How to use Dependency Injection with Static Methods?

Question

Imagine there is a Customer class with an instance Load() method.

When the Load() method is called, it retrieves order details by e.g.

var orders = Order.GetAll(customerId, ...);

GetAll() is a static method of the Order class and the input parameters are fields defined in the Customer class.

As you can see, Order is a dependency of the Customer class, however, I can't just create an IOrder and inject it there as interfaces can't have static methods.

Therefore, the question is how could I introduce dependency injection in this example?

I don't want to make GetAll() an instance method since it's a static method and need to keep it that way.

For example, I have used utility classes in my design, most of which just contain static methods.

Answer 1

If you must keep the static method, I would wrap the static calls in a Repository object.

Like this:

interface IOrderRepository {
   IEnumerable<IOrder> GetAll(customerId, ..);
}

class OrderRepository : IOrderRepository {
   IEnumerable<IOrder> GetAll(customerId, ...)
   {
     Order.GetAll(customerId,...); // The original static call.
   }
}

Now you inject this repository into your Customer class.

(I'm assuming you're doing this so you can inject fake IOrders at runtime for testing purposes. I should say that in general, static methods are a serious obstacle to testing.)

Answer 2

Seeing as your aggregate root for fetching orders is your customer model I would strongly advise you create a customer repository and inject that to whatever service requires it.

Here is an example:

public class CustomerService
{
    private readonly ICustomerRepository _customerRepository;

    public CustomerService(ICustomerRepository customerRepository)
    {
        if (customerRepository == null)
        {
            throw new ArgumentNullException("customerRepository");
        }

        _customerRepository = customerRepository;
    }

    public IEnumerable<IOrder> GetOrdersForCustomerId(int customerId)
    {
        return _customerRepository.GetOrdersForCustomerId(customerId);
    }
}

public interface ICustomerRepository
{
    IEnumerable<IOrder> GetOrdersForCustomerId(int customerId);
}

class CustomerRepository : ICustomerRepository
{
    public IEnumerable<IOrder> GetOrdersForCustomerId(int customerId)
    {
        throw new NotImplementedException();
    }
}