Interface segregation (ISP).md

Interface Segregation Principle (ISP)

In this example, the IMachine interface includes Print, Fax, and Scan methods. However, the SimplePrinter only needs the Print method, but it is forced to implement Fax and Scan as well. This results in empty or dummy methods, which can be confusing and lead to maintenance issues. You might not want the SimplePrinter to have Fax and Scan capabilities, but since these methods are required by the IMachine interface, you need to implement them all.

class Document { }

interface IMachine {
	void Print(Document document);
	void Fax(Document document);
	void Scan(Document document);
}

class SimplePrinter : IMachine {
	public void Print(Document document) {
		Console.WriteLine("Printing");
	}

	public void Fax(Document document) {}
	public void Scan(Document document) {}
}

class MultiFunctionPrinter : IMachine {
	public void Print(Document document) {
		Console.WriteLine("Printing with colors the document");
	}

	public void Fax(Document document) {
		Console.WriteLine("Sending fax");
	}

	public void Scan(Document document) {
		Console.WriteLine("Scanning the document");
	}
}

This code violates the Interface Segregation Principle (ISP) because it forces the SimplePrinter class to implement methods that it doesn't need.

By having a large, monolithic interface, any change in the IMachine interface affects all implementing classes, even those that don't use the new or changed methods. This increases the risk of bugs and makes the code harder to maintain and extend.

To address this issue, you can split the IMachine interface into smaller, more specific interfaces that only include methods relevant to the classes implementing them. This way, SimplePrinter would only implement a IPrinter interface with a Print method, and other devices could implement IScanner and IFax interfaces as needed.

💡 Notice how MultiFunctionMachine is composed of other components.

SimplePrinter printer = new();

Photocopier scanner = new();

IMultiFunctionDevice multi = new MultiFunctionMachine(
	printer, 
	scanner
);

interface IPrinter {
    void Print(Document document);
}

interface IScanner {
    void Scan(Document document);
}

interface IFax {
    void Fax(Document document);
}

interface IMultiFunctionDevice : IPrinter, IScanner, IFax {}

class SimplePrinter : IPrinter {
    public void Print(Document document) {
        Console.WriteLine("Printing the document");
    }
}

class Photocopier : IPrinter, IScanner {
    public void Print(Document document) =>
        throw new NotImplementedException();

    public void Scan(Document document) =>
        throw new NotImplementedException();
}

struct MultiFunctionMachine(
	IPrinter printer, 
	IScanner scanner
) : IMultiFunctionDevice {
    public void Print(Document document) => printer.Print(document);
    public void Scan(Document document) => scanner.Scan(document);
    public void Fax(Document document) => throw new NotImplementedException();
}