How can an object be used without instantiating it, by extension methods?

Question

I am following along in an ASP.NET MVC book. This is one of the unit tests:

    [TestMethod]
    public void Can_Generate_Page_Links()
    {
        // arrange
        HtmlHelper myHelper = null;

        PagingInfo pagingInfo = new PagingInfo
        {
            CurrentPage = 2,
            TotalItems = 28,
            ItemsPerPage = 10
        };
        Func<int, string> pageUrlDelegate = i => "Page" + i;

        // act 
        MvcHtmlString result = myHelper.PageLinks(pagingInfo, pageUrlDelegate);

        // assert
        Assert.AreEqual(result.ToString(), 
            @"<a href=""Page1"">1</a>" + 
            @"<a href=""Page2"">2</a>" +
            @"<a href=""Page3"">3</a>"
        );
    }

Notice how myHelper is set to null and then used? The only code that does anything to HtmlHelper is this extension method:

    public static MvcHtmlString PageLinks(this HtmlHelper html, PagingInfo pagingInfo, Func<int, string> pageUrl)
    {
        StringBuilder result = new StringBuilder();
        for (int i = 1; i <= pagingInfo.TotalPages; i++)
        {
            TagBuilder tag = new TagBuilder("a");
            tag.MergeAttribute("href", pageUrl(i));
            tag.InnerHtml = i.ToString();
            if (i == pagingInfo.CurrentPage)
                tag.AddCssClass("selected");
            result.Append(tag.ToString());
        }

        return MvcHtmlString.Create(result.ToString());
    }

Does the extension method have something to do with allowing us to use an object that was set to null? If not, what is happening here?

Yes, that did answer my question. So, think of it as static because it never does a null check. — user1873073, Dec 29 '13 at 02:00

score 3 · Accepted Answer · answered Dec 29 '13 at 01:55

You are correct.

Only because extension methods are syntactic sugar. When compiled, it is essentially a call like this:

YourStaticClass.PageLinks(myHelper, pagingInfo, pageUrlDelegate);

Notice in the PageLinks method, the html parameter is never used. If it were, it would throw a NullReferenceException.

How can an object be used without instantiating it, by extension methods?

1 Answers1