I have been trying to declare a base implementation in an interface using the its own property but its not working

public interface ITutorial
{
    void SetTutorial()
    {
        if (!isTutorial)
        {
            isTutorial = true;
        }
    }

    bool isTutorial { get; set;}
}

I expected isTutorial to return true after calling SetTutorial, but it returns false. How do I do this?