How to resolve generic interface to generic implementation when using dependency injection?

Question

I have created a generic repository that I want to be used in a service.

public abstract class AbstractBaseRepository<TEntity, TEntityKey>
        : IBaseRepository<TEntity, TEntityKey>
        where TEntity : class, IBaseEntity<TEntityKey>, new() { /* some code */ }

And the interface:

public interface IBaseRepository<TEntity, TEntityKey> { /* some code */ }

On my service I inject the repository like this:

public class TenantsService : AbstractBaseService<TenantEntity, int>
{
    public TenantsService(IBaseRepository<TenantEntity, int> tenantsRepository)
        : base(tenantsRepository) { }
}

On my startup, on the ConfigureServices method, I have:

services.AddScoped(typeof(IBaseRepository<,>), typeof(AbstractBaseRepository<,>));

I added this startup code based on the following two answers:

https://stackoverflow.com/a/33567396

https://stackoverflow.com/a/43094684

When I run the application I am getting the following error:

Cannot instantiate implementation type 'Playground.Repositories.Base.AbstractBaseRepository`2[TEntity,TEntityKey]' for service type 'Playground.Repositories.Base.IBaseRepository`2[TEntity,TEntityKey]'

Have you tried AddSingleton in place of AddScoped in startup.cs ? — Wit Wikky
– Wit Wikky, Commented Nov 1, 2018 at 12:31
You're asking the DI container to instantiate an abstract class, which, by definition, can't be instantiated. — Kirk Larkin
– Kirk Larkin, Commented Nov 1, 2018 at 12:34
That makes sense. Isn't there a way to specify that we want the interface to resolve to any class that extends a given class? In this case any class that extends the AbstractBaseRepository? — João Pinto
– João Pinto, Commented Nov 1, 2018 at 13:08
For extra functionality like that, see Default service container replacement in the docs. Another option is to use something like Scrutor. — Kirk Larkin
– Kirk Larkin, Commented Nov 1, 2018 at 14:33
What you're asking for is whether there is an assembly scanning feature that allows you to auto-register all non-generic implementations of your generic abstraction. Answer is: no, such feature does not exist out of the box. I agree with @KirkLarkin, Scrutor adds this missing functionality on top of ASP.NET Core's built-in container. — Steven
– Steven, Commented Nov 1, 2018 at 16:28

Shelby115 · Accepted Answer · 2018-11-01 12:42:49Z

1

Try this:

services.AddScoped(typeof(IBaseRepository<TenantEntity, int>), typeof(TenantsService));

As Kirk Larkin mentioned in the comments, you're telling it to instantiate an abstract class for services.AddScoped(typeof(IBaseRepository<,>), typeof(AbstractBaseRepository<,>)); which it can't do.

answered Nov 1, 2018 at 12:42

Shelby115

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