DesignPatterns | The 23 Gang of Four Design Patterns | Architecture library

As the linked issue says in the comments:

Class __SingletonObject is not be instantiated

I would point out a couple issues of semantics and terminology.

• It's confusing to have a method named fly that can be implemented as not flying. Naming the method tryToFly or documenting the method as merely an attempt are two ways of addressing this confusion. The software principle to reference here is Liskov Substitution.
• The base class does not implement one of the strategies; rather, it composes a strategy. The purpose of the Strategy pattern is to avoid subclassing through composition.
• To reiterate one of the comments, Duck should accept an instance of IFly directly in its constructor (or a setter method) rather than switching on an enum. Another goal of the Strategy pattern is to avoid branching logic.

The essence of the pattern is that you've avoided creating multiple subclasses of Duck by instead creating multiple implementations of IFly. This has the advantage that those IFly implementations can be reused without a complex inheritance hierarchy, e.g. WILD and CITY can share one strategy.

As mentioned in the comments, strategies also have the advantage that a Duck could change its strategy at runtime. For example, IFly might be implemented by Soar and Glide so that a Duck would switch between these different strategies depending on the wind.