C#•15mo ago

❔ Can someone help me understand this statement around dependency inversion with an example?

I was refreshing my memory on SOLID based on conversations in #chat today, and a particular statement is hurting my head this evening:

High-level modules should not depend on low-level modules. Both should depend on abstractions.

I'm wondering, can someone share a simple example that demonstrates how both can depend on abstractions? I think I'm just overthinking it at the moment.

4 Replies

Hazel 🌊💃OP•15mo ago

For clarity, the both part throws me off. I found another article that says what I currently do is fine So something like:

public interface IUserService {
    Task<User> Get(Guid id, CancellationToken cancellationToken);
}
public class CachedUserService : IUserService {
    ...
    public async Task<User> Get(Guid id, CancellationToken cancellationToken) {
        if (_cache.TryGetValue(id, out User cachedUser))
            return cachedUser;
    
        var request = new UserByIdRequest(id);
        var user = await _mediator.Send(request);
        return user;
    }
}
public class Sample {
    private readonly IUserService _userService;
    public Sample(IUserService userService) =>
        _userService = userService;

    // terrible example
    public async Task PrintUsername(Guid userId) {
         User user = await _userService.Get(userId);
         if (user is null) {
             Console.WriteLine($"User `{userId}` not found.");
             return;
         }

         Console.WriteLine($"User `{userId}` has a username of `{user.Username ?? "null"}`.");
    }
}

public interface IUserService {
    Task<User> Get(Guid id, CancellationToken cancellationToken);
}
public class CachedUserService : IUserService {
    ...
    public async Task<User> Get(Guid id, CancellationToken cancellationToken) {
        if (_cache.TryGetValue(id, out User cachedUser))
            return cachedUser;
    
        var request = new UserByIdRequest(id);
        var user = await _mediator.Send(request);
        return user;
    }
}
public class Sample {
    private readonly IUserService _userService;
    public Sample(IUserService userService) =>
        _userService = userService;

    // terrible example
    public async Task PrintUsername(Guid userId) {
         User user = await _userService.Get(userId);
         if (user is null) {
             Console.WriteLine($"User `{userId}` not found.");
             return;
         }

         Console.WriteLine($"User `{userId}` has a username of `{user.Username ?? "null"}`.");
    }
}

My understanding is that this is fine since I'm consuming the user service through an interface instead of interacting with CachedUserService directly.

x0rld 👻 🎃•15mo ago

yeap your Sample doesn't need to know if there is any cache system

Omnissiah•15mo ago

yeah, essentially you have a layer of implementations which inherits from an accompanying layer of abstractions when you have to use something from that layer you register dependencies in a module and you're done ymmv depending on where those abstractions are, sometimes architecture could be more sophisticated than that

Accord•15mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Can someone help me understand this statement around dependency inversion with an example?