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?

Solution 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>

Solution 2

Encountered the exact same error after switching to VS 2017 & C# 7.1 and adding references to System.ValueTuple into my .NET 4.6 ASP.NET project

Problem solved by activating Roslyn in ASP.NET projects without fiddling with references in web.config

Added benefit of this is you can use new C# features in aspx pages and MVC views

Comments

[Luke Puplett]

Thanks Dave and apologies for taking ages to tick your answer.

[Gutemberg Ribeiro]

I'm having the exactly same problem, but this time, its not an MVC application. Instead, it is an Azure Worker role that hosts a Workflow application and the same error happens when I'm loading the workflow from a byte[] definition. I've posted a question here too: stackoverflow.com/questions/28198348/… any clues? There is no compilation tag in APP.config :(

[Teoman shipahi]

When you say it will be fixed on next release, so all these references will be removed by the update? Or should we remove them manually?

[John Tseng]

I had to add this to the compilation tag that already existed in Views/Web.config

[Admin]

Where can we find snapshots for versions different to 4.5 e.g. 4.5.1? Where can we find information about what fixes is MS planning regarding that issue and when to expect them?

[Silvos]

The references in the web.config are still required at 2018, when you are calling code from a PCL in a runtime compiled (ASPX, ASCX, ...) file. (tested with WebForms ASP.Net 4.7. VS 2017 15.8.3 and Microsoft.CodeDom.Providers.DotNetCompilerPlatform v 2.0.0 installed )