How do I fix the "compilerVersion" IIS error?

I'm getting this exception:

System.Configuration.ConfigurationErrorsException: The value for the 'compilerVersion' attribute in the provider options must be 'v4.0' or later if you are compiling for version 4.0 or later of the .NET Framework.

What should I do to resolve this?

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this should help

<!--  -->
  <compiler language="c#;cs;csharp" extension=".cs" warningLevel="4"
    type="Microsoft.CSharp.CSharpCodeProvider,System, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089">
   <providerOption name="CompilerVersion" value="v4.0"/>
   <providerOption name="WarnAsError" value="false"/>
<!--  -->

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In my case, I was trying to run a child application using 4.0, but the parent application needed to still use 2.0. Wrapping the compilers information in the parent web.config with a <location path="." inheritInChildApplications="false"> tag fixed it.

Parent Web.config:

<location path="." inheritInChildApplications="false">
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider,System, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4">
        <providerOption name="CompilerVersion" value="v3.5" />
        <providerOption name="WarnAsError" value="false" />

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Remove this section from web.config

<compilation debug="true" strict="true" explicit="true"  targetFramework="4.0" />

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In my case it was a child site under the Default Website and although the default website settings were set at ASP.NET 4.0, the web.config file was set for 2.0 and 3.5. Changing the web.config file to use 4.0 fixed it. Use this for reference: ASP.NET 4 Breaking Changes

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  • Thanks for this answer. Worked great.
  • 4igi answer has the C# 4.0 answer.
  • As a side note to "2.0 was used instead of 3.5": The solution above worked for me too in this scenario. But I finally found out WHY the wrong compiler was being used: I'm using a "Web Deployment Project" to compile the web application and it seems it doesn't understand "CodeFile=" in aspx/ascx files (and doesn't find any ".cs" files). The legacy name "CodeBehind=" works though and the "web.config"-hack above is no longer needed. *** MSVC2010, .NET 2.0 with 3.5 Code ***
  • Looking at John K's references, most of the compiler element options are exactly that - optional! So, I had a similar issue with C# and vb in the same website project. The VB stuff was not working, most likely due to setting incorrect version, but we wanted any and all builds of this warning type to be ignored, so I added the compiler element with only the option I wanted: nowarn. <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" warningLevel="1" compilerOptions="/nowarn:618,612,40000" />
  • This one helped my situation. We did not have any problems with a mixed .Net versions on the server until a vendor MSI was run.
  • Nice...this fixed my problem (ASP.NET 4 site is child of ASP.NET 2.0)
  • I didn't see you that you answered your own question before I finished filling mine out :)
  • Yea, but yours was more complete.
  • Also need to remove <httpRuntime targetFramework="4.5" /> in 4.5.
  • For me, I had a nested <assemblies> block in <compilation>. So I just removed all the attributes and ended up with <compilation><assemblies>...</assemblies></compilation>.
  • This helped me. Thanks. I had copied a 3.5 project and changed it to 4.0, but couldn't get it to work correctly until I did this.