无法使用类型创建抽象类或接口接口的实例

Question

I know what this error is, my question is slightly different.

This is one of these brain fade moments.

I've been reading this interesting article on Repositoy and Unit of Work patterns:

http://www.asp.net/mvc/tutorials/getting-started-with-ef-using-mvc/implementing-the-repository-and-unit-of-work-patterns-in-an-asp-net-mvc-application

I am having a play with this code and I'm now trying to tweak it so I can use IoC with DI.

Now, here is where I have just hit a mental blank.

I am changing all my classes to use interfaces etc. So given this code snippet:

    public class UnitOfWork : IDisposable
    {
        private SchoolContext context = new SchoolContext();
        private GenericRepository<Department> departmentRepository;
        private GenericRepository<Course> courseRepository;

        public GenericRepository<Department> DepartmentRepository
        {
            get
            {

                if (this.departmentRepository == null)
                {
                    this.departmentRepository = new GenericRepository<Department>(context);
                }
                return departmentRepository;
            }
        }
    }
}

How do I edit this line:

this.departmentRepository = new GenericRepository<Department>(context);

So I don't have to reference the actual class so I can ensure it is decoupled and avoid producing the syntax error:

Cannot create an instance of the abstract class or interface interface with type

Many thanks in advance!

Answer 1

In brief, you should declare the field as an interface type. The interface could have a name like IGenericRepository<T> . Then the class GenericRepository<T> should implement that interface. Then you register the class with your IoC framework, which basically tells the framework, "when I ask for an instance of this interface, give me an instance of that class". Then you do something like this:

this.departmentRepository = InversionOfControl.Resolve<IGenericRepository<Department>>();

(The resolve method has a signature like public T Resolve<T>() )

Answer 2

http://www.tugberkugurlu.com/archive/generic-repository-pattern-entity-framework-asp-net-mvc-and-unit-testing-triangle

Seems GenericRepository be abstract class which cannot be instantiated.

http://msdn.microsoft.com/en-us/library/sf985hc5(v=vs.71).aspx

You need derived class for GenericRepository, ie

DepartmentRepository : GenericRepository<Department>