System.Web.Abstractions missing on a ASP.NET web site

I recently re-enabled feature on a ASP.NET site. This feature (a set of pages) had been running OK about six months ago for an initial pilot project but was disabled until now until a decision was made to develop the feature fully. In intervening time the web site had other modification and had been rebuilt (it was targeted at .NET 3.5) without issue.

When I re-enabled the feature (renamed a .ASPX file) I got the error

Could not load file or assembly 'System.Web.Abstractions, Version=, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

Now this surprised me, I had not change the .NET version targeting or changed development environment. In the end I fixed it by copying up the missing DLL to the bin directory. I had tried adding the following to the web.config to no effect

  <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
    <assemblyIdentity name="System.Web.Abstractions" publicKeyToken="31BF3856AD364E35"/>
    <bindingRedirect oldVersion="" newVersion=""/>

On think a bit more I think the issue must have been caused by:

  • I may have upgraded to .NET to 3.5. SP1 in the time frame (but I thought I did it earlier)
  • I have installed MVC on my development PC, and most other posts that mention this issue also mention MVC, usually beta versions.

EIther way it is fixed now