class IShapeProviderState {
    Shape GetShape(ShapeProvider provider);
}
class Square : Shape {

}
class Rect : Shape {

}
class SquareProvider : IShapeProvider {
    public void GetShape() {
        return new Square();
    }
}
class ShapeProviderShapesLeft : IShapeProviderState {
    public Shape GetShape(ShapeProvider provider) {
        provider.RemoveShape();
        return provider.CreateShape();
    }
}
class ShapeProviderShapesEnd : IShapeProviderState {
    public Shape GetShape(ShapeProvider provider) {
        throw new Exception("Figures ended");
    }
}

class ShapeProvider {
    int shapesLeft = 10;
    public IShapeProviderState State;
    public IShapeProvider Provider {get;set;} //по клику ставим какой-либо провайдер
    public ShapeProvider() : this(null) {

    }
    public ShapeProvider(IShapeProvider provider) {
        Provider = provider;
    }
    public Shape GetShape() {
        return State.GetShape(this);
    }
    public Shape CreateShape() {
        if(Provider != null)
            return Provider.GetShape();
        else
            throw new Exeption("ShapeProvider wasn't set");
    }
    public Shape CreateShape(IShapeProvider provider) {
        if(provider != null) {
            return provider.GetShape();
        } else {
            throw new Exeption("ShapeProvider wasn't set");
        }
    }
    private int ShapesLeft {
        get {
            return shapesLeft;
        }
        set {
            shapesLeft = value;
            AmountHasChanged();
        }
    }
    public void RemoveShape() {
        --ShapesLeft; //вызываем set
    }
    public void AddShape() {
        ++ShapesLeft; //вызываем set
    }
    private void AmountHasChanged() {
        if(shapesLeft == 0)  {
            State = new ShapeProviderShapesEnd();
        }else if(shapesLeft > 0) {
            State = new ShapeProviderShapesLeft();
        }
    }
}