Today the ASP.NET and Web Tools 2012.2 update was released. Go download it right now!

It contains a lot of new and updated Visual Studio tooling features including:

  • First class LESS editor
  • Knockout.js Intellisense
  • Paste JSON as classes
  • CoffeeScript editor
  • Mustache/Handlebars/JsRender syntax highlighting
  • Page Inspector
    • Live CSS auto-sync as you type
    • JavaScript selection mapping and callstack

Some of them are features that started their lives in Web Essentials 2012 and are now ported into an official release. Every time we move features from the experimental Web Essentials extension into the official product, we try to make the transition as smooth as possible.

However, this time we moved some substantial features that are mutually exclusive – they register the same MEF components and that leads to this rather ugly exception:

An exception has been encountered. This may be caused by an extension

This exception occurs when Web Tools 2012.2 is installed and you haven’t updated Web Essentials to version 2.5. The solution is simply to update Web Essentials. Go do that now. If you don’t have Web Essentials installed at all, you won’t get this error because then there is no conflict.


Comment by Egor

Hi Mads,

After installing new version of Web Essentials(2.5.1) and then Web Tools 2012.2, LESS files are displayed as regular text files, but solution explorer shows structure(Classes, IDs, Mixins etc) correctly.
When I try to use 'Open With', it has 'LESS Editor(Default)' selected. Uninstallation of Web Essentials does not help.

Is any way to fix it?



Comment by Victor

Try starting Visual Studio with the /setup flag. (devenv /setup)
This solved the issue here.

Note: You'll need administrator privileges.


Comment by Nick

I'm having the same problem with the no longer getting a LESS Editor. Using 'Open With' I don't even have LESS Editor in the list anymore, and looking at the change log for Web Essentials at http://vswebessentials.com/changelog it's actually showing the LESS/CoffeeScript editors as being 'Removed', so why do you list 'First class LESS editor' as one of the new features?


Comment by Nick

Apparently it was removed from Web Essentials 2012.2 and instead added to Web Tools 2012.2, installing that now. Not a lot of transparency on that though, and because of the identical version numbers I hadn't realized the two were different components.


Comment by Raj Krishnan

I tried installing the web tools 2012.2 and repeated it several times based on release notes. Still I can't find the project types. The extension says it is installed but I don't see the new project types and other features.

Raj Krishnan

Comment by Doug McDonald

Hi Mads, just a quick note on a similar topic. When i updated my web essentials to 2012.2 I get the following error when opening my solutions:

Could not load type 'Microsoft.Less.Core.LessMixinDeclaration' from assembly 'Microsoft.VisualStudio.Web.Extensions, Version=, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

Uninstalling web essentials allows the solutions to load and installing Web Tool 2012.2 and then web essentials afterwards sorts it. Just a heads up if it's worth adding to the list of potential exceptions.



Comment by sebastienf

Same issue that all previous comments : in the solution explorer I have the structure of the less file, but the file itself is displayed as a "simple" text file. Runned the devenv.exe /setup with administrator privileges, nothing happened.


Comment by Daniel

Guys as you know Web essentials can't be installed on Express version, can we install ASP.NET and Web Tools on VS 2012 Express for Web?

It would be great if everyone get hands on cool new stuff.

@Mads, can you please add Coffee script, SCSS, TypeScript etc templates so they appear in "New File dialog"?


Comment by Sunjay

Does Web Essentials 2012 work with WebMatrix 3? if so how do we install the extension.