Question

I used this article to create an Image helper:
http://www.asp.net/mvc/tutorials/older-versions/views/using-the-tagbuilder-class-to-build-html-helpers-cs. It says it replaces periods, but has no mention of other limitations.

It all works fine unless I try using GUID string as the id.

The Razor code:

@Html.Image(id, ....) //passes a GUID string to helper fine

The ImageHelper is exactly as the webpage describes:

builder.GenerateId(id);
//breakpoint after this shows the builder object 
//does not create an id attribute when id is a GUID string.
//using id.ToString() doesn't work for GUIDs either

Using the GUID string in MergeAttributes works fine:

builder.MergeAttribute("title", id); //the GUID string is img title no problem
builder.MergeAttribute("class", id); //the GUID string is class as expected 

Is there some limitation or workaround when using GUID strings as an ID?

Answer 1

IDs must start with a letter, not a number. Often, a GUID will start with a number, hence the problem. TagBuilder.GenerateId calls CreateSanitizedId, which performs this check:

if (!TagBuilder.Html401IdUtil.IsLetter(c1))
    return (string) null;

Your best bet is to just add a prefix, like “guid” or something to the beginning of the ID.