How to solve No parameterless constructor defined for this object error in Mvc?

问题

I am creating one demo application to learn how to use repository pattern for performing Insert operation.I am using Nop Commerce**(http://www.nopcommerce.com) **code for repository pattern

Error:No parameterless constructor defined for this object

I have seen this link:MVC: No parameterless constructor defined for this object

This is my Structure:

My Repository interface:

public partial interface IRepository<T>
    {
        void Insert(T entity);
     }

My Service Layer:

 public partial interface IEmployeeService
    {
        void InsertCategory(EmployeeMaster employeeMaster);
    }

My Class which will implement that interface(service):

public partial class EmployeeService : IEmployeeService
    {
        #region Fields
        private readonly IRepository<EmployeeMaster> _employeeMasterRepository;
        #endregion

        #region Ctor
        public EmployeeService
            (
            IRepository<EmployeeMaster> employeeMasterRepository
            )
         {
             this._employeeMasterRepository = employeeMasterRepository;
         }

        #endregion



public virtual void InsertCategory(EmployeeMaster employeeMaster)
        {
            if (employeeMaster == null)
                throw new ArgumentNullException("employeeMaster");

            _employeeMasterRepository.Insert(employeeMaster);
}

This is my controller:

public class HomeController : Controller
    {
        #region Fields
        private readonly IEmployeeService  _employeeService;
        #endregion

 #region Constructors
        public HomeController
        (
            IEmployeeService employeeService
        )
        {
            this._employeeService = employeeService;
        }
        #endregion

Getting Error:No parameterless constructor defined for this object

I have studied regarding this error and all the sources are saying that use Dependency injection to solve this error.

Can anybody guide me how to use dependency injection to solve this error??

回答1:

You're correct - you'll need to use something like Unity, Ninject or Autofac to get this running.

Essentially - the steps are roughly the same regardless of the tech used.

Using unity as an example:

1) In your MVC project, use NuGet to add "Unity Bootstrapper for MVC". (right click references, manage nuget packages, search for unity online, select "Unity Bootstrapper for MVC"

This will add a few things to your project - the most interesting is the unityConfig.cs in App_start.

2) In unityConfig.cs - find RegisterTypes method

RegisterTypes is where you tell the unity container "when you need one of X, use implementation Y" - in your case "when you need an IEmployeeService, use EmployeeService"

3) In register types - add your type mappings - for example, in your case:

container.RegisterType<IEmployeeService, EmployeeService>();

AND

4) Bit more complicated: as you have IRepository and you need this to resolve your employeeService correctly you'll also have to register a generic type against a generic implementation (it's a little odd). Looking at your types, it's going to be something like:

container.RegisterType(typeof(IRepository<>), typeof(Repository<>));

(You'll need to resolve the namespaces according to your projects).

So - what this is saying is...

Right MVC - when you need an IEmployeeService use employeeService, and when you need an IRepository<>, use Repository<>

Now - because of the webActivator that was added during the NuGet installation (right next door to UnityConfig.cs), that should be it - the webActivator sets the DependencyResolver.Current to something that uses this unityContainer.

Beacause of this, MVC will now use this new unityContainer whenever it tries to instantiate your controllers, resolving the type mappings on the way.

The webActivator and unityConfig classes do alot of the heavy-lifting for you, just elaborate registerTypes and you should at least have a foot-hold in the solution.

HTH - good luck.

PS - DI itself is a massive subject - far too much for here, you'll bump into all sort of weird and wonderful things (supplying Ctor args for resolutions, lifetime management just for a start!) - it's fun, but not trivial, so you'll have to excuse me if this doesn't work "out of the box" (it's almost impractical to think it would) - this is merely just a vague attempt to give you a foothold in the subject!

For more info on unity - I suggest you read up here (Dev guide to using Unity on MSDN)

回答2:

Maybe you're taking as a sample a project that is too complex for you at this moment. Nopcommerce is a big and full featured product that has a lot of elements, so it easy to get lost. Not the best sample to learn how the repository pattern works, but I certainly recommend you to check it again once you have the basic concepts clear to see them used in a real scenario.

NopCommerce uses Autofac for dependency injection, a very popular IoC container. You can look for a class called DependencyRegistrar in the project Nop.Web.Framework to see how it is used, if you're curious about it. You can get more examples on how to use dependency injection with Autofac in their repository and their getting started guide.

My recommendation is to look for an easier to follow example. For starters, any of the popular IoC containers will be OK until you have your own criteria to choose one. For instance you can follow the Autofac's guide for MVC

来源：https://stackoverflow.com/questions/28694532/how-to-solve-no-parameterless-constructor-defined-for-this-object-error-in-mvc

标签

asp.net-mvc

dependency-injection

nopcommerce