Documentation: Improve discoverability of "htmlAttributes" entry in ViewDataDictionary

[Eilon]

From @dougbu on October 6, 2014 17:5

@Html.Editor() and @Html.EditorFor() special-case an entry named "htmlAttributes" in the ViewDataDictionary (optionally augmented using additionalViewData parameters) when executing their default templates. MVC 5.2 issue #2142 is an indication this feature needs additional documentation. Users will otherwise search for ways to control generated HTML attributes when using these methods.

Couple of details:

• "htmlAttributes" key is case-insensitive
• entry is handled very similarly to the htmlAttributes object passed to input helpers such as @Html.HiddenFor() and @Html.TextBox()

Copied from original issue: aspnet/Mvc#1233

[ardalis]

Where should this be documented? https://docs.microsoft.com/en-us/aspnet/core/mvc/views/overview seems like the closest place, currently.

[dougbu]

@ardalis that page doesn't mention @Html.Editor(...) or @Html.EditorFor(...) and those are the only methods that support "htmlAttributes" keys in the ViewDataDictionary. I suggest https://docs.microsoft.com/en-us/aspnet/core/mvc/views/working-with-forms

Side notes (FYI):

• Doesn't really matter what adds the key to ViewDataDictionary. It's most commonly done in the @Html.Editor[For]() call (using the additionalViewData parameter) because it's rare for the same HTML attributes to be correct for multiple calls in a view.
• @Html.Display() and @Html.DisplayFor() also support additionalViewData parameters but don't check the merged ViewDataDictionary for the "htmlAttributes" key.

[Rick-Anderson]

@ardalis can you do this under PO?