CodeDom provider can't be located after another application deployed

Question

I had two web applications deployed in a site under IIS 7.5

site.com/myApp site.com/anotherMyApp

Now another devs deployed their app as root application

site.com/

Now when I access site.com/myApp or /anotherMyApp it throws:

The CodeDom provider type "Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" could not be located.

Source error :

Line 54:   <system.codedom>
Line 55:     <compilers>
Line 56:       <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701" />
Line 57:       <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+" />
Line 58:     </compilers>

Source File: C:\inetpub\wwwrootprettysite\THEIR APPLICATION\web.config

Questions

Why is this happening?
Why is the "source error" in their application's webconfig and how does it affect my application?
How do I fix my applications without breaking theirs?

To clarify :

• Application hierarchy is correct, their application should be
  installed in root.
• .NET framework installed is 4.5.2
• Their application is using Roslyn compiler which is supplied in bin/roslyn
• My applications web.configs have no <system.codedom> section.

Council

Answer 1

Your app inherits the config from its parent app, and your app doesn't have Microsoft.CodeDom.Providers.DotNetCompilerPlatform. That's why you see that error. There are several options.

1. Override system.codedom setting in your web.config
2. Install Microsoft.CodeDom.Providers.DotNetCompilerPlatform nupkg in your app

Frangipani

How do you exactly "override" system.codedom setting ? – Antetype

Just add <system.codedom> into the web.config under your web app folder. If you just want to use old codedom provider, you can add the following setting. <system.codedom> <compilers> <compiler language="c#;cs;csharp" extension=".cs" warningLevel="4" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"> <providerOption name="CompilerVersion" value="v4.0" /> <providerOption name="WarnAsError" value="false" /> </compiler> //.... – Frangipani