public void ensure_initialized_fails_if_not_yet_initialized()
        {
            var sut = new InitializationGuard(() => Observable.Return(Unit.Default));
            var ex  = Assert.Throws <InitializationException>(() => sut.EnsureInitialized());

            Assert.Equal("Not yet initialized.", ex.Message);
        }
        public void ensure_initialized_succeeds_if_initialized()
        {
            var sut = new InitializationGuard(() => Observable.Return(Unit.Default));

            sut
            .Initialize()
            .Subscribe();

            sut.EnsureInitialized();
        }