Update:

I've now also tried this from scratch to eliminate as many variables as possible. Here are the repro steps:

1. Created a brand new empty ASP.NET solution/project
2. Added the UmbracoCms nuget package (7.4.3)
3. Built the solution
4. Setup my IIS Site - "UmbracoDemo" with host name umbraco-demo.local
5. Browsed to http://umbraco-demo.local and ran the Umbraco installer

NOTE: So far everything works as expected. I can log into the back office or browse the new Umbraco site..

1. Created a new ASP.NET MVC application in a new VS solution called "DemoApp"
2. Built the new solution and ran it immediately with IIS Express -- the MVC app works correctly
3. In IIS Manager, I right-clicked my UmbracoDemo site and added an "Application" called "DemoApp", pointing it to the DemoApp MVC project directory
4. Added "~/DemoApp" to the umbracoReservedPaths setting in Umbraco's web.config
5. Browsed to http://umbraco-demo.local/DemoApp

Again I get the error:

Could not load file or assembly 'Umbraco.ModelsBuilder' or one of its dependencies. The system cannot find the file specified.

Additional info:

I don't think this is specifically related to ModelsBuilder. Just to see what would happen, I added this to my "DemoApp" web.config:

<assemblies>
  <remove assembly="Umbraco.ModelsBuilder" />
</assemblies>

Now the error message says:

Could not load file or assembly 'UrlRewritingNet.UrlRewriter' or one of its dependencies. The system cannot find the file specified.

So naturally, I tried adding:

<remove assembly="UrlRewritingNet.UrlRewriter" />

But this had no impact, the error remained.

Hi Brian,

I found your post as I was getting the same error. For me the child application was taking on the configuration from the main web.config which was for the Umbraco site root.

I found information on this stackoverflow post here - http://stackoverflow.com/questions/782252/avoid-web-config-inheritance-in-child-web-application-using-inheritinchildapplic#answer-782277

For me I just wanted to access the application without any Umbraco dependencies and added <clear/> to my DemoApp's 'web.config' file beneath the <configSections> tag and this resolved the YSOD.

Also it didn't have any effect meantime but would be good to read about the UmbracoReservedPaths key in the Umbraco web.config, I believe your demo app directory should be added here but would need to read up again to find exactly why.

Hope this helps.

After several hours I finally got this to work with a nested MVC application. I added the path to the umbracoReservedPaths, in the parent application.

Added the following to the nested MVC web.config, not touching parent web.config.

<system.web>
<compilation debug="false" targetFramework="4.5.2">
  <assemblies>
    <remove assembly="Umbraco.ModelsBuilder"/>
    <remove assembly="System.Web.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
    <remove assembly="UrlRewritingNet.UrlRewriter"/>
  </assemblies>
  <buildProviders>
    <clear/>
  </buildProviders>
</compilation>
<httpModules>
  <remove name="UrlRewriteModule"/>
</httpModules>
<httpRuntime targetFramework="4.5.2" />
<roleManager enabled="false">
  <providers>
    <clear/>
  </providers>
</roleManager>

Hope this helps someone else that is trying to do the same thing.

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