Base Classes with Optional Features

How should you use inheritance when a base class can’t implement a feature and derived classes don’t have to either?

Make the method in the base class virtual and have it throw a NotSupportedException. Also, expose a virtual Boolean method in derived classes. Clients can use this method to check an optional feature’s availability.

If only a few derived classes will implement an optional feature, place them in an interface.


public class File {
    // Read is an optional feature.
    public virtual void Read() {
        throw new NotSupportedException("Reading isn't supported.");

    public virtual bool CanRead() {
        return false;

public class TextFile : File {
    public override void Read() {
        // Implement the Read feature here.

    public override bool CanRead() {
        // Inform clients that the Read feature is available.
        return true;

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