WCF - 依赖注入和 SOLID 原理的接口或 Class

Question

I am migrating an old.Net Framework 4.7.2 MVC application to.Net 6 and I am using some web services. As Asp.Net 6 MVC is setup for Dependency Injection, I am trying to follow the principles of SOLID in my updated design.

I know in the service container I can add the service as:

builder.Services.AddTransient<IWebService, WebServiceClient>();

Then in the dependent class:

private readonly IWebService _ws;

public SomeClass(IWebService ws)
{
   _ws = ws;
}

This all works fine, but when using the interface of a web service, there are no methods exposed for OpenAsync or CloseAsync , but if I change it to this:

private readonly WebServiceClient _ws;

public SomeClass(IWebService ws)
{
    _ws = (WebServiceClient)ws;
}

I now have access to the concrete class and it exposes OpenAsync and CloseAsync

Question(s) :

1. Is it safe to use the Interface and not be able to call OpenAsync or CloseAsync ? Will this potentially cause unreleased resources and risk the object not being disposed?

2. If I go with the second example and use the concrete class web service client instead of the Interface, is this breaking the D in SOLID -

Entities must depend on abstractions, not on concretions. It states that the high-level module must not depend on the low-level module, but they should depend on abstractions.

Answer 1

Even with letting SOLID aside, using interfaces instead of concrete classes allows you to perform unit testing with mocked dependencies. You can't mock concrete classes but only interfaces. When you will try to mock the interface in your unit tests, your concrete class under your interface will be replaced by a mocked version of it with your custom behavior. With your code in the second example, your application will crash because the mock can't be casted into WebServiceClient.

Talking about SOLID, your not breaking the D really, but the L. Liskov Substitution means that you should be able to replace an interface implementation at any time without breaking your code. Therefore, if you put any implementation other than WebServiceClient under your interface, your cast will make the application crash.

So anyway, the second example is a really bad idea, both for solid and unit testing.

Now, about not calling OpenAsync and CallAsync. I assume WebServiceClient and IWebService are both classes you have access to (I mean, they are not from an external library). Why don't you just declare OpenAsync and CloseAsync in your interface so you can call them from it?