After Windows update "The type or namespace name 'Html' does not exist in the namespace 'System.Web.Mvc'"

Solution 1:

Holy crap, thanks to @Nevada-Williford for the hint. Going in and setting my System.Web.Mvc reference to <Private>True</Private> (Copy Local = True) fixed it. Note, that before the update everything was working, after the update I had to modify my csproj to get it working again.

Working theory on what's going on:

Copy Local = True and <Private>True</Private> used to be almost, but not exactly, the same thing. The former was a Visual Studio setting, the latter an msbuild setting. If the msbuild setting was absent, the Visual Studio setting would be applied (as long as you were in VS). In this update I think they changed it so Copy Local just reflects the presence attribute.

In our project we do not have that attribute set explicitly but Copy Local = True so prior to the update System.Web.Mvc.dll gets copied to the bin directory. After the update, since the attribute is missing Copy Local shows False and you have to set it to True to make sure you get a local copy.

Manually setting Copy Local = True (or adding that xml element to msbuild) fixes the issue.

Edit: While this appears to be the answer to the specific question, anyone coming here should read the comment thread and other answers - especially dmatson's - for more context, caveats, and related bugs.

Solution 2:

This was broken for any users without CopyLocal=true (or, in MSBuild speak, <Private>True</Private>) by MS14-059. MVC templates do set <Private>True</Private> by default, but if you use NuGet to update the MVC version, you lose that setting (see NuGet bug #4344).

There are two aspects to the problem:

1. Razor doesn't include a reference to MVC by default, so its compilation won't work unless some version of the MVC DLL exists in your bin folder.
2. If you deploy to a separate machine that doesn't have this update installed, the MVC DLL isn't included in your output anymore, so MVC will be missing.

You're seeing problem #1. To resolve both problems, I'd recommend making both of the following changes:

1. Add the following configuration to Views\Web.config:

   <system.web>
     <compilation>
       <assemblies>
         <add assembly="System.Web.Mvc, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
       </assemblies>
     </compilation>
   </system.web>
   

2. Set CopyLocal=true in the VS UI for the project reference, or manually add the following line below in the Reference in your .csproj file:

   <Private>True</Private>
   

So your full reference should look something like the following:

<Reference Include="System.Web.Mvc, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <Private>True</Private>
  <HintPath>..\..\packages\Microsoft.AspNet.Mvc.5.0.0\lib\net45\System.Web.Mvc.dll</HintPath>
</Reference>

Note that NuGet will remove the CopyLocal/Private setting if you update packages again in the future. (For example, if you update to MVC 5.2 today). If that version of MVC is ever GAC'd, problem #1 above will not recur as long as you've added the configuration in step A above, but problem #2 could still happen again. To ensure this doesn't happen in the future, I'd recommend manually setting CopyLocal back to true any time you do a NuGet package update.

Solution 3:

1. You can go to References of current Project.
2. Right click in dll System.Web.Mvc and choose Properties
3. A Properties window will open
4. Change Copy Local to True