调用 HtmlHelper 的单元测试中的 NullReferenceException

Question

我编写了一个 HtmlHelper，它又调用 Html.TextBoxFor()。但是，当我执行测试时，我收到空引用异常。我假设我错误地创建了 HtmlHelper，但我不知道出了什么问题。

我已经尽可能简化了代码：

public static HtmlHelper CreateHtmlHelper(ViewDataDictionary viewData = null, TModel model = null) where TModel : class, new()
{
    if (viewData == null)
        viewData = new ViewDataDictionary(model ?? new TModel());

    Mock mockViewContext = new Mock(
                                            new ControllerContext(
                                                new Mock().Object,
                                                new RouteData(),
                                                new Mock().Object),
                                            new Mock().Object,
                                            viewData,
                                            new TempDataDictionary(),
                                            new StringWriter());

    var mockViewDataContainer = new Mock();

    mockViewDataContainer
        .Setup(v => v.ViewData)
        .Returns(viewData);

    return new HtmlHelper(mockViewContext.Object, mockViewDataContainer.Object);
}

[TestMethod]
public void ReadOnlyTextBox_Returns_ReadOnly_Textbox()
{
    var cl = new Class1();
    var helper = CreateHtmlHelper(model: cl);
    MvcHtmlString result = helper.ReadOnlyTextBox(m => m.First);

    Assert.IsNotNull(result);
}

public static MvcHtmlString ReadOnlyTextBox(this HtmlHelper helper, Expression> expr)
{
    // Do some stuff
    return helper.TextBoxFor(expr, new { @readonly = "readonly", @disabled = "disabled" });
}

异常是在 helper.TextBoxFor() 处调用的。有人看到任何看起来不对劲的地方吗？ 我从 http://joyofexcellence.com/blog/index.php/2010/02/27/testing-htmlhelper-in-mvc-2-rc-2/