title: CS1617: Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default
body: <p>I have run the vs 2015 project version in vs2013. when i build the application without any error. when running the application . i got the followng Message. how could i run the  app in vs 2013.</p>
  
  <p>Error:CS1617: Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default.</p>
  
  <p><strong>In webconfig: compiler section like this</strong></p>
  
  <pre><code>&lt;compilers&gt;
    &lt;compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701"&gt;
        &lt;providerOption name="CompilerVersion" value="v4.0" /&gt;
    &lt;/compiler&gt;
    &lt;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=\&amp;quot;Web\&amp;quot; /optionInfer+" /&gt;
  &lt;/compilers&gt;
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/FY4E1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FY4E1.png" alt="enter image description here"></a></p>