What does the web.config compilation/assemblies element do?

Question

I had a problem where, working on a new MVC project, as soon as I added Html.Partial("Name") to my _Layout.cshtml page, ASP.NET would throw this exception on page load:

error CS0012: The type 'System.Object' is not in a referenced assembly is defined. Add a reference to the Assembly "System.Runtime, Version = 4.0.0.0, Culture = neutral, PublicKeyToken = b03f5f7f11d50a3a" added.

The fix is to add this to the web.config, compilation element:

  <assemblies>     <add assembly="System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />   </assemblies> 

I'd love to know why this fixes the issue.

What's odd is that the new MVC app I'm using has a load of WebForms ASPX pages that work a treat. I've copied a ton of legacy WebForms stuff into this new MVC project, acres of code, and everything works, and without the Html.Partial call, even my MVC view renders.

So what on Earth is going on? What does this config do and why aren't the normal project references and target framework settings enough?

Answer 1

This is caused by a disconnect and limitation between the build time and the runtime environments of an ASP.NET/MVC project. Most code in a Web project is compiled at build time (ie F5) with exception to views (both Razor and inline ASPX code) which are built dynamically at runtime (ie within IIS).

For the most part, we attempt to make these two environments exactly the same and you will not typically notice any difference. Unfortunately. you've run into one of those cases where there is a difference and you've needed to manually add a reference to make the runtime mimic build-time.

What's happening, is that you likely have a dependency on a portable library (at a guess, Microsoft.Net.Http). Portable Libraries are a little different to normal libraries, and we need extra references to make the compiler happy. At build-time we detect a dependency on a portable library and automatically add all the references to C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\Facades. Unfortunately, the runtime doesn't do this and you need to manually add these references until we fix this in a future release.

For future reference, you can just grab the following section and paste this directly into Web.Config (this represents the snapshot for 4.5):

 <compilation>       <assemblies>         <add assembly="System.Collections, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Collections.Concurrent, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ComponentModel, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ComponentModel.Annotations, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ComponentModel.EventBasedAsync, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Diagnostics.Contracts, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Diagnostics.Debug, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Diagnostics.Tools, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Diagnostics.Tracing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Dynamic.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Globalization, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.IO, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Linq.Expressions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Linq.Parallel, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Linq.Queryable, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Net.NetworkInformation, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Net.Primitives, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Net.Requests, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ObjectModel, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Reflection, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Reflection.Emit, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Reflection.Emit.ILGeneration, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Reflection.Emit.Lightweight, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Reflection.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Reflection.Primitives, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Resources.ResourceManager, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Runtime.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Runtime.InteropServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Runtime.InteropServices.WindowsRuntime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Runtime.Numerics, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Runtime.Serialization.Json, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Runtime.Serialization.Primitives, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Runtime.Serialization.Xml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Security.Principal, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ServiceModel.Duplex, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ServiceModel.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ServiceModel.NetTcp, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ServiceModel.Primitives, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.ServiceModel.Security, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Text.Encoding, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Text.Encoding.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Text.RegularExpressions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Threading, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Threading.Tasks, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Threading.Tasks.Parallel, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Xml.ReaderWriter, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Xml.XDocument, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />         <add assembly="System.Xml.XmlSerializer, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />       </assemblies>     </compilation>