The Dependency Inversion Principle with .NET Framework classes

Question

I'm trying to understand SOLID principles, in particular The Dependency Inversion Principle.

In this is SO answer it is explained very well.

I think I have understood that I can't create any instance of a class inside my class. Is it right?

But if I have to save to disk some content, can I create an instance of System.IO.File or do I have to inject it?

I don't understand where is the limit, if I can't instance my own classes or if I can't either instance .NET Framework classes (or whatever other framework).

UPDATE:
I think File is a bad example because is declared as static.

By the way, does this principle apply to static classes?

Answer 1

The S of SOLID stands for SRP (Single Responsibility Principle). You won't violate it by using System.IO.File inside a class directly, once you keep that class with one single responsibility.

It's a good idea trying to abstract the purpose behind using System.IO.File. Let's suppose you need it to generate a log. Then you would probably do something like:

public interface IMyLogger
{
    void GenerateLog(IEnumerable<string> content);
}

public class FileLogger: IMyLogger
{
    public void GenerateLog(IEnumerable<string> content)
    {
        System.IO.File.WriteAllLines("C:/Log", content);
    }
}

Maybe it's not just a log, it's something more important, like generating a file so other system/app (even external) read it and do some job.

If you are trying to use a DDD approach, the interface could belong to your domain, and the implementation could belong in the application. Then you register your interface as a service and inject it.

The class which needs an IMyLogger actually doesn't need to know how is the log being generated, it just needs the job to be done.

You can apply the same idea when you need to send an email inside some business logic in your domain. Instead of making a connection to an Exchange inside your domain directly, create an interface INotifier and a MailNotifier implementing it to be injected.