How do I fix the "compilerVersion" IIS error?

Question

I had a similar problem and had to tell ASP.NET in configuration to use the 3.5 compiler as follows by modifying Web.config.

I've copied and pasted the following from my code. You have to change value="v3.5" to value="v4.0". The compiler type strings might also change.

<configuration>

  <!--  ... other configuraiton stuff ... -->

  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider,System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4">
        <providerOption name="CompilerVersion" value="v3.5"/>
        <providerOption name="WarnAsError" value="false"/>
      </compiler>
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.VisualBasic.VBCodeProvider, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4">
        <providerOption name="CompilerVersion" value="v3.5"/>
        <providerOption name="OptionInfer" value="true"/>
        <providerOption name="WarnAsError" value="false"/>
      </compiler>
    </compilers>
  </system.codedom>

</configuration>

In my case the 2.0 compiler was being used instead of 3.5. I was working in an IIS 7, ASP.NET Website project.

You might glean additional insight from:

• System.CodeDom Namespace or
• compiler Element for compilers for compilation - ASP.NET Settings Schema.

this should help

<configuration>
<!--  -->
<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>
 </compilers>
</system.codedom>
<!--  -->
</configuration>

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">
  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider,System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4">
        <providerOption name="CompilerVersion" value="v3.5" />
        <providerOption name="WarnAsError" value="false" />
      </compiler>
    </compilers>
  </system.codedom>

Remove this section from web.config

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

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

We had accidentally copy-pasted Web.config into C:\inetpub\wwwroot. This accidentally copy-pasted Web.config had mismatching configuration and caused the compilerVersion error for us. Deleting the Web.config solved the problem for us.