I — Interface Segregation Principle — SOLID Principles with Examples

4. I — Interface Segregation Principle

Intermediate★ Must knowLesson 4 of 8

Clients should not be forced to depend on methods they don't use. Split fat interfaces into role interfaces.

csharp

// ❌ Fat interface — a basic printer must implement Fax and Scan
public interface IMachine { void Print(Doc d); void Scan(Doc d); void Fax(Doc d); }
public class BasicPrinter : IMachine
{
    public void Print(Doc d) { }
    public void Scan(Doc d) => throw new NotSupportedException();   // also an LSP smell
    public void Fax(Doc d) => throw new NotSupportedException();
}

// ✅ Role interfaces
public interface IPrinter { void Print(Doc d); }
public interface IScanner { void Scan(Doc d); }
public interface IFax { void Fax(Doc d); }
public class BasicPrinter2 : IPrinter { public void Print(Doc d) { } }
public class OfficeMachine : IPrinter, IScanner, IFax { /* all three */ }

Real-world .NET example: IReadOnlyList<T> vs IList<T>; IEnumerable<T> vs ICollection<T>.