Service using disposable repository

namespace MyNamespace
{
    public class MyDbService : IMyDbService
    {
        IDbRepository<DbItem> _repository;
    readonly Type _repositoryType;

        public MyDbService()
        {
            _repositoryType = typeof(new DbRepository<DbItem>());
        }

        public MyDbService(IDbRepository<DbItem> repository)
        {
            _repository = typeof(repository);
        }

        public DbItem SomeFunction()
        {
        using (_repository = instantiateRepo())
        {
        // some actions on _repository returning DbItem
        }
        }

    IDbRepository instantiateRepo()
    {
        // returns IDbRepository concrete instance using _repositoryType
    }
    }
}