Html Encoding Code Blocks With ASP.NET 4
This is the first in a three part series related to HTML encoding
blocks, aka the
<%: ... %> syntax.
- Html Encoding Code Blocks With ASP.NET 4
- Html Encoding Nuggets With ASP.NET MVC 2
- Using AntiXss as the default encoder for ASP.NET
One great new feature being introduced in ASP.NET 4 is a new code block (often called a Code Nugget by members of the Visual Web Developer team) syntax which provides a convenient means to HTML encode output in an ASPX page or view.
<%: CodeExpression %>
I often tell people it’s
<%= but with the
= seen from the front.
Let’s look at an example of how this might be used in an ASP.NET MVC view. Suppose you have a form which allows the user to submit their first and last name. After submitting the form, the same view is used to display the submitted values.
First Name: <%: Model.FirstName %> Last Name: <%: Model.FirstName %> <form method="post"> <%: Html.TextBox("FirstName") %> <%: Html.TextBox("LastName") %> </form>
By using the the new syntax,
properly HTML encoded which helps in mitigating Cross Site Scripting
Expressing Intent with the new
If you’re paying close attention, you might be asking yourself
Html.TextBox is supposed to return HTML that is already sanitized.
Wouldn’t using this syntax with
Html.TextBox cause double encoding?”
ASP.NET 4 also introduces a new interface,
IHtmlString along with a
HtmlString. Any method that returns a value
that implements the
IHtmlString interface will not get encoded by this
In ASP.NET MVC 2, all helpers which return HTML now take advantage of this new interface which means that when you’re writing a view, you can simply use this new syntax all the time and it will just work.By adopting this habit, you’ve effectively changed the act of HTML encoding from an opt-in model to an opt-out model.
There were four primary goals we wanted to satisfy with the new syntax.
Obvious at a glance. When you look at a page or a view, it should be immediately obvious which code blocks are HTML encoded and which are not. You shouldn’t have to refer back to flags in
pagedirective (which could turn encoding on or off) to figure out whether the code is actually being encoded. Also, it’s not uncommon to review code changes via check-in emails which only show a DIFF. This is one reason we didn’t reuse existing syntax.
Not only that, code review becomes a bit easier with this new syntax. For example, it would be easy to do a global search for
<%=in a code base and review those lines with more scrutiny (though we hope there won’t be any to review). Also, when you receive a check-in email which shows a DIFF, you have most of the context you need to review that code.
Evokes a similar meaning to <%=. We could have used something entirely new, but we didn’t have the time to drastically change the syntax. We also wanted something that had a similar feel to
<%=which evokes the sense that it’s related to output. Yeah, it’s a bit touchy feely and arbitrary, but I think it helps people feel immediately familiar with the syntax.
Replaces the old syntax and allows developers to show their intent. One issue with the current implementation of output code blocks is there’s no way for developers to indicate that a method is returning already sanitized HTML. Having this in place helps enable our goal of completely replacing the old syntax with this new syntax in practice.
This also means we need to work hard to make sure all new samples, books, blog posts, etc. eventually use the new syntax when targeting ASP.NET 4.
Hopefully, the next generation of ASP.NET developers will experience this as being the default output code block syntax and
<%=will just be a bad memory for us old-timers like punch cards, manual memory allocations, and
Do While Not rs.EOF.
Make it easy to migrate from ASP.NET 3.5. We strongly considered just changing the existing
<%=syntax to encode by default. We eventually decided against this for several reasons, some of which are listed in the above goals. Doing so would make it tricky and painful to upgrade an existing application from earlier versions of ASP.NET.
Also, we didn’t want to impose an additional burden for those who already do practice good encoding. For those who don’t already practice good encoding, this additional burden might prevent them from porting their app and thus they wouldn’t get the benefit anyways.
When Can I Use This?
This is a new feature of ASP.NET 4. If you’re developing on ASP.NET 3.5,
you will have to continue to use the existing
<%= syntax and remember
to encode the output yourself.
In ASP.NET 4 Beta 2, you will have the ability to try this out yourself with ASP.NET MVC 2 Preview 2. If you’re running on ASP.NET 3.5, you’ll have to use the old syntax.
What about ASP.NET MVC 2?
As mentioned, ASP.NET MVC 2 supports this new syntax in its helper when running on ASP.NET 4.
In order to make this possible, we are making a breaking change such
that the relevant helper methods (ones that return HTML as a string)
will return a type that implements
In a follow-up blog post, I’ll write about the specifics of that change.
It was an interesting challenge given that
IHtmlString is new to
ASP.NET 4, but ASP.NET MVC 2 is actually compiled against ASP.NET 3.5