ok, I am making a Api, trying to use DI.

My Controller:

 [Route("api/[controller]")]
    [ApiController]
    public class TerminalsController : ControllerBase
    {
        private readonly IServiceWrapper _service;

        public TerminalsController(IServiceWrapper service)
        {
            _service = service;
        }

        [HttpPost]
        public async Task<ActionResult> Post([FromBody] Message object)
        {
            try
            {

                Result result = await _service.Terminal.UpsertInfo(ternminalObject);

                if (result.shopId != -1 || result.deviceId != -1 || result.companyId != -1)
                {
                    return Ok(result);
                }
                else
                {
                    return BadRequest("Can not save info from session on database");
                }
            }
            catch (Exception ex)
            {
                return StatusCode(500, "Internal server error");
            }
        }
    }

And the code of my service:

public class TerminalService : ITerminalService
    {
        private readonly IRepositoryWrapper _repository;

        public TerminalService(IRepositoryWrapper repository) 
        {
            _repository = repository;
        }

        public async Task<Result> UpsertInfo(company company)
        {
            try
            {
                var company = await _repository.Company.GetById(int.Parse(company.Id))

                return result;
            }
            catch (Exception ex)
            {
                throw ex;
            }

        }
    }

When my code rise the line

 var company = await _repository.Company.GetById(int.Parse(company.Id))

I get the error

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Here there are my others class:

My factory:

 public class DbClientFactory<T>
    {
        private static Lazy<T> _factoryLazy = new Lazy<T>(
            () => (T)FormatterServices.GetUninitializedObject(typeof(T)),
            LazyThreadSafetyMode.ExecutionAndPublication);

        public static T Instance
        {
            get
            {
                return _factoryLazy.Value;
            }
        }
    }

The factory instace the service and the repositories.

This is my StartUp.cs:

 public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
            SqlHelper.connectionString = Environment.GetEnvironmentVariable("CONNECTION_STRING");
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            services.ConfigureCors();
            services.AddMvc();
            services.ConfigureServiceWrapper();
            services.ConfigureRepositoryWrapper();
            services.AddControllers().AddNewtonsoftJson();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseHttpsRedirection();

            app.UseStaticFiles();
            app.UseCors("CorsPolicy");
            app.UseForwardedHeaders(new ForwardedHeadersOptions
            {
                ForwardedHeaders = ForwardedHeaders.All
            });

            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }

The ConfigureRepositoryWrapper and the ConfigureServiceWrapper are in the ServiceExtensions.cs:

 public static class ServiceExtensions
    {
        public static void ConfigureCors(this IServiceCollection services)
        {
            services.AddCors(options =>
            {
                options.AddPolicy("CorsPolicy",
                    builder => builder.AllowAnyOrigin()
                    .AllowAnyMethod());
            });
        }
        public static void ConfigureRepositoryWrapper(this IServiceCollection services)
        {
            services.AddScoped<IRepositoryWrapper, RepositoryWrapper>();
        }
        public static void ConfigureServiceWrapper(this IServiceCollection services)
        {
            services.AddScoped<IServiceWrapper, ServiceWrapper>();
        }
    }

The implement of ServiceWrapper is:

 public class ServiceWrapper : IServiceWrapper
    {
        private ITerminalService _terminal;

        public ITerminalService Terminal {
            get
            {
                if (_terminal == null)
                {
                    _terminal = DbClientFactory<TerminalService>.Instance;
                }
                return _terminal;
            }
        }
    }

And the implement of RepositoryWrapper is:

public class RepositoryWrapper : IRepositoryWrapper
    {
        private IDeviceRepository _device;
        private IShopRepository _shop;
        private ICompanyRepository _company;
        public IDeviceRepository Device
        {
            get
            {
                if (_device == null)
                {
                    _device = DbClientFactory<DeviceRepository>.Instance;
                }
                return _device;
            }
        }

        public IShopRepository Shop
        {
            get
            {
                if (_shop == null)
                {
                    _shop = DbClientFactory<ShopRepository>.Instance;
                }
                return _shop;
            }
        }

        public ICompanyRepository Company {
            get {
                if (_company == null)
                {
                    _company = DbClientFactory<CompanyRepository>.Instance;
                }
                return _company;
            }
        }
    }

I really dont know what is wrong here...

Thank you!

Answer 1

You are getting this error because company.Id does not exist.

var company = await _repository.Company.GetById(int.Parse(company.Id));

You are creating/fetching an object named company and at the same already have an object passed in that is named company, you can not do have two variables or objects in the same scope with the same name. Rename one of the objects from company to something else.

Also you likely are not passing in an object in the parameter, you are probably passing in null which is the reason for company.Id not existing. Check the value you are passing in, and rename one of the items.

var newCompany = await _repository.Company.GetById(int.Parse(company.Id));

If the exception is happening on _repository, the reason is that you are trying to pass a dependency into a non service class. To fix it, pass the dependency into the controller, then when initializing TerminalService just pass in the _repository. Also make sure that _service.Terminal is instantiated.

public class TerminalsController : ControllerBase
{
     private readonly IServiceWrapper _service;
     private readonly IRepositoryWrapper _repository;

    public TerminalsController(IServiceWrapper service, IRepositoryWrapper repository)
    {
         _service = service;
         _repository = repository;
    }

    [HttpPost]
    public async Task<ActionResult> Post([FromBody] Message object)
    {
        _service.Terminal = new TerminalService(_repository);
        Result result = await _service.Terminal.UpsertInfo(ternminalObject);

        //remaining code    
    }
}