I'm using ASP.NET MVC2 and C#, but this question applies to ASP.NET in general.
This breaks:
<body id="<asp:ContentPlaceHolder ID="BodyID' runat="server" />">
Intellisense underlines the body tag and the opening quote immediately after id=, and complains:
Validation (HTML 4.01): Element 'body' is missing the '>' character from its start tag.
The asp element is ignored, and the id attribute is empty in the rendered HTML. (Same problem whether I use double or single quotes inside the ASP element, tho the latter breaks syntax hilighting in VS.)
This works (assuming I set the session variable):
<body id="<%: Session["BodyID"] %>">
Why is it that inline evaluation is supported inside HTML attributes, but ASP controls won't render inside attributes?
Here's my use case: based on data passed from the controller, the view knows what type of data it's rendering. The view injects data into various places in the master page. I can inject a title into the head, and markup into the body -- but I also want to inject data into some attributres. IDs and class names are the obvious examples, but there are others.
I want to do this while still maintaining valid markup; no tricks like dynamically rendering the entire body tag -- I want a page that looks like a valid HTML or XML doc at all times in Visual Studio.
Using inline eval is OK, but it requires me to set properties, which I do in the model or controller. In some cases that's necessary, but in others the values are static -- I have a view that's purpose-built, and I just need to inject a static value from the view into the master page. I don't want to go thru all the overhead of creating an abstract controller class, having all my controllers inherit from it, etc. just to get to the same functionality I already have when I'm injecting markup.
Side question (yes, I should open a separate question for it): What's the order of evaluation of ASP controls and inline code nuggets? I assume the code nuggets are resolved first, before the ASP controls, so I could e.g. put a code nugget inside an ASP control declaration. But I can't find docs that detail the process -- can anyone point them out to me?
Thanks!
UPDATE: Pauli mentioned that you can indeed use ContentPlaceholders anywhere you like, as long as they're not inside an element that's already marked runat="server". I tested again, and see he's correct -- I missed that initially. Visual Studio is still confused and gives an HTML validation warning, but the expected value appears in the attribute when the page is rendered. So, the answer to the question is "But you can!"
- you shouldn't use WebForms controls with MVC
- use '' around your ID and runat, since the parser is confused of all your conflicting "'s
Since the body tag doesn't have runat="server" its not treated as a server control, but just clear text and you can therefor put a contentplaceholder, or whatever other control wherever you want.
来源：https://stackoverflow.com/questions/4752023/why-cant-i-use-a-contentplaceholder-inside-html-attributes-in-asp-net